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1 | =head1 NAME |
2 | |
3 | perlthrtut - tutorial on threads in Perl |
4 | |
5 | =head1 DESCRIPTION |
6 | |
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7 | B<NOTE>: this tutorial describes the new Perl threading flavour |
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8 | introduced in Perl 5.6.0 called interpreter threads, or B<ithreads> |
9 | for short. In this model each thread runs in its own Perl interpreter, |
10 | and any data sharing between threads must be explicit. |
11 | |
12 | There is another older Perl threading flavour called the 5.005 model, |
13 | unsurprisingly for 5.005 versions of Perl. The old model is known to |
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14 | have problems, is deprecated, and support for it will be removed in release |
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15 | 5.10. You are strongly encouraged to migrate any existing 5.005 |
16 | threads code to the new model as soon as possible. |
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17 | |
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18 | You can see which (or neither) threading flavour you have by |
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19 | running C<perl -V> and looking at the C<Platform> section. |
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20 | If you have C<useithreads=define> you have ithreads, if you |
21 | have C<use5005threads=define> you have 5.005 threads. |
22 | If you have neither, you don't have any thread support built in. |
23 | If you have both, you are in trouble. |
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24 | |
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25 | The user-level interface to the 5.005 threads was via the L<Threads> |
26 | class, while ithreads uses the L<threads> class. Note the change in case. |
27 | |
28 | =head1 Status |
29 | |
30 | The ithreads code has been available since Perl 5.6.0, and is considered |
31 | stable. The user-level interface to ithreads (the L<threads> classes) |
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32 | appeared in the 5.8.0 release, and as of this time is considered stable |
33 | although it should be treated with caution as with all new features. |
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34 | |
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35 | =head1 What Is A Thread Anyway? |
36 | |
37 | A thread is a flow of control through a program with a single |
38 | execution point. |
39 | |
40 | Sounds an awful lot like a process, doesn't it? Well, it should. |
41 | Threads are one of the pieces of a process. Every process has at least |
42 | one thread and, up until now, every process running Perl had only one |
43 | thread. With 5.8, though, you can create extra threads. We're going |
44 | to show you how, when, and why. |
45 | |
46 | =head1 Threaded Program Models |
47 | |
48 | There are three basic ways that you can structure a threaded |
49 | program. Which model you choose depends on what you need your program |
50 | to do. For many non-trivial threaded programs you'll need to choose |
51 | different models for different pieces of your program. |
52 | |
53 | =head2 Boss/Worker |
54 | |
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55 | The boss/worker model usually has one "boss" thread and one or more |
56 | "worker" threads. The boss thread gathers or generates tasks that need |
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57 | to be done, then parcels those tasks out to the appropriate worker |
58 | thread. |
59 | |
60 | This model is common in GUI and server programs, where a main thread |
61 | waits for some event and then passes that event to the appropriate |
62 | worker threads for processing. Once the event has been passed on, the |
63 | boss thread goes back to waiting for another event. |
64 | |
65 | The boss thread does relatively little work. While tasks aren't |
66 | necessarily performed faster than with any other method, it tends to |
67 | have the best user-response times. |
68 | |
69 | =head2 Work Crew |
70 | |
71 | In the work crew model, several threads are created that do |
72 | essentially the same thing to different pieces of data. It closely |
73 | mirrors classical parallel processing and vector processors, where a |
74 | large array of processors do the exact same thing to many pieces of |
75 | data. |
76 | |
77 | This model is particularly useful if the system running the program |
78 | will distribute multiple threads across different processors. It can |
79 | also be useful in ray tracing or rendering engines, where the |
80 | individual threads can pass on interim results to give the user visual |
81 | feedback. |
82 | |
83 | =head2 Pipeline |
84 | |
85 | The pipeline model divides up a task into a series of steps, and |
86 | passes the results of one step on to the thread processing the |
87 | next. Each thread does one thing to each piece of data and passes the |
88 | results to the next thread in line. |
89 | |
90 | This model makes the most sense if you have multiple processors so two |
91 | or more threads will be executing in parallel, though it can often |
92 | make sense in other contexts as well. It tends to keep the individual |
93 | tasks small and simple, as well as allowing some parts of the pipeline |
94 | to block (on I/O or system calls, for example) while other parts keep |
95 | going. If you're running different parts of the pipeline on different |
96 | processors you may also take advantage of the caches on each |
97 | processor. |
98 | |
99 | This model is also handy for a form of recursive programming where, |
100 | rather than having a subroutine call itself, it instead creates |
101 | another thread. Prime and Fibonacci generators both map well to this |
102 | form of the pipeline model. (A version of a prime number generator is |
103 | presented later on.) |
104 | |
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105 | =head1 What kind of threads are Perl threads? |
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106 | |
107 | If you have experience with other thread implementations, you might |
108 | find that things aren't quite what you expect. It's very important to |
109 | remember when dealing with Perl threads that Perl Threads Are Not X |
110 | Threads, for all values of X. They aren't POSIX threads, or |
111 | DecThreads, or Java's Green threads, or Win32 threads. There are |
112 | similarities, and the broad concepts are the same, but if you start |
113 | looking for implementation details you're going to be either |
114 | disappointed or confused. Possibly both. |
115 | |
116 | This is not to say that Perl threads are completely different from |
117 | everything that's ever come before--they're not. Perl's threading |
118 | model owes a lot to other thread models, especially POSIX. Just as |
119 | Perl is not C, though, Perl threads are not POSIX threads. So if you |
120 | find yourself looking for mutexes, or thread priorities, it's time to |
121 | step back a bit and think about what you want to do and how Perl can |
122 | do it. |
123 | |
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124 | However it is important to remember that Perl threads cannot magically |
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125 | do things unless your operating systems threads allows it. So if your |
