PERLDEBGUTS(1) Perl Programmers Reference Guide PERLDEBGUTS(1)NAMEperldebguts - Guts of Perl debugging
DESCRIPTION
This is not the perldebug(1) manpage, which tells you how
to use the debugger. This manpage describes low-level
details ranging between difficult and impossible for any
one who isn't incredibly intimate with Perl's guts to
understand. Caveat lector.
Debugger Internals
Perl has special debugging hooks at compile-time and run-
time used to create debugging environments. These hooks
are not to be confused with the perl -Dxxx command
described in the perlrun manpage, which is usable only if
a special Perl is built per the instructions in the
INSTALL podpage in the Perl source tree.
For example, whenever you call Perl's built-in "caller"
function from the package DB, the arguments that the cor
responding stack frame was called with are copied to the
@DB::args array. The general mechanisms is enabled by
calling Perl with the -d switch, the following additional
features are enabled (cf. the section on "$^P" in the per
lvar manpage):
Perl inserts the contents of "$ENV{PERL5DB}" (or
"BEGIN {require 'perl5db.pl'}" if not present) before
the first line of your program.
Each array "@{"_<$filename"}" holds the lines of
$filename for a file compiled by Perl. The same for
"eval"ed strings that contain subroutines, or which
are currently being executed. The $filename for
"eval"ed strings looks like "(eval 34)". Code asser
tions in regexes look like "(re_eval 19)".
Values in this array are magical in numeric context:
they compare equal to zero only if the line is not
breakable.
Each hash "%{"_<$filename"}" contains breakpoints and
actions keyed by line number. Individual entries (as
opposed to the whole hash) are settable. Perl only
cares about Boolean true here, although the values
used by perl5db.pl have the form ""$break_condi
tion\0$action"".
The same holds for evaluated strings that contain sub
routines, or which are currently being executed. The
$filename for "eval"ed strings looks like "(eval 34)"
or "(re_eval 19)".
Each scalar "${"_<$filename"}" contains ""_<$file
name"". This is also the case for evaluated strings
that contain subroutines, or which are currently being
executed. The $filename for "eval"ed strings looks
like "(eval 34)" or "(re_eval 19)".
After each "require"d file is compiled, but before it
is executed, "DB::postponed(*{"_<$filename"})" is
called if the subroutine "DB::postponed" exists.
Here, the $filename is the expanded name of the
"require"d file, as found in the values of %INC.
After each subroutine "subname" is compiled, the exis
tence of "$DB::postponed{subname}" is checked. If
this key exists, "DB::postponed(subname)" is called if
the "DB::postponed" subroutine also exists.
A hash "%DB::sub" is maintained, whose keys are sub
routine names and whose values have the form "file
name:startline-endline". "filename" has the form
"(eval 34)" for subroutines defined inside "eval"s, or
"(re_eval 19)" for those within regex code assertions.
When the execution of your program reaches a point
that can hold a breakpoint, the "DB::DB()" subroutine
is called any of the variables $DB::trace, $DB::sin
gle, or $DB::signal is true. These variables are not
"local"izable. This feature is disabled when execut
ing inside "DB::DB()", including functions called from
it unless "$^D & (1<<30)" is true.
When execution of the program reaches a subroutine
call, a call to "&DB::sub"(args) is made instead, with
"$DB::sub" holding the name of the called subroutine.
This doesn't happen if the subroutine was compiled in
the "DB" package.)
Note that if "&DB::sub" needs external data for it to
work, no subroutine call is possible until this is done.
For the standard debugger, the "$DB::deep" variable (how
many levels of recursion deep into the debugger you can go
before a mandatory break) gives an example of such a
dependency.
Writing Your Own Debugger
The minimal working debugger consists of one line
sub DB::DB {}
which is quite handy as contents of "PERL5DB" environment
variable:
$ PERL5DB="sub DB::DB {}" perl -d your-script
Another brief debugger, slightly more useful, could be
created with only the line:
sub DB::DB {print ++$i; scalar <STDIN>}
This debugger would print the sequential number of encoun
tered statement, and would wait for you to hit a newline
before continuing.
The following debugger is quite functional:
{
package DB;
sub DB {}
sub sub {print ++$i, " $sub\n"; &$sub}
}
It prints the sequential number of subroutine call and the
name of the called subroutine. Note that "&DB::sub"
should be compiled into the package "DB".
