From: Ilya Zakharevich Date: Tue, 9 Jun 1998 22:19:02 +0000 (-0400) Subject: -DL and PERL_DEBUG_MSTATS unravelled X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=a77df738193280ffa098f7f89c9640b77c4b9d4c;p=p5sagit%2Fp5-mst-13.2.git -DL and PERL_DEBUG_MSTATS unravelled Message-Id: <199806100219.WAA04865@monk.mps.ohio-state.edu> p4raw-id: //depot/perl@1110 --- diff --git a/pod/perldebug.pod b/pod/perldebug.pod index cb042e9..888ad7f 100644 --- a/pod/perldebug.pod +++ b/pod/perldebug.pod @@ -1097,10 +1097,289 @@ convenient as arguments to C>, CE> commands. You did try the B<-w> switch, didn't you? -=head1 BUGS +=head2 BUGS You cannot get the stack frame information or otherwise debug functions that were not compiled by Perl, such as C or C++ extensions. If you alter your @_ arguments in a subroutine (such as with B or B, the stack backtrace will not show the original values. + +=head1 Debugging Perl memory usage + +Perl is I frivolous with memory. There is a saying that to +estimate memory usage of Perl, assume a reasonable algorithm of +allocation, and multiply your estimages by 10. This is not absolutely +true, but may give you a good grasp of what happens. + +Say, an integer cannot take less than 20 bytes of memory, a float +cannot take less than 24 bytes, a string cannot take less than 32 +bytes (all these examples assume 32-bit architectures, the result are +much worse on 64-bit architectures). If a variable is accessed in two +of three different ways (which require an integer, a float, or a +string), the memory footprint may increase by another 20 bytes. A +sloppy malloc() implementation will make these numbers yet more. + +On the opposite end of the scale, a declaration like + + sub foo; + +may take (on some versions of perl) up to 500 bytes of memory. + +Off-the-cuff anecdotal estimates of a code bloat give a factor around +8. This means that the compiled form of reasonable (commented +indented etc.) code will take approximately 8 times more than the +disk space the code takes. + +There are two Perl-specific ways to analyze the memory usage: +$ENV{PERL_DEBUG_MSTATS} and B<-DL> switch. First one is available +only if perl is compiled with Perl's malloc(), the second one only if +Perl compiled with C<-DDEBUGGING> (as with giving C<-D optimise=-g> +option to F). + +=head2 Using C<$ENV{PERL_DEBUG_MSTATS}> + +If your perl is using Perl's malloc(), and compiled with correct +switches (this is the default), then it will print memory usage +statistics after compiling your code (if C<$ENV{PERL_DEBUG_MSTATS}> > +1), and before termination of the script (if +C<$ENV{PERL_DEBUG_MSTATS}> >= 1). The report format is similar to one +in the following example: + + env PERL_DEBUG_MSTATS=2 perl -e "require Carp" + Memory allocation statistics after compilation: (buckets 4(4)..8188(8192) + 14216 free: 130 117 28 7 9 0 2 2 1 0 0 + 437 61 36 0 5 + 60924 used: 125 137 161 55 7 8 6 16 2 0 1 + 74 109 304 84 20 + Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048. + Memory allocation statistics after execution: (buckets 4(4)..8188(8192) + 30888 free: 245 78 85 13 6 2 1 3 2 0 1 + 315 162 39 42 11 + 175816 used: 265 176 1112 111 26 22 11 27 2 1 1 + 196 178 1066 798 39 + Total sbrk(): 215040/47:145. Odd ends: pad+heads+chain+tail: 0+2192+0+6144. + +It is possible to ask for such a statistic at arbitrary moment by +usind Devel::Peek::mstats() (module Devel::Peek is available on CPAN). + +Here is the explanation of different parts of the format: + +=over + +=item C + +Perl's malloc() uses bucketed allocations. Every request is rounded +up to the closest bucket size available, and a bucket of these size is +taken from the pool of the buckets of this size. + +The above line describes limits of buckets currently in use. Each +bucket has two sizes: memory footprint, and the maximal size of user +data which may be put into this bucket. Say, in the above example the +smallest bucket is both sizes 4. The biggest bucket has usable size +8188, and the memory footprint 8192. + +With debugging Perl some buckets may have negative usable size. This +means that these buckets cannot (and will not) be used. For greater +buckets the memory footprint may be one page greater than a power of +2. In such a case the corresponding power of two is printed instead +in the C field above. + +=item Free/Used + +The following 1 or 2 rows of numbers correspond to the number of +buckets of each size between C and C. In the +first row the sizes (memory footprints) of buckets are powers of two +(or possibly one page greater). In the second row (if present) the +memory footprints of the buckets are between memory footprints of two +buckets "above". + +Say, with the above example the memory footprints are (with current +algorith) + + free: 8 16 32 64 128 256 512 1024 2048 4096 8192 + 4 12 24 48 80 + +With non-C perl the buckets starting from C<128>-long ones +have 4-byte overhead, thus 8192-long bucket may take up to +8188-byte-long allocations. + +=item C + +The first two fields give the total amount of memory perl sbrk()ed, +and number of sbrk()s used. The third number is what perl thinks +about continuity of returned chunks. As far