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126 | system blocks the entire process on sleep(), Perl usually will as well. |
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127 | |
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128 | Perl Threads Are Different. |
129 | |
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130 | =head1 Thread-Safe Modules |
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131 | |
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132 | The addition of threads has changed Perl's internals |
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133 | substantially. There are implications for people who write |
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134 | modules with XS code or external libraries. However, since perl data is |
135 | not shared among threads by default, Perl modules stand a high chance of |
136 | being thread-safe or can be made thread-safe easily. Modules that are not |
137 | tagged as thread-safe should be tested or code reviewed before being used |
138 | in production code. |
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139 | |
140 | Not all modules that you might use are thread-safe, and you should |
141 | always assume a module is unsafe unless the documentation says |
142 | otherwise. This includes modules that are distributed as part of the |
143 | core. Threads are a new feature, and even some of the standard |
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144 | modules aren't thread-safe. |
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145 | |
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146 | Even if a module is thread-safe, it doesn't mean that the module is optimized |
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147 | to work well with threads. A module could possibly be rewritten to utilize |
148 | the new features in threaded Perl to increase performance in a threaded |
149 | environment. |
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150 | |
151 | If you're using a module that's not thread-safe for some reason, you |
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152 | can protect yourself by using it from one, and only one thread at all. |
153 | If you need multiple threads to access such a module, you can use semaphores and |
154 | lots of programming discipline to control access to it. Semaphores |
155 | are covered in L</"Basic semaphores">. |
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156 | |
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157 | See also L</"Thread-Safety of System Libraries">. |
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158 | |
159 | =head1 Thread Basics |
160 | |
161 | The core L<threads> module provides the basic functions you need to write |
162 | threaded programs. In the following sections we'll cover the basics, |
163 | showing you what you need to do to create a threaded program. After |
164 | that, we'll go over some of the features of the L<threads> module that |
165 | make threaded programming easier. |
166 | |
167 | =head2 Basic Thread Support |
168 | |
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169 | Thread support is a Perl compile-time option - it's something that's |
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170 | turned on or off when Perl is built at your site, rather than when |
171 | your programs are compiled. If your Perl wasn't compiled with thread |
172 | support enabled, then any attempt to use threads will fail. |
173 | |
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174 | Your programs can use the Config module to check whether threads are |
175 | enabled. If your program can't run without them, you can say something |
176 | like: |
177 | |
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178 | $Config{useithreads} or die "Recompile Perl with threads to run this program."; |
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179 | |
180 | A possibly-threaded program using a possibly-threaded module might |
181 | have code like this: |
182 | |
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183 | use Config; |
184 | use MyMod; |
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185 | |
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186 | BEGIN { |
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187 | if ($Config{useithreads}) { |
188 | # We have threads |
189 | require MyMod_threaded; |
190 | import MyMod_threaded; |
191 | } else { |
192 | require MyMod_unthreaded; |
193 | import MyMod_unthreaded; |
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194 | } |
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195 | } |
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196 | |
197 | Since code that runs both with and without threads is usually pretty |
198 | messy, it's best to isolate the thread-specific code in its own |
199 | module. In our example above, that's what MyMod_threaded is, and it's |
200 | only imported if we're running on a threaded Perl. |
201 | |
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202 | =head2 A Note about the Examples |
203 | |
204 | Although thread support is considered to be stable, there are still a number |
205 | of quirks that may startle you when you try out any of the examples below. |
206 | In a real situation, care should be taken that all threads are finished |
207 | executing before the program exits. That care has B<not> been taken in these |
208 | examples in the interest of simplicity. Running these examples "as is" will |
209 | produce error messages, usually caused by the fact that there are still |
210 | threads running when the program exits. You should not be alarmed by this. |
211 | Future versions of Perl may fix this problem. |
212 | |
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213 | =head2 Creating Threads |
214 | |
215 | The L<threads> package provides the tools you need to create new |
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216 | threads. Like any other module, you need to tell Perl that you want to use |
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217 | it; C<use threads> imports all the pieces you need to create basic |
218 | threads. |
219 | |
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220 | The simplest, most straightforward way to create a thread is with new(): |
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221 | |
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222 | use threads; |
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223 | |
224 | $thr = threads->new(\&sub1); |
225 | |
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226 | sub sub1 { |
227 | print "In the thread\n"; |
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228 | } |
229 | |
230 | The new() method takes a reference to a subroutine and creates a new |
231 | thread, which starts executing in the referenced subroutine. Control |
232 | then passes both to the subroutine and the caller. |
233 | |
234 | If you need to, your program can pass parameters to the subroutine as |
235 | part of the thread startup. Just include the list of parameters as |
236 | part of the C<threads::new> call, like this: |
237 | |
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238 | use threads; |
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239 | |
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240 | $Param3 = "foo"; |
241 | $thr = threads->new(\&sub1, "Param 1", "Param 2", $Param3); |
242 | $thr = threads->new(\&sub1, @ParamList); |