At the start, the debugger reads your rc file (./.perldb
or ~/.perldb under Unix), which can set important options.
This file may define a subroutine "&afterinit" to be exe
cuted after the debugger is initialized.
After the rc file is read, the debugger reads the
PERLDB_OPTS environment variable and parses this as the
remainder of a "O ..." line as one might enter at the
debugger prompt.
The debugger also maintains magical internal variables,
such as "@DB::dbline", "%DB::dbline", which are aliases
for "@{"::_<current_file"}" "%{"::_<current_file"}". Here
"current_file" is the currently selected file, either
explicitly chosen with the debugger's "f" command, or
implicitly by flow of execution.
Some functions are provided to simplify customization.
See the Options entry in the perldebug manpage for
description of options parsed by
"DB::parse_options(string)". The function
"DB::dump_trace(skip[, count])" skips the specified number
of frames and returns a list containing information about
the calling frames (all of them, if "count" is missing).
Each entry is reference to a hash with keys "context"
(either ".", "$", or "@"), "sub" (subroutine name, or info
about "eval"), "args" ("undef" or a reference to an
array), "file", and "line".
The function "DB::print_trace(FH, skip[, count[, short]])"
prints formatted info about caller frames. The last two
functions may be convenient as arguments to "<", "<<" com
mands.
Note that any variables and functions that are not docu
mented in this manpages (or in the perldebug manpage) are
considered for internal use only, and as such are subject
to change without notice.
Frame Listing Output Examples
The "frame" option can be used to control the output of
frame information. For example, contrast this expression
trace:
$ perl -de 42
Stack dump during die enabled outside of evals.
Loading DB routines from perl5db.pl patch level 0.94
Emacs support available.
Enter h or `h h' for help.
main::(-e:1): 0
DB<1> sub foo { 14 }
DB<2> sub bar { 3 }
DB<3> t print foo() * bar()
main::((eval 172):3): print foo() + bar();
main::foo((eval 168):2):
main::bar((eval 170):2):
42
with this one, once the "O"ption "frame=2" has been set:
DB<4> O f=2
frame = '2'
DB<5> t print foo() * bar()
3: foo() * bar()
entering main::foo
2: sub foo { 14 };
exited main::foo
entering main::bar
2: sub bar { 3 };
exited main::bar
42
By way of demonstration, we present below a laborious
listing resulting from setting your "PERLDB_OPTS" environ
ment variable to the value "f=n N", and running perl -d -V
from the command line. Examples use various values of "n"
are shown to give you a feel for the difference between
settings. Long those it may be, this is not a complete
listing, but only excerpts.
1
entering main::BEGIN
entering Config::BEGIN
Package lib/Exporter.pm.
Package lib/Carp.pm.
Package lib/Config.pm.
entering Config::TIEHASH
entering Exporter::import
entering Exporter::export
entering Config::myconfig
entering Config::FETCH
entering Config::FETCH
entering Config::FETCH
entering Config::FETCH
2
entering main::BEGIN
entering Config::BEGIN
Package lib/Exporter.pm.
Package lib/Carp.pm.
exited Config::BEGIN
Package lib/Config.pm.
entering Config::TIEHASH
exited Config::TIEHASH
entering Exporter::import
entering Exporter::export
exited Exporter::export
exited Exporter::import
exited main::BEGIN
entering Config::myconfig
entering Config::FETCH
exited Config::FETCH
entering Config::FETCH
exited Config::FETCH
entering Config::FETCH
4
in $=main::BEGIN() from /dev/null:0
in $=Config::BEGIN() from lib/Config.pm:2
Package lib/Exporter.pm.
Package lib/Carp.pm.
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:644
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
in @=Config::myconfig() from /dev/null:0
in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574
6
in $=main::BEGIN() from /dev/null:0
in $=Config::BEGIN() from lib/Config.pm:2
Package lib/Exporter.pm.
Package lib/Carp.pm.
out $=Config::BEGIN() from lib/Config.pm:0
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:644
out $=Config::TIEHASH('Config') from lib/Config.pm:644
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
out $=main::BEGIN() from /dev/null:0
in @=Config::myconfig() from /dev/null:0
in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
out $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
14
in $=main::BEGIN() from /dev/null:0
in $=Config::BEGIN() from lib/Config.pm:2
Package lib/Exporter.pm.