as this number is +positive, malloc() will assume that it is probable that sbrk() will +provide continuous memory. + +The amounts sbrk()ed by external libraries is not counted. + +=item C + +The amount of sbrk()ed memory needed to keep buckets aligned. + +=item C + +While memory overhead of bigger buckets is kept inside the bucket, for +smaller buckets it is kept in separate areas. This field gives the +total size of these areas. + +=item C + +malloc() may want to subdivide a bigger bucket into smaller buckets. +If only a part of the deceased-bucket is left non-subdivided, the rest +is kept as an element of a linked list. This field gives the total +size of these chunks. + +=item C + +To minimize amount of sbrk()s malloc() asks for more memory. This +field gives the size of the yet-unused part, which is sbrk()ed, but +never touched. + +=back + +=head2 Example of using B<-DL> switch + +Below we show how to analyse memory usage by + + do 'lib/auto/POSIX/autosplit.ix'; + +The file in question contains a header and 146 lines similar to + + sub getcwd ; + +B I + +Here is the itemized list of Perl allocations performed during parsing +of this file: + + !!! "after" at test.pl line 3. + Id subtot 4 8 12 16 20 24 28 32 36 40 48 56 64 72 80 80+ + 0 02 13752 . . . . 294 . . . . . . . . . . 4 + 0 54 5545 . . 8 124 16 . . . 1 1 . . . . . 3 + 5 05 32 . . . . . . . 1 . . . . . . . . + 6 02 7152 . . . . . . . . . . 149 . . . . . + 7 02 3600 . . . . . 150 . . . . . . . . . . + 7 03 64 . -1 . 1 . . 2 . . . . . . . . . + 7 04 7056 . . . . . . . . . . . . . . . 7 + 7 17 38404 . . . . . . . 1 . . 442 149 . . 147 . + 9 03 2078 17 249 32 . . . . 2 . . . . . . . . + + +To see this list insert two C statements around the call: + + warn('!'); + do 'lib/auto/POSIX/autosplit.ix'; + warn('!!! "after"'); + +and run it with B<-DL> option. The first warn() will print memory +allocation info before the parsing of the file, and will memorize the +statistics at this point (we ignore what it prints). The second warn() +will print increments w.r.t. this memorized statistics. This is the +above printout. + +Different Is on the left correspond to different subsystems of +perl interpreter, they are just first argument given to perl memory +allocation API New(). To find what C<9 03> means C the perl +source for C<903>. You will see that it is F, function +savepvn(). This function is used to store a copy of existing chunk of +memory. Using C debugger, one can see that it is called either +directly from gv_init(), or via sv_magic(), and gv_init() is called +from gv_fetchpv() - which is called from newSUB(). + +B to reach this place in debugger and skip all the calls to +savepvn during the compilation of the main script, set a C breakpoint +in Perl_warn(), C this point is reached, I set +breakpoint in Perl_savepvn(). Note that you may need to skip a +handful of Perl_savepvn() which do not correspond to mass production +of CVs (there are more C<903> allocations than 146 similar lines of +F). Note also that C prefixes are +added by macroization code in perl header files to avoid conflicts +with external libraries. + +Anyway, we see that C<903> ids correspond to creation of globs, twice +per glob - for glob name, and glob stringification magic. + +Here are explanations for other Is above: + +=over + +=item C<717> + +is for creation of bigger C structures. In the above case it +creates 3 C per subroutine, one for a list of lexical variable +names, one for a scratchpad (which contains lexical variables and +C), and one for the array of scratchpads needed for +recursion. + +It also creates a C and a C per subroutine (all called from +start_subparse()). + +=item C<002> + +Creates C array corresponding to the C of scratchpads, and the +scratchpad itself (the first fake entry of this scratchpad is created +though the subroutine itself is not defined yet). + +It also creates C arrays to keep data for the stash (this is one HV, +but it grows, thus there are 4 big allocations: the big chunks are not +freeed, but are kept as additional arenas for C allocations). + +=item C<054> + +creates a C for the name of the glob for the subroutine (this +name is a key in a I). + +Big allocations with this I correspond to allocations of new +arenas to keep C. + +=item C<602> + +creates a C for the glob for the subroutine. + +=item C<702> + +creates the C for the glob for the subroutine. + +=item C<704> + +creates I which keep SVs. + +=back + +=head2 B<-DL> details + +If Perl is run with B<-DL> option, then warn()s which start with `!' +behave specially. They print a list of I of memory +allocations, and statistics of allocations of different sizes for +these categories. + +If warn() string starts with + +=over + +=item C + +print changed categories only, print the differences in counts of allocations; + +=item C + +print grown categories only; print the absolute values of counts, and totals; + +=item C + +print nonempty categories, print the absolute values of counts and totals. + +=back + +=head2 Limitations of B<-DL> statistic + +If an extension or an external library does not use Perl API to +allocate memory, these allocations are not counted. + +=cut