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243 | $thr = threads->new(\&sub1, qw(Param1 Param2 Param3)); |
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244 | |
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245 | sub sub1 { |
246 | my @InboundParameters = @_; |
247 | print "In the thread\n"; |
248 | print "got parameters >", join("<>", @InboundParameters), "<\n"; |
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249 | } |
250 | |
251 | |
252 | The last example illustrates another feature of threads. You can spawn |
253 | off several threads using the same subroutine. Each thread executes |
254 | the same subroutine, but in a separate thread with a separate |
255 | environment and potentially separate arguments. |
256 | |
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257 | C<create()> is a synonym for C<new()>. |
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258 | |
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259 | =head2 Waiting For A Thread To Exit |
260 | |
261 | Since threads are also subroutines, they can return values. To wait |
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262 | for a thread to exit and extract any values it might return, you can |
263 | use the join() method: |
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264 | |
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265 | use threads; |
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266 | |
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267 | $thr = threads->new(\&sub1); |
268 | |
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269 | @ReturnData = $thr->join; |
270 | print "Thread returned @ReturnData"; |
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271 | |
272 | sub sub1 { return "Fifty-six", "foo", 2; } |
273 | |
274 | In the example above, the join() method returns as soon as the thread |
275 | ends. In addition to waiting for a thread to finish and gathering up |
276 | any values that the thread might have returned, join() also performs |
277 | any OS cleanup necessary for the thread. That cleanup might be |
278 | important, especially for long-running programs that spawn lots of |
279 | threads. If you don't want the return values and don't want to wait |
280 | for the thread to finish, you should call the detach() method |
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281 | instead, as described next. |
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282 | |
283 | =head2 Ignoring A Thread |
284 | |
285 | join() does three things: it waits for a thread to exit, cleans up |
286 | after it, and returns any data the thread may have produced. But what |
287 | if you're not interested in the thread's return values, and you don't |
288 | really care when the thread finishes? All you want is for the thread |
289 | to get cleaned up after when it's done. |
290 | |
291 | In this case, you use the detach() method. Once a thread is detached, |
292 | it'll run until it's finished, then Perl will clean up after it |
293 | automatically. |
294 | |
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295 | use threads; |
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296 | |
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297 | $thr = threads->new(\&sub1); # Spawn the thread |
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298 | |
299 | $thr->detach; # Now we officially don't care any more |
300 | |
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301 | sub sub1 { |
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302 | $a = 0; |
303 | while (1) { |
304 | $a++; |
305 | print "\$a is $a\n"; |
306 | sleep 1; |
307 | } |
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308 | } |
309 | |
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310 | Once a thread is detached, it may not be joined, and any return data |
311 | that it might have produced (if it was done and waiting for a join) is |
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312 | lost. |
313 | |
314 | =head1 Threads And Data |
315 | |
316 | Now that we've covered the basics of threads, it's time for our next |
317 | topic: data. Threading introduces a couple of complications to data |
318 | access that non-threaded programs never need to worry about. |
319 | |
320 | =head2 Shared And Unshared Data |
321 | |
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322 | The biggest difference between Perl ithreads and the old 5.005 style |
323 | threading, or for that matter, to most other threading systems out there, |
324 | is that by default, no data is shared. When a new perl thread is created, |
325 | all the data associated with the current thread is copied to the new |
326 | thread, and is subsequently private to that new thread! |
327 | This is similar in feel to what happens when a UNIX process forks, |
328 | except that in this case, the data is just copied to a different part of |
329 | memory within the same process rather than a real fork taking place. |
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330 | |
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331 | To make use of threading however, one usually wants the threads to share |
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332 | at least some data between themselves. This is done with the |
333 | L<threads::shared> module and the C< : shared> attribute: |
334 | |
335 | use threads; |
336 | use threads::shared; |
337 | |
338 | my $foo : shared = 1; |
339 | my $bar = 1; |
340 | threads->new(sub { $foo++; $bar++ })->join; |
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341 | |
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342 | print "$foo\n"; #prints 2 since $foo is shared |
343 | print "$bar\n"; #prints 1 since $bar is not shared |
344 | |
345 | In the case of a shared array, all the array's elements are shared, and for |
346 | a shared hash, all the keys and values are shared. This places |
347 | restrictions on what may be assigned to shared array and hash elements: only |
348 | simple values or references to shared variables are allowed - this is |
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349 | so that a private variable can't accidentally become shared. A bad |
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350 | assignment will cause the thread to die. For example: |
351 | |
352 | use threads; |
353 | use threads::shared; |
354 | |
355 | my $var = 1; |
356 | my $svar : shared = 2; |
357 | my %hash : shared; |
358 | |
359 | ... create some threads ... |
360 | |
361 | $hash{a} = 1; # all threads see exists($hash{a}) and $hash{a} == 1 |
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362 | $hash{a} = $var # okay - copy-by-value: same effect as previous |
363 | $hash{a} = $svar # okay - copy-by-value: same effect as previous |
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364 | $hash{a} = \$svar # okay - a reference to a shared variable |
365 | $hash{a} = \$var # This will die |
366 | delete $hash{a} # okay - all threads will see !exists($hash{a}) |
367 | |
368 | Note that a shared variable guarantees that if two or more threads try to |
369 | modify it at the same time, the internal state of the variable will not |
370 | become corrupted. However, there are no guarantees beyond this, as |
371 | explained in the next section. |
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372 | |
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373 | =head2 Thread Pitfalls: Races |