Package lib/Carp.pm.
out $=Config::BEGIN() from lib/Config.pm:0
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:644
out $=Config::TIEHASH('Config') from lib/Config.pm:644
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
out $=main::BEGIN() from /dev/null:0
in @=Config::myconfig() from /dev/null:0
in $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
30
in $=CODE(0x15eca4)() from /dev/null:0
in $=CODE(0x182528)() from lib/Config.pm:2
Package lib/Exporter.pm.
out $=CODE(0x182528)() from lib/Config.pm:0
scalar context return from CODE(0x182528): undef
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:628
out $=Config::TIEHASH('Config') from lib/Config.pm:628
scalar context return from Config::TIEHASH: empty hash
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
scalar context return from Exporter::export: ''
out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
scalar context return from Exporter::import: ''
In all cases shown above, the line indentation shows the
call tree. If bit 2 of "frame" is set, a line is printed
on exit from a subroutine as well. If bit 4 is set, the
arguments are printed along with the caller info. If bit
8 is set, the arguments are printed even if they are tied
or references. If bit 16 is set, the return value is
printed, too.
When a package is compiled, a line like this
Package lib/Carp.pm.
is printed with proper indentation.
Debugging regular expressions
There are two ways to enable debugging output for regular
expressions.
If your perl is compiled with "-DDEBUGGING", you may use
the -Dr flag on the command line.
Otherwise, one can "use re 'debug'", which has effects at
compile time and run time. It is not lexically scoped.
Compile-time output
The debugging output at compile time looks like this:
compiling RE `[bc]d(ef*g)+h[ij]k$'
size 43 first at 1
1: ANYOF(11)
11: EXACT <d>(13)
13: CURLYX {1,32767}(27)
15: OPEN1(17)
17: EXACT <e>(19)
19: STAR(22)
20: EXACT <f>(0)
22: EXACT <g>(24)
24: CLOSE1(26)
26: WHILEM(0)
27: NOTHING(28)
28: EXACT <h>(30)
30: ANYOF(40)
40: EXACT <k>(42)
42: EOL(43)
43: END(0)
anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
stclass `ANYOF' minlen 7
The first line shows the pre-compiled form of the regex.
The second shows the size of the compiled form (in arbi
trary units, usually 4-byte words) and the label id of the
first node that does a match.
The last line (split into two lines above) contains opti
mizer information. In the example shown, the optimizer
found that the match should contain a substring "de" at
offset 1, plus substring "gh" at some offset between 3 and
infinity. Moreover, when checking for these substrings
(to abandon impossible matches quickly), Perl will check
for the substring "gh" before checking for the substring
"de". The optimizer may also use the knowledge that the
match starts (at the "first" id) with a character class,
and the match cannot be shorter than 7 chars.
The fields of interest which may appear in the last line
are
""anchored"" STRING ""at"" POS
""floating"" STRING ""at"" POS1..POS2
See above.
""matching floating/anchored""
Which substring to check first.
""minlen""
The minimal length of the match.
""stclass"" TYPE
Type of first matching node.
""noscan""
Don't scan for the found substrings.
""isall""
Means that the optimizer info is all that the regular
expression contains, and thus one does not need to
enter the regex engine at all.
""GPOS""
Set if the pattern contains "\G".
""plus""
Set if the pattern starts with a repeated char (as in
"x+y").
""implicit""
Set if the pattern starts with ".*".
""with eval""
Set if the pattern contain eval-groups, such as "(?{
code })" and "(??{ code })".
""anchored(TYPE)""
If the pattern may match only at a handful of places,
(with "TYPE" being "BOL", "MBOL", or "GPOS". See the
table below.
If a substring is known to match at end-of-line only, it
may be followed by "$", as in "floating `k'$".
The optimizer-specific info is used to avoid entering (a
slow) regex engine on strings that will not definitely
match. If "isall" flag is set, a call to the regex engine
may be avoided even when the optimizer found an
appropriate place for the match.
The rest of the output contains the list of nodes of the
compiled form of the regex. Each line has format
" "id: TYPE OPTIONAL-INFO (next-id)
Types of nodes
Here are the possible types, with short descriptions:
# TYPE arg-description [num-args] [longjump-len] DESCRIPTION
# Exit points
END no End of program.
SUCCEED no Return from a subroutine, basically.
# Anchors:
BOL no Match "" at beginning of line.
MBOL no Same, assuming multiline.
SBOL no Same, assuming singleline.
EOS no Match "" at end of string.
EOL no Match "" at end of line.