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374 | |
375 | While threads bring a new set of useful tools, they also bring a |
376 | number of pitfalls. One pitfall is the race condition: |
377 | |
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378 | use threads; |
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379 | use threads::shared; |
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380 | |
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381 | my $a : shared = 1; |
382 | $thr1 = threads->new(\&sub1); |
383 | $thr2 = threads->new(\&sub2); |
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384 | |
385 | $thr1->join; |
386 | $thr2->join; |
387 | print "$a\n"; |
388 | |
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389 | sub sub1 { my $foo = $a; $a = $foo + 1; } |
390 | sub sub2 { my $bar = $a; $a = $bar + 1; } |
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391 | |
392 | What do you think $a will be? The answer, unfortunately, is "it |
393 | depends." Both sub1() and sub2() access the global variable $a, once |
394 | to read and once to write. Depending on factors ranging from your |
395 | thread implementation's scheduling algorithm to the phase of the moon, |
396 | $a can be 2 or 3. |
397 | |
398 | Race conditions are caused by unsynchronized access to shared |
399 | data. Without explicit synchronization, there's no way to be sure that |
400 | nothing has happened to the shared data between the time you access it |
401 | and the time you update it. Even this simple code fragment has the |
402 | possibility of error: |
403 | |
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404 | use threads; |
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405 | my $a : shared = 2; |
406 | my $b : shared; |
407 | my $c : shared; |
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408 | my $thr1 = threads->create(sub { $b = $a; $a = $b + 1; }); |
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409 | my $thr2 = threads->create(sub { $c = $a; $a = $c + 1; }); |
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410 | $thr1->join; |
411 | $thr2->join; |
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412 | |
413 | Two threads both access $a. Each thread can potentially be interrupted |
414 | at any point, or be executed in any order. At the end, $a could be 3 |
415 | or 4, and both $b and $c could be 2 or 3. |
416 | |
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417 | Even C<$a += 5> or C<$a++> are not guaranteed to be atomic. |
418 | |
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419 | Whenever your program accesses data or resources that can be accessed |
420 | by other threads, you must take steps to coordinate access or risk |
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421 | data inconsistency and race conditions. Note that Perl will protect its |
422 | internals from your race conditions, but it won't protect you from you. |
423 | |
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424 | =head1 Synchronization and control |
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425 | |
426 | Perl provides a number of mechanisms to coordinate the interactions |
427 | between themselves and their data, to avoid race conditions and the like. |
428 | Some of these are designed to resemble the common techniques used in thread |
429 | libraries such as C<pthreads>; others are Perl-specific. Often, the |
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430 | standard techniques are clumsy and difficult to get right (such as |
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431 | condition waits). Where possible, it is usually easier to use Perlish |
432 | techniques such as queues, which remove some of the hard work involved. |
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433 | |
434 | =head2 Controlling access: lock() |
435 | |
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436 | The lock() function takes a shared variable and puts a lock on it. |
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437 | No other thread may lock the variable until the variable is unlocked |
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438 | by the thread holding the lock. Unlocking happens automatically |
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439 | when the locking thread exits the block that contains the call to the |
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440 | C<lock()> function. Using lock() is straightforward: this example has |
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441 | several threads doing some calculations in parallel, and occasionally |
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442 | updating a running total: |
443 | |
444 | use threads; |
445 | use threads::shared; |
446 | |
447 | my $total : shared = 0; |
448 | |
449 | sub calc { |
450 | for (;;) { |
451 | my $result; |
452 | # (... do some calculations and set $result ...) |
453 | { |
454 | lock($total); # block until we obtain the lock |
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455 | $total += $result; |
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456 | } # lock implicitly released at end of scope |
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457 | last if $result == 0; |
458 | } |
459 | } |
460 | |
461 | my $thr1 = threads->new(\&calc); |
462 | my $thr2 = threads->new(\&calc); |
463 | my $thr3 = threads->new(\&calc); |
464 | $thr1->join; |
465 | $thr2->join; |
466 | $thr3->join; |
467 | print "total=$total\n"; |
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468 | |
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469 | lock() blocks the thread until the variable being locked is |
470 | available. When lock() returns, your thread can be sure that no other |
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471 | thread can lock that variable until the block containing the |
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472 | lock exits. |
473 | |
474 | It's important to note that locks don't prevent access to the variable |
475 | in question, only lock attempts. This is in keeping with Perl's |
476 | longstanding tradition of courteous programming, and the advisory file |
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477 | locking that flock() gives you. |
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478 | |
479 | You may lock arrays and hashes as well as scalars. Locking an array, |
480 | though, will not block subsequent locks on array elements, just lock |
481 | attempts on the array itself. |
482 | |
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483 | Locks are recursive, which means it's okay for a thread to |
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484 | lock a variable more than once. The lock will last until the outermost |
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485 | lock() on the variable goes out of scope. For example: |
486 | |
487 | my $x : shared; |
488 | doit(); |
489 | |
490 | sub doit { |
491 | { |
492 | { |
493 | lock($x); # wait for lock |
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494 | lock($x); # NOOP - we already have the lock |
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495 | { |
496 | lock($x); # NOOP |
497 | { |
498 | lock($x); # NOOP |
499 | lockit_some_more(); |
500 | } |
501 | } |
502 | } # *** implicit unlock here *** |
503 | } |
504 | } |
505 | |
506 | sub lockit_some_more { |
507 | lock($x); # NOOP |
508 | } # nothing happens here |
509 | |
510 | Note that there is no unlock() function - the only way to unlock a |
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511 | variable is to allow it to go out of scope. |
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512 | |
513 | A lock can either be used to guard the data contained within the variable |
514 | being locked, or it can be used to guard something else, like a section |
515 | of code. In this latter case, the variable in question does not hold any |
516 | useful data, and exists only for the purpose of being locked. In this |
517 | respect, the variable behaves like the mutexes and basic semaphores of |
518 | traditional thread libraries. |
c975c451 |
519 | |
bfce6503 |
520 | =head2 A Thread Pitfall: Deadlocks |
c975c451 |
521 | |
bfce6503 |
522 | Locks are a handy tool to synchronize access to data, and using them |
c975c451 |
523 | properly is the key to safe shared data. Unfortunately, locks aren't |
f3278b06 |
524 | without their dangers, especially when multiple locks are involved. |
bfce6503 |
525 | Consider the following code: |
c975c451 |
526 | |
0b390a82 |
527 | use threads; |
528 | |
529 | my $a : shared = 4; |
530 | my $b : shared = "foo"; |
531 | my $thr1 = threads->new(sub { |
532 | lock($a); |
533 | sleep 20; |
534 | lock($b); |
535 | }); |
536 | my $thr2 = threads->new(sub { |
537 | lock($b); |
538 | sleep 20; |
539 | lock($a); |
c975c451 |
540 | }); |
541 | |
542 | This program will probably hang until you kill it. The only way it |
bfce6503 |
543 | won't hang is if one of the two threads acquires both locks |
c975c451 |
544 | first. A guaranteed-to-hang version is more complicated, but the |
545 | principle is the same. |
546 | |
bfce6503 |
547 | The first thread will grab a lock on $a, then, after a pause during which |
548 | the second thread has probably had time to do some work, try to grab a |
549 | lock on $b. Meanwhile, the second thread grabs a lock on $b, then later |
550 | tries to grab a lock on $a. The second lock attempt for both threads will |
551 | block, each waiting for the other to release its lock. |
c975c451 |
552 | |
553 | This condition is called a deadlock, and it occurs whenever two or |
554 | more threads are trying to get locks on resources that the others |
555 | own. Each thread will block, waiting for the other to release a lock |
556 | on a resource. That never happens, though, since the thread with the |
557 | resource is itself waiting for a lock to be released. |
558 | |
559 | There are a number of ways to handle this sort of problem. The best |
560 | way is to always have all threads acquire locks in the exact same |
561 | order. If, for example, you lock variables $a, $b, and $c, always lock |
562 | $a before $b, and $b before $c. It's also best to hold on to locks for |
563 | as short a period of time to minimize the risks of deadlock. |
564 | |
48b96218 |
565 | The other synchronization primitives described below can suffer from |
bfce6503 |
566 | similar problems. |
567 | |
c975c451 |
568 | =head2 Queues: Passing Data Around |
569 | |
570 | A queue is a special thread-safe object that lets you put data in one |
571 | end and take it out the other without having to worry about |
572 | synchronization issues. They're pretty straightforward, and look like |
573 | this: |
574 | |
0b390a82 |
575 | use threads; |
83272a45 |
576 | use Thread::Queue; |
c975c451 |
577 | |
0b390a82 |
578 | my $DataQueue = Thread::Queue->new; |
579 | $thr = threads->new(sub { |
580 | while ($DataElement = $DataQueue->dequeue) { |
c975c451 |
581 | print "Popped $DataElement off the queue\n"; |
0b390a82 |
582 | } |
583 | }); |
c975c451 |
584 | |
0b390a82 |
585 | $DataQueue->enqueue(12); |
586 | $DataQueue->enqueue("A", "B", "C"); |
587 | $DataQueue->enqueue(\$thr); |
588 | sleep 10; |
c975c451 |
589 | $DataQueue->enqueue(undef); |
8f95bfb9 |
590 | $thr->join; |
c975c451 |
591 | |
83272a45 |
592 | You create the queue with C<new Thread::Queue>. Then you can |
6eded8f3 |
593 | add lists of scalars onto the end with enqueue(), and pop scalars off |
594 | the front of it with dequeue(). A queue has no fixed size, and can grow |
595 | as needed to hold everything pushed on to it. |
c975c451 |
596 | |
597 | If a queue is empty, dequeue() blocks until another thread enqueues |
598 | something. This makes queues ideal for event loops and other |
599 | communications between threads. |
600 | |
c975c451 |
601 | =head2 Semaphores: Synchronizing Data Access |
602 | |
bfce6503 |
603 | Semaphores are a kind of generic locking mechanism. In their most basic |
fa11829f |
604 | form, they behave very much like lockable scalars, except that they |
bfce6503 |
605 | can't hold data, and that they must be explicitly unlocked. In their |
606 | advanced form, they act like a kind of counter, and can allow multiple |
607 | threads to have the 'lock' at any one time. |
2605996a |
608 | |
bfce6503 |
609 | =head2 Basic semaphores |
2605996a |
610 | |
bfce6503 |
611 | Semaphores have two methods, down() and up(): down() decrements the resource |
612 | count, while up increments it. Calls to down() will block if the |
c975c451 |
613 | semaphore's current count would decrement below zero. This program |
614 | gives a quick demonstration: |
615 | |
536bca94 |
616 | use threads; |
0b390a82 |
617 | use Thread::Semaphore; |
bfce6503 |
618 | |
0b390a82 |
619 | my $semaphore = new Thread::Semaphore; |
bfce6503 |
620 | my $GlobalVariable : shared = 0; |
2605996a |
621 | |
0b390a82 |
622 | $thr1 = new threads \&sample_sub, 1; |
623 | $thr2 = new threads \&sample_sub, 2; |
c975c451 |
624 | $thr3 = new threads \&sample_sub, 3; |
2605996a |
625 | |
0b390a82 |
626 | sub sample_sub { |
627 | my $SubNumber = shift @_; |
628 | my $TryCount = 10; |
629 | my $LocalCopy; |
630 | sleep 1; |
631 | while ($TryCount--) { |
632 | $semaphore->down; |
633 | $LocalCopy = $GlobalVariable; |
634 | print "$TryCount tries left for sub $SubNumber (\$GlobalVariable is $GlobalVariable)\n"; |
635 | sleep 2; |
636 | $LocalCopy++; |
637 | $GlobalVariable = $LocalCopy; |
638 | $semaphore->up; |
639 | } |
c975c451 |
640 | } |
6eded8f3 |
641 | |
8f95bfb9 |
642 | $thr1->join; |
643 | $thr2->join; |
644 | $thr3->join; |
2605996a |
645 | |
c975c451 |
646 | The three invocations of the subroutine all operate in sync. The |
647 | semaphore, though, makes sure that only one thread is accessing the |
648 | global variable at once. |
2605996a |
649 | |
bfce6503 |
650 | =head2 Advanced Semaphores |
2605996a |
651 | |
c975c451 |
652 | By default, semaphores behave like locks, letting only one thread |
653 | down() them at a time. However, there are other uses for semaphores. |
2605996a |
654 | |
6eded8f3 |
655 | Each semaphore has a counter attached to it. By default, semaphores are |
656 | created with the counter set to one, down() decrements the counter by |
657 | one, and up() increments by one. However, we can override any or all |
658 | of these defaults simply by passing in different values: |
659 | |
660 | use threads; |
83272a45 |
661 | use Thread::Semaphore; |
662 | my $semaphore = Thread::Semaphore->new(5); |
6eded8f3 |
663 | # Creates a semaphore with the counter set to five |
664 | |
665 | $thr1 = threads->new(\&sub1); |
666 | $thr2 = threads->new(\&sub1); |
667 | |
668 | sub sub1 { |
669 | $semaphore->down(5); # Decrements the counter by five |
670 | # Do stuff here |
671 | $semaphore->up(5); # Increment the counter by five |