MEOL no Same, assuming multiline.
SEOL no Same, assuming singleline.
BOUND no Match "" at any word boundary
BOUNDL no Match "" at any word boundary
NBOUND no Match "" at any word non-boundary
NBOUNDL no Match "" at any word non-boundary
GPOS no Matches where last m//g left off.
# [Special] alternatives
ANY no Match any one character (except newline).
SANY no Match any one character.
ANYOF sv Match character in (or not in) this class.
ALNUM no Match any alphanumeric character
ALNUML no Match any alphanumeric char in locale
NALNUM no Match any non-alphanumeric character
NALNUML no Match any non-alphanumeric char in locale
SPACE no Match any whitespace character
SPACEL no Match any whitespace char in locale
NSPACE no Match any non-whitespace character
NSPACEL no Match any non-whitespace char in locale
DIGIT no Match any numeric character
NDIGIT no Match any non-numeric character
# BRANCH The set of branches constituting a single choice are hooked
# together with their "next" pointers, since precedence prevents
# anything being concatenated to any individual branch. The
# "next" pointer of the last BRANCH in a choice points to the
# thing following the whole choice. This is also where the
# final "next" pointer of each individual branch points; each
# branch starts with the operand node of a BRANCH node.
#
BRANCH node Match this alternative, or the next...
# BACK Normal "next" pointers all implicitly point forward; BACK
# exists to make loop structures possible.
# not used
BACK no Match "", "next" ptr points backward.
# Literals
EXACT sv Match this string (preceded by length).
EXACTF sv Match this string, folded (prec. by length).
EXACTFL sv Match this string, folded in locale (w/len).
# Do nothing
NOTHING no Match empty string.
# A variant of above which delimits a group, thus stops optimizations
TAIL no Match empty string. Can jump here from outside.
# STAR,PLUS '?', and complex '*' and '+', are implemented as circular
# BRANCH structures using BACK. Simple cases (one character
# per match) are implemented with STAR and PLUS for speed
# and to minimize recursive plunges.
#
STAR node Match this (simple) thing 0 or more times.
PLUS node Match this (simple) thing 1 or more times.
CURLY sv 2 Match this simple thing {n,m} times.
CURLYN no 2 Match next-after-this simple thing
# {n,m} times, set parens.
CURLYM no 2 Match this medium-complex thing {n,m} times.
CURLYX sv 2 Match this complex thing {n,m} times.
# This terminator creates a loop structure for CURLYX
WHILEM no Do curly processing and see if rest matches.
# OPEN,CLOSE,GROUPP ...are numbered at compile time.
OPEN num 1 Mark this point in input as start of #n.
CLOSE num 1 Analogous to OPEN.
REF num 1 Match some already matched string
REFF num 1 Match already matched string, folded
REFFL num 1 Match already matched string, folded in loc.
# grouping assertions
IFMATCH off 1 2 Succeeds if the following matches.
UNLESSM off 1 2 Fails if the following matches.
SUSPEND off 1 1 "Independent" sub-regex.
IFTHEN off 1 1 Switch, should be preceded by switcher .
GROUPP num 1 Whether the group matched.
# Support for long regex
LONGJMP off 1 1 Jump far away.
BRANCHJ off 1 1 BRANCH with long offset.
# The heavy worker
EVAL evl 1 Execute some Perl code.
# Modifiers
MINMOD no Next operator is not greedy.
LOGICAL no Next opcode should set the flag only.
# This is not used yet
RENUM off 1 1 Group with independently numbered parens.
# This is not really a node, but an optimized away piece of a "long" node.
# To simplify debugging output, we mark it as if it were a node
OPTIMIZED off Placeholder for dump.
Run-time output
First of all, when doing a match, one may get no run-time
output even if debugging is enabled. This means that the
regex engine was never entered and that all of the job was
therefore done by the optimizer.