672 | } |
673 | |
8f95bfb9 |
674 | $thr1->detach; |
675 | $thr2->detach; |
6eded8f3 |
676 | |
677 | If down() attempts to decrement the counter below zero, it blocks until |
678 | the counter is large enough. Note that while a semaphore can be created |
679 | with a starting count of zero, any up() or down() always changes the |
680 | counter by at least one, and so $semaphore->down(0) is the same as |
681 | $semaphore->down(1). |
2605996a |
682 | |
c975c451 |
683 | The question, of course, is why would you do something like this? Why |
684 | create a semaphore with a starting count that's not one, or why |
685 | decrement/increment it by more than one? The answer is resource |
686 | availability. Many resources that you want to manage access for can be |
687 | safely used by more than one thread at once. |
2605996a |
688 | |
c975c451 |
689 | For example, let's take a GUI driven program. It has a semaphore that |
690 | it uses to synchronize access to the display, so only one thread is |
691 | ever drawing at once. Handy, but of course you don't want any thread |
692 | to start drawing until things are properly set up. In this case, you |
693 | can create a semaphore with a counter set to zero, and up it when |
694 | things are ready for drawing. |
2605996a |
695 | |
c975c451 |
696 | Semaphores with counters greater than one are also useful for |
697 | establishing quotas. Say, for example, that you have a number of |
698 | threads that can do I/O at once. You don't want all the threads |
699 | reading or writing at once though, since that can potentially swamp |
700 | your I/O channels, or deplete your process' quota of filehandles. You |
701 | can use a semaphore initialized to the number of concurrent I/O |
702 | requests (or open files) that you want at any one time, and have your |
703 | threads quietly block and unblock themselves. |
2605996a |
704 | |
c975c451 |
705 | Larger increments or decrements are handy in those cases where a |
706 | thread needs to check out or return a number of resources at once. |
2605996a |
707 | |
bfce6503 |
708 | =head2 cond_wait() and cond_signal() |
709 | |
710 | These two functions can be used in conjunction with locks to notify |
711 | co-operating threads that a resource has become available. They are |
712 | very similar in use to the functions found in C<pthreads>. However |
713 | for most purposes, queues are simpler to use and more intuitive. See |
714 | L<threads::shared> for more details. |
2605996a |
715 | |
536bca94 |
716 | =head2 Giving up control |
717 | |
718 | There are times when you may find it useful to have a thread |
719 | explicitly give up the CPU to another thread. You may be doing something |
720 | processor-intensive and want to make sure that the user-interface thread |
721 | gets called frequently. Regardless, there are times that you might want |
722 | a thread to give up the processor. |
723 | |
724 | Perl's threading package provides the yield() function that does |
725 | this. yield() is pretty straightforward, and works like this: |
726 | |
0b390a82 |
727 | use threads; |
536bca94 |
728 | |
729 | sub loop { |
0b390a82 |
730 | my $thread = shift; |
731 | my $foo = 50; |
732 | while($foo--) { print "in thread $thread\n" } |
733 | threads->yield; |
734 | $foo = 50; |
735 | while($foo--) { print "in thread $thread\n" } |
536bca94 |
736 | } |
737 | |
738 | my $thread1 = threads->new(\&loop, 'first'); |
739 | my $thread2 = threads->new(\&loop, 'second'); |
740 | my $thread3 = threads->new(\&loop, 'third'); |
741 | |
742 | It is important to remember that yield() is only a hint to give up the CPU, |
743 | it depends on your hardware, OS and threading libraries what actually happens. |
744 | B<On many operating systems, yield() is a no-op.> Therefore it is important |
745 | to note that one should not build the scheduling of the threads around |
746 | yield() calls. It might work on your platform but it won't work on another |
747 | platform. |
748 | |
c975c451 |
749 | =head1 General Thread Utility Routines |
750 | |
751 | We've covered the workhorse parts of Perl's threading package, and |
752 | with these tools you should be well on your way to writing threaded |
753 | code and packages. There are a few useful little pieces that didn't |
754 | really fit in anyplace else. |
755 | |
756 | =head2 What Thread Am I In? |
757 | |
bfce6503 |
758 | The C<< threads->self >> class method provides your program with a way to |
759 | get an object representing the thread it's currently in. You can use this |
6eded8f3 |
760 | object in the same way as the ones returned from thread creation. |
c975c451 |
761 | |
762 | =head2 Thread IDs |
763 | |
764 | tid() is a thread object method that returns the thread ID of the |
765 | thread the object represents. Thread IDs are integers, with the main |
766 | thread in a program being 0. Currently Perl assigns a unique tid to |
767 | every thread ever created in your program, assigning the first thread |
768 | to be created a tid of 1, and increasing the tid by 1 for each new |
769 | thread that's created. |
770 | |
771 | =head2 Are These Threads The Same? |
772 | |
0b390a82 |
773 | The equal() method takes two thread objects and returns true |
c975c451 |
774 | if the objects represent the same thread, and false if they don't. |
775 | |
776 | Thread objects also have an overloaded == comparison so that you can do |
777 | comparison on them as you would with normal objects. |
778 | |
779 | =head2 What Threads Are Running? |
780 | |
bfce6503 |
781 | C<< threads->list >> returns a list of thread objects, one for each thread |
c975c451 |
782 | that's currently running and not detached. Handy for a number of things, |
783 | including cleaning up at the end of your program: |
784 | |
0b390a82 |
785 | # Loop through all the threads |
786 | foreach $thr (threads->list) { |
787 | # Don't join the main thread or ourselves |
788 | if ($thr->tid && !threads::equal($thr, threads->self)) { |
789 | $thr->join; |
790 | } |
c975c451 |
791 | } |
792 | |
bfce6503 |
793 | If some threads have not finished running when the main Perl thread |
794 | ends, Perl will warn you about it and die, since it is impossible for Perl |
6eded8f3 |
795 | to clean up itself while other threads are running |
c975c451 |
796 | |
797 | =head1 A Complete Example |
798 | |
799 | Confused yet? It's time for an example program to show some of the |
800 | things we've covered. This program finds prime numbers using threads. |
801 | |
802 | 1 #!/usr/bin/perl -w |
803 | 2 # prime-pthread, courtesy of Tom Christiansen |
804 | 3 |
805 | 4 use strict; |
806 | 5 |
807 | 6 use threads; |
83272a45 |
808 | 7 use Thread::Queue; |
c975c451 |
809 | 8 |
83272a45 |
810 | 9 my $stream = new Thread::Queue; |
c975c451 |
811 | 10 my $kid = new threads(\&check_num, $stream, 2); |
812 | 11 |
813 | 12 for my $i ( 3 .. 1000 ) { |
814 | 13 $stream->enqueue($i); |
0b390a82 |
815 | 14 } |
c975c451 |
816 | 15 |
817 | 16 $stream->enqueue(undef); |
8f95bfb9 |
818 | 17 $kid->join; |
c975c451 |
819 | 18 |
820 | 19 sub check_num { |
821 | 20 my ($upstream, $cur_prime) = @_; |
822 | 21 my $kid; |
83272a45 |
823 | 22 my $downstream = new Thread::Queue; |
c975c451 |
824 | 23 while (my $num = $upstream->dequeue) { |
825 | 24 next unless $num % $cur_prime; |
826 | 25 if ($kid) { |
827 | 26 $downstream->enqueue($num); |
828 | 27 } else { |
829 | 28 print "Found prime $num\n"; |
830 | 29 $kid = new threads(\&check_num, $downstream, $num); |
831 | 30 } |
0b390a82 |
832 | 31 } |
c975c451 |
833 | 32 $downstream->enqueue(undef) if $kid; |
8f95bfb9 |
834 | 33 $kid->join if $kid; |
c975c451 |
835 | 34 } |
836 | |
837 | This program uses the pipeline model to generate prime numbers. Each |
838 | thread in the pipeline has an input queue that feeds numbers to be |
839 | checked, a prime number that it's responsible for, and an output queue |
9e75ef81 |
840 | into which it funnels numbers that have failed the check. If the thread |
c975c451 |
841 | has a number that's failed its check and there's no child thread, then |
842 | the thread must have found a new prime number. In that case, a new |
843 | child thread is created for that prime and stuck on the end of the |
844 | pipeline. |
845 | |
6eded8f3 |
846 | This probably sounds a bit more confusing than it really is, so let's |
c975c451 |
847 | go through this program piece by piece and see what it does. (For |
848 | those of you who might be trying to remember exactly what a prime |
849 | number is, it's a number that's only evenly divisible by itself and 1) |
850 | |
851 | The bulk of the work is done by the check_num() subroutine, which |
852 | takes a reference to its input queue and a prime number that it's |
853 | responsible for. After pulling in the input queue and the prime that |
854 | the subroutine's checking (line 20), we create a new queue (line 22) |
855 | and reserve a scalar for the thread that we're likely to create later |
856 | (line 21). |
857 | |
858 | The while loop from lines 23 to line 31 grabs a scalar off the input |
859 | queue and checks against the prime this thread is responsible |
860 | for. Line 24 checks to see if there's a remainder when we modulo the |
861 | number to be checked against our prime. If there is one, the number |
862 | must not be evenly divisible by our prime, so we need to either pass |
863 | it on to the next thread if we've created one (line 26) or create a |
864 | new thread if we haven't. |
865 | |
866 | The new thread creation is line 29. We pass on to it a reference to |
867 | the queue we've created, and the prime number we've found. |
868 | |
869 | Finally, once the loop terminates (because we got a 0 or undef in the |
870 | queue, which serves as a note to die), we pass on the notice to our |
6eded8f3 |
871 | child and wait for it to exit if we've created a child (lines 32 and |
c975c451 |
872 | 37). |
873 | |
874 | Meanwhile, back in the main thread, we create a queue (line 9) and the |
875 | initial child thread (line 10), and pre-seed it with the first prime: |
876 | 2. Then we queue all the numbers from 3 to 1000 for checking (lines |
877 | 12-14), then queue a die notice (line 16) and wait for the first child |
878 | thread to terminate (line 17). Because a child won't die until its |
879 | child has died, we know that we're done once we return from the join. |
880 | |
881 | That's how it works. It's pretty simple; as with many Perl programs, |
882 | the explanation is much longer than the program. |
883 | |
536bca94 |
884 | =head1 Different implementations of threads |
885 | |
886 | Some background on thread implementations from the operating system |
887 | viewpoint. There are three basic categories of threads: user-mode threads, |
888 | kernel threads, and multiprocessor kernel threads. |
889 | |
890 | User-mode threads are threads that live entirely within a program and |
891 | its libraries. In this model, the OS knows nothing about threads. As |
892 | far as it's concerned, your process is just a process. |
893 | |
894 | This is the easiest way to implement threads, and the way most OSes |
895 | start. The big disadvantage is that, since the OS knows nothing about |
896 | threads, if one thread blocks they all do. Typical blocking activities |
897 | include most system calls, most I/O, and things like sleep(). |
898 | |
899 | Kernel threads are the next step in thread evolution. The OS knows |
900 | about kernel threads, and makes allowances for them. The main |
901 | difference between a kernel thread and a user-mode thread is |
902 | blocking. With kernel threads, things that block a single thread don't |
903 | block other threads. This is not the case with user-mode threads, |
904 | where the kernel blocks at the process level and not the thread level. |
905 | |
906 | This is a big step forward, and can give a threaded program quite a |
907 | performance boost over non-threaded programs. Threads that block |
908 | performing I/O, for example, won't block threads that are doing other |
909 | things. Each process still has only one thread running at once, |
910 | though, regardless of how many CPUs a system might have. |
911 | |
912 | Since kernel threading can interrupt a thread at any time, they will |
913 | uncover some of the implicit locking assumptions you may make in your |
914 | program. For example, something as simple as C<$a = $a + 2> can behave |
915 | unpredictably with kernel threads if $a is visible to other |
916 | threads, as another thread may have changed $a between the time it |
917 | was fetched on the right hand side and the time the new value is |
918 | stored. |
919 | |
920 | Multiprocessor kernel threads are the final step in thread |
921 | support. With multiprocessor kernel threads on a machine with multiple |
922 | CPUs, the OS may schedule two or more threads to run simultaneously on |
923 | different CPUs. |
924 | |
925 | This can give a serious performance boost to your threaded program, |
926 | since more than one thread will be executing at the same time. As a |
927 | tradeoff, though, any of those nagging synchronization issues that |
928 | might not have shown with basic kernel threads will appear with a |
929 | vengeance. |
930 | |
931 | In addition to the different levels of OS involvement in threads, |
932 | different OSes (and different thread implementations for a particular |
933 | OS) allocate CPU cycles to threads in different ways. |
934 | |
935 | Cooperative multitasking systems have running threads give up control |
936 | if one of two things happen. If a thread calls a yield function, it |
937 | gives up control. It also gives up control if the thread does |
938 | something that would cause it to block, such as perform I/O. In a |
939 | cooperative multitasking implementation, one thread can starve all the |
940 | others for CPU time if it so chooses. |
941 | |
942 | Preemptive multitasking systems interrupt threads at regular intervals |
943 | while the system decides which thread should run next. In a preemptive |
944 | multitasking system, one thread usually won't monopolize the CPU. |
945 | |
946 | On some systems, there can be cooperative and preemptive threads |
947 | running simultaneously. (Threads running with realtime priorities |
948 | often behave cooperatively, for example, while threads running at |
949 | normal priorities behave preemptively.) |
950 | |
951 | Most modern operating systems support preemptive multitasking nowadays. |
952 | |
bfce6503 |
953 | =head1 Performance considerations |
954 | |
955 | The main thing to bear in mind when comparing ithreads to other threading |
956 | models is the fact that for each new thread created, a complete copy of |
957 | all the variables and data of the parent thread has to be taken. Thus |
958 | thread creation can be quite expensive, both in terms of memory usage and |
959 | time spent in creation. The ideal way to reduce these costs is to have a |