If the regex engine was entered, the output may look like
this:
Matching `[bc]d(ef*g)+h[ij]k$' against `abcdefg__gh__'
Setting an EVAL scope, savestack=3
2 <ab> <cdefg__gh_> | 1: ANYOF
3 <abc> <defg__gh_> | 11: EXACT <d>
4 <abcd> <efg__gh_> | 13: CURLYX {1,32767}
4 <abcd> <efg__gh_> | 26: WHILEM
0 out of 1..32767 cc=effff31c
4 <abcd> <efg__gh_> | 15: OPEN1
4 <abcd> <efg__gh_> | 17: EXACT <e>
5 <abcde> <fg__gh_> | 19: STAR
EXACT <f> can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
6 <bcdef> <g__gh__> | 22: EXACT <g>
7 <bcdefg> <__gh__> | 24: CLOSE1
7 <bcdefg> <__gh__> | 26: WHILEM
1 out of 1..32767 cc=effff31c
Setting an EVAL scope, savestack=12
7 <bcdefg> <__gh__> | 15: OPEN1
7 <bcdefg> <__gh__> | 17: EXACT <e>
restoring \1 to 4(4)..7
failed, try continuation...
7 <bcdefg> <__gh__> | 27: NOTHING
7 <bcdefg> <__gh__> | 28: EXACT <h>
failed...
failed...
The most significant information in the output is about
the particular node of the compiled regex that is cur
rently being tested against the target string. The format
of these lines is
" "STRING-OFFSET <PRE-STRING> <POST-STRING> |ID:
TYPE
The TYPE info is indented with respect to the backtracking
level. Other incidental information appears interspersed
within.
Debugging Perl memory usage
Perl is a profligate wastrel when it comes to memory use.
There is a saying that to estimate memory usage of Perl,
assume a reasonable algorithm for memory allocation, mul
tiply that estimate by 10, and while you still may miss
the mark, at least you won't be quite so astonished. This
is not absolutely true, but may provide a good grasp of
what happens.
Assume that 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 quite a bit 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 yet
another 20 bytes. A sloppy malloc(3) implementation can
inflate these numbers dramatically.
On the opposite end of the scale, a declaration like
sub foo;
may take up to 500 bytes of memory, depending on which
release of Perl you're running.
Anecdotal estimates of source-to-compiled code bloat sug
gest an eightfold increase. This means that the compiled
form of reasonable (normally commented, properly indented
etc.) code will take about eight times more space in mem
ory than the code took on disk.
There are two Perl-specific ways to analyze memory usage:
$ENV{PERL_DEBUG_MSTATS} and -DL command-line switch. The
first is available only if Perl is compiled with Perl's
malloc(); the second only if Perl was built with "-DDEBUG
GING". See the instructions for how to do this in the
INSTALL podpage at the top level of the Perl source tree.
Using "$ENV{PERL_DEBUG_MSTATS}"
If your perl is using Perl's malloc() and was compiled
with the necessary switches (this is the default), then it
will print memory usage statistics after compiling your
code when "$ENV{PERL_DEBUG_MSTATS} > 1", and before termi
nation of the program when "$ENV{PERL_DEBUG_MSTATS} >= 1".
The report format is similar to the following example:
$ 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
points in your execution using the mstat() function out of
the standard Devel::Peek module.
Here is some explanation of that format:
""buckets SMALLEST(APPROX)..GREATEST(APPROX)""
Perl's malloc() uses bucketed allocations. Every
request is rounded up to the closest bucket size
available, and a bucket is taken from the pool of
buckets of that size.
The line above describes the limits of buckets cur
rently in use. Each bucket has two sizes: memory
footprint and the maximal size of user data that can
fit into this bucket. Suppose in the above example
that the smallest bucket were size 4. The biggest
bucket would have usable size 8188, and the memory
footprint would be 8192.
In a Perl built for debugging, some buckets may have
negative usable size. This means that these buckets
cannot (and will not) be used. For larger buckets,
the memory footprint may be one page greater than a
power of 2. If so, case the corresponding power of
two is printed in the "APPROX" field above.
Free/Used
The 1 or 2 rows of numbers following that correspond
to the number of buckets of each size between "SMALL
EST" and "GREATEST". In the first row, the sizes
(memory footprints) of buckets are powers of two--or
possibly one page greater. In the second row, if pre
sent, the memory footprints of the buckets are between
the memory footprints of two buckets "above".
For example, suppose under the previous example, the
memory footprints were
free: 8 16 32 64 128 256 512 1024 2048 4096 8192
4 12 24 48 80
With non-"DEBUGGING" perl, the buckets starting from
"128" have a 4-byte overhead, and thus a 8192-long
bucket may take up to 8188-byte allocations.
""Total sbrk(): SBRKed/SBRKs:CONTINUOUS""
The first two fields give the total amount of memory
perl sbrk(2)ed (ess-broken? :-) and number of sbrk(2)s
used. The third number is what perl thinks about con
tinuity of returned chunks. So long as this number is
positive, malloc() will assume that it is probable
that sbrk(2) will provide continuous memory.