960 | relatively short number of long-lived threads, all created fairly early |
961 | on - before the base thread has accumulated too much data. Of course, this |
962 | may not always be possible, so compromises have to be made. However, after |
963 | a thread has been created, its performance and extra memory usage should |
964 | be little different than ordinary code. |
965 | |
966 | Also note that under the current implementation, shared variables |
967 | use a little more memory and are a little slower than ordinary variables. |
968 | |
cf5baa48 |
969 | =head1 Process-scope Changes |
970 | |
971 | Note that while threads themselves are separate execution threads and |
972 | Perl data is thread-private unless explicitly shared, the threads can |
973 | affect process-scope state, affecting all the threads. |
974 | |
975 | The most common example of this is changing the current working |
976 | directory using chdir(). One thread calls chdir(), and the working |
977 | directory of all the threads changes. |
bdcfa4c7 |
978 | |
cf5baa48 |
979 | Even more drastic example of a process-scope change is chroot(): |
980 | the root directory of all the threads changes, and no thread can |
981 | undo it (as opposed to chdir()). |
982 | |
983 | Further examples of process-scope changes include umask() and |
984 | changing uids/gids. |
985 | |
986 | Thinking of mixing fork() and threads? Please lie down and wait |
a95a5f75 |
987 | until the feeling passes. Be aware that the semantics of fork() vary |
988 | between platforms. For example, some UNIX systems copy all the current |
989 | threads into the child process, while others only copy the thread that |
990 | called fork(). You have been warned! |
cf5baa48 |
991 | |
b03ad8f6 |
992 | Similarly, mixing signals and threads should not be attempted. |
993 | Implementations are platform-dependent, and even the POSIX |
994 | semantics may not be what you expect (and Perl doesn't even |
995 | give you the full POSIX API). |
996 | |
cf5baa48 |
997 | =head1 Thread-Safety of System Libraries |
998 | |
999 | Whether various library calls are thread-safe is outside the control |
1000 | of Perl. Calls often suffering from not being thread-safe include: |
bdcfa4c7 |
1001 | localtime(), gmtime(), get{gr,host,net,proto,serv,pw}*(), readdir(), |
cf5baa48 |
1002 | rand(), and srand() -- in general, calls that depend on some global |
1003 | external state. |
80bbcbc4 |
1004 | |
cf5baa48 |
1005 | If the system Perl is compiled in has thread-safe variants of such |
80bbcbc4 |
1006 | calls, they will be used. Beyond that, Perl is at the mercy of |
cf5baa48 |
1007 | the thread-safety or -unsafety of the calls. Please consult your |
80bbcbc4 |
1008 | C library call documentation. |
1009 | |
af685957 |
1010 | On some platforms the thread-safe library interfaces may fail if the |
1011 | result buffer is too small (for example the user group databases may |
1012 | be rather large, and the reentrant interfaces may have to carry around |
1013 | a full snapshot of those databases). Perl will start with a small |
1014 | buffer, but keep retrying and growing the result buffer |
1015 | until the result fits. If this limitless growing sounds bad for |
1016 | security or memory consumption reasons you can recompile Perl with |
1017 | PERL_REENTRANT_MAXSIZE defined to the maximum number of bytes you will |
1018 | allow. |
bdcfa4c7 |
1019 | |
c975c451 |
1020 | =head1 Conclusion |
1021 | |
1022 | A complete thread tutorial could fill a book (and has, many times), |
6eded8f3 |
1023 | but with what we've covered in this introduction, you should be well |
1024 | on your way to becoming a threaded Perl expert. |
c975c451 |
1025 | |
1026 | =head1 Bibliography |
1027 | |
1028 | Here's a short bibliography courtesy of Jürgen Christoffel: |
1029 | |
1030 | =head2 Introductory Texts |
1031 | |
1032 | Birrell, Andrew D. An Introduction to Programming with |
1033 | Threads. Digital Equipment Corporation, 1989, DEC-SRC Research Report |
1034 | #35 online as |
6eded8f3 |
1035 | http://gatekeeper.dec.com/pub/DEC/SRC/research-reports/abstracts/src-rr-035.html |
1036 | (highly recommended) |
c975c451 |
1037 | |
1038 | Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A |
1039 | Guide to Concurrency, Communication, and |
1040 | Multithreading. Prentice-Hall, 1996. |
1041 | |
1042 | Lewis, Bill, and Daniel J. Berg. Multithreaded Programming with |
1043 | Pthreads. Prentice Hall, 1997, ISBN 0-13-443698-9 (a well-written |
1044 | introduction to threads). |
1045 | |
1046 | Nelson, Greg (editor). Systems Programming with Modula-3. Prentice |
1047 | Hall, 1991, ISBN 0-13-590464-1. |
1048 | |
1049 | Nichols, Bradford, Dick Buttlar, and Jacqueline Proulx Farrell. |
1050 | Pthreads Programming. O'Reilly & Associates, 1996, ISBN 156592-115-1 |
1051 | (covers POSIX threads). |
1052 | |
1053 | =head2 OS-Related References |
1054 | |
1055 | Boykin, Joseph, David Kirschen, Alan Langerman, and Susan |
1056 | LoVerso. Programming under Mach. Addison-Wesley, 1994, ISBN |
1057 | 0-201-52739-1. |
1058 | |
1059 | Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall, |
1060 | 1995, ISBN 0-13-219908-4 (great textbook). |
1061 | |
1062 | Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts, |
1063 | 4th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4 |
1064 | |
1065 | =head2 Other References |
1066 | |
1067 | Arnold, Ken and James Gosling. The Java Programming Language, 2nd |
1068 | ed. Addison-Wesley, 1998, ISBN 0-201-31006-6. |
1069 | |
b03ad8f6 |
1070 | comp.programming.threads FAQ, |
1071 | L<http://www.serpentine.com/~bos/threads-faq/> |
1072 | |
c975c451 |
1073 | Le Sergent, T. and B. Berthomieu. "Incremental MultiThreaded Garbage |
1074 | Collection on Virtually Shared Memory Architectures" in Memory |
1075 | Management: Proc. of the International Workshop IWMM 92, St. Malo, |
1076 | France, September 1992, Yves Bekkers and Jacques Cohen, eds. Springer, |
1077 | 1992, ISBN 3540-55940-X (real-life thread applications). |
1078 | |
5e549d84 |
1079 | Artur Bergman, "Where Wizards Fear To Tread", June 11, 2002, |
1080 | L<http://www.perl.com/pub/a/2002/06/11/threads.html> |
1081 | |
c975c451 |
1082 | =head1 Acknowledgements |
1083 | |
1084 | Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy |
1085 | Sarathy, Ilya Zakharevich, Benjamin Sugars, Jürgen Christoffel, Joshua |
1086 | Pritikin, and Alan Burlison, for their help in reality-checking and |
1087 | polishing this article. Big thanks to Tom Christiansen for his rewrite |
1088 | of the prime number generator. |
1089 | |
1090 | =head1 AUTHOR |
1091 | |
9316ed2f |
1092 | Dan Sugalski E<lt>dan@sidhe.org<gt> |
c975c451 |
1093 | |
1094 | Slightly modified by Arthur Bergman to fit the new thread model/module. |
1095 | |
cf5baa48 |
1096 | Reworked slightly by Jörg Walter E<lt>jwalt@cpan.org<gt> to be more concise |
1097 | about thread-safety of perl code. |
1098 | |
536bca94 |
1099 | Rearranged slightly by Elizabeth Mattijsen E<lt>liz@dijkmat.nl<gt> to put |
1100 | less emphasis on yield(). |
1101 | |
c975c451 |
1102 | =head1 Copyrights |
1103 | |
bfce6503 |
1104 | The original version of this article originally appeared in The Perl |
1105 | Journal #10, and is copyright 1998 The Perl Journal. It appears courtesy |
1106 | of Jon Orwant and The Perl Journal. This document may be distributed |
1107 | under the same terms as Perl itself. |
2605996a |
1108 | |
53d7eaa8 |
1109 | For more information please see L<threads> and L<threads::shared>. |