Memory allocated by external libraries is not counted.
""pad: 0""
The amount of sbrk(2)ed memory needed to keep buckets
aligned.
""heads: 2192""
Although 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.
""chain: 0""
malloc() may want to subdivide a bigger bucket into
smaller buckets. If only a part of the deceased
bucket is left unsubdivided, the rest is kept as an
element of a linked list. This field gives the total
size of these chunks.
""tail: 6144""
To minimize the number of sbrk(2)s, malloc() asks for
more memory. This field gives the size of the yet
unused part, which is sbrk(2)ed, but never touched.
Example of using -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 simi
lar to
sub getcwd;
WARNING: The discussion below supposes 32-bit architec
ture. In newer releases of Perl, memory usage of the con
structs discussed here is greatly improved, but the story
discussed below is a real-life story. This story is mer
cilessly terse, and assumes rather more than cursory
knowledge of Perl internals. Type space to continue, `q'
to quit. (Actually, you just want to skip to the next
section.)
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 "warn('!...')" statements
around the call:
warn('!');
do 'lib/auto/POSIX/autosplit.ix';
warn('!!! "after"');
and run it with Perl's -DL option. The first warn() will
print memory allocation info before parsing the file and
will memorize the statistics at this point (we ignore what
it prints). The second warn() prints increments with
respect to these memorized data. This is the printout
shown above.
Different Ids on the left correspond to different subsys
tems of the perl interpreter. They are just the first
argument given to the perl memory allocation API named
New(). To find what "9 03" means, just grep the perl
source for "903". You'll find it in util.c, function
savepvn(). (I know, you wonder why we told you to grep
and then gave away the answer. That's because grepping
the source is good for the soul.) This function is used
to store a copy of an existing chunk of memory. Using a C
debugger, one can see that the function was called either
directly from gv_init() or via sv_magic(), and that
gv_init() is called from gv_fetchpv()--which was itself
called from newSUB(). Please stop to catch your breath
now.
NOTE: To reach this point in the debugger and skip the
calls to savepvn() during the compilation of the main pro
gram, you should set a C breakpoint in Perl_warn(), con
tinue until this point is reached, and then set a C break
point in Perl_savepvn(). Note that you may need to skip a
handful of Perl_savepvn() calls that do not correspond to
mass production of CVs (there are more "903" allocations
than 146 similar lines of lib/auto/POSIX/autosplit.ix).
Note also that "Perl_" prefixes are added by macroization
code in perl header files to avoid conflicts with external
libraries.
Anyway, we see that "903" ids correspond to creation of
globs, twice per glob - for glob name, and glob stringifi
cation magic.
Here are explanations for other Ids above:
""717""
Creates bigger "XPV*" structures. In the case above,
it creates 3 "AV"s per subroutine, one for a list of
lexical variable names, one for a scratchpad (which
contains lexical variables and "targets"), and one for
the array of scratchpads needed for recursion.
It also creates a "GV" and a "CV" per subroutine, all
called from start_subparse().
""002""
Creates a C array corresponding to the "AV" of
scratchpads and the scratchpad itself. The first fake
entry of this scratchpad is created though the subrou
tine 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 freed, but are
kept as additional arenas for "SV" allocations.
""054""
Creates a "HEK" for the name of the glob for the sub
routine. This name is a key in a stash.
Big allocations with this Id correspond to allocations
of new arenas to keep "HE".
""602""
Creates a "GP" for the glob for the subroutine.
""702""
Creates the "MAGIC" for the glob for the subroutine.
""704""
Creates arenas which keep SVs.
-DL details
If Perl is run with -DL option, then warn()s that start
with `!' behave specially. They print a list of cate_
gories of memory allocations, and statistics of alloca
tions of different sizes for these categories.
If warn() string starts with
""!!!""
print changed categories only, print the differences
in counts of allocations.
""!!""
print grown categories only; print the absolute values
of counts, and totals.
""!""
print nonempty categories, print the absolute values
of counts and totals.
Limitations of -DL statistics
If an extension or external library does not use the Perl
API to allocate memory, such allocations are not counted.
SEE ALSO
the perldebug manpage, the perlguts manpage, the perlrun
manpage the re manpage, and the Devel::Dprof manpage.
2001-04-07 perl v5.6.1 PERLDEBGUTS(1)
