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PERLDEBGUTS(1) Perl Programmers Reference Guide PERLDEBGUTS(1)
NAME
perldebguts - 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 concerning the debugger's internals, which range
from difficult to impossible to understand for anyone who
isn't incredibly intimate with Perl's guts. Caveat lec-
tor.
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 perlrun, 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
corresponding stack frame was called with are copied to
the @DB::args array. These mechanisms are enabled by
calling Perl with the -d switch. Specifically, the fol-
lowing additional features are enabled (cf. "$^P" in perl-
var):
o Perl inserts the contents of $ENV{PERL5DB} (or "BEGIN
{require 'perl5db.pl'}" if not present) before the
first line of your program.
o Each array "@{"_<$filename"}" holds the lines of
$filename for a file compiled by Perl. The same is
also true for "eval"ed strings that contain subrou-
tines, or which are currently being executed. The
$filename for "eval"ed strings looks like "(eval 34)".
Code assertions in regexes look like "(re_eval 19)".
Values in this array are magical in numeric context:
they compare equal to zero only if the line is not
breakable.
o Each hash "%{"_<$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
$filename for "eval"ed strings looks like "(eval 34)"
or "(re_eval 19)".
o Each scalar "${"_<$filename"}" contains "_<$filename".
This is also the case for evaluated strings that con-
tain subroutines, or which are currently being exe-
cuted. The $filename for "eval"ed strings looks like
"(eval 34)" or "(re_eval 19)".
o After each "require"d file is compiled, but before it
is executed, "DB::postponed(*{"_<$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.
o 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.
o A hash %DB::sub is maintained, whose keys are subrou-
tine 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.
o When the execution of your program reaches a point
that can hold a breakpoint, the "DB::DB()" subroutine
is called if any of the variables $DB::trace,
$DB::single, or $DB::signal is true. These variables
are not "local"izable. This feature is disabled when
executing inside "DB::DB()", including functions
called from it unless "$^D & (1<<30)" is true.
o 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 without it. As an example,
the standard debugger's &DB::sub depends on the $DB::deep
variable (it defines how many levels of recursion deep
into the debugger you can go before a mandatory break).
If $DB::deep is not defined, subroutine calls are not pos-
sible, even though &DB::sub exists.
Writing Your Own Debugger
Environment Variables
The "PERL5DB" environment variable can be used to define a
debugger. For example, the minimal "working" debugger (it
actually doesn't do anything) consists of one line:
sub DB::DB {}
It can easily be defined like this:
$ PERL5DB="sub DB::DB {}" perl -d your-script
Another brief debugger, slightly more useful, can be cre-
ated with only the line:
sub DB::DB {print ++$i; scalar }
This debugger prints a number which increments for each
statement encountered and waits for you to hit a newline
before continuing to the next statement.
The following debugger is actually useful:
{
package DB;
sub DB {}
sub sub {print ++$i, " $sub\n"; &$sub}
}
It prints the sequence number of each subroutine call and
the name of the called subroutine. Note that &DB::sub is
being compiled into the package "DB" through the use of
the "package" directive.
When it starts, the debugger reads your rc file (./.perldb
or ~/.perldb under Unix), which can set important options.
(A subroutine (&afterinit) can be defined here as well; it
is executed after the debugger completes its own initial-
ization.)
After the rc file is read, the debugger reads the
PERLDB_OPTS environment variable and uses it to set debug-
ger options. The contents of this variable are treated as
if they were the argument of an "o ..." debugger command
(q.v. in "Options" in perldebug).
Debugger internal variables In addition to the file and
subroutine-related variables mentioned above, the debugger
also maintains various magical internal variables.
o @DB::dbline is an alias for "@{"::_ 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 $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
scalar context return from Exporter::import: ''
In all cases shown above, the line indentation shows the
call tree. If bit 2 of "frame" is set, a line is printed
on exit from a subroutine as well. If bit 4 is set, the
arguments are printed along with the caller info. If bit
8 is set, the arguments are printed even if they are tied
or references. If bit 16 is set, the return value is
printed, too.
When a package is compiled, a line like this
Package lib/Carp.pm.
is printed with proper indentation.
Debugging regular expressions
There are two ways to enable debugging output for regular
expressions.
If your perl is compiled with "-DDEBUGGING", you may use
the -Dr flag on the command line.
Otherwise, one can "use re 'debug'", which has effects at
compile time and run time. It is not lexically scoped.
Compile-time output
The debugging output at compile time looks like this:
Compiling REx `[bc]d(ef*g)+h[ij]k$'
size 45 Got 364 bytes for offset annotations.
first at 1
rarest char g at 0
rarest char d at 0
1: ANYOF[bc](12)
12: EXACT (14)
14: CURLYX[0] {1,32767}(28)
16: OPEN1(18)
18: EXACT (20)
20: STAR(23)
21: EXACT (0)
23: EXACT (25)
25: CLOSE1(27)
27: WHILEM[1/1](0)
28: NOTHING(29)
29: EXACT (31)
31: ANYOF[ij](42)
42: EXACT (44)
44: EOL(45)
45: END(0)
anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
stclass `ANYOF[bc]' minlen 7
Offsets: [45]
1[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 5[1]
0[0] 12[1] 0[0] 6[1] 0[0] 7[1] 0[0] 9[1] 8[1] 0[0] 10[1] 0[0]
11[1] 0[0] 12[0] 12[0] 13[1] 0[0] 14[4] 0[0] 0[0] 0[0] 0[0]
0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 18[1] 0[0] 19[1] 20[0]
Omitting $` $& $' support.
The first line shows the pre-compiled form of the regex.
The second shows the size of the compiled form (in arbi-
trary units, usually 4-byte words) and the total number of
bytes allocated for the offset/length table, usually
4+"size"*8. The next line shows the label id of the first
node that does a match.
The
anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
stclass `ANYOF[bc]' minlen 7
line (split into two lines above) contains optimizer
information. In the example shown, the optimizer found
that the match should contain a substring "de" at offset
1, plus substring "gh" at some offset between 3 and infin-
ity. Moreover, when checking for these substrings (to
abandon impossible matches quickly), Perl will check for
the substring "gh" before checking for the substring "de".
The optimizer may also use the knowledge that the match
starts (at the "first" id) with a character class, and no
string shorter than 7 characters can possibly match.
The fields of interest which may appear in this line are
"anchored" STRING "at" POS
"floating" STRING "at" POS1..POS2
See above.
"matching floating/anchored"
Which substring to check first.
"minlen"
The minimal length of the match.
"stclass" TYPE
Type of first matching node.
"noscan"
Don't scan for the found substrings.
"isall"
Means that the optimizer information is all that the
regular expression contains, and thus one does not
need to enter the regex engine at all.
"GPOS"
Set if the pattern contains "\G".
"plus"
Set if the pattern starts with a repeated char (as in
"x+y").
"implicit"
Set if the pattern starts with ".*".
"with eval"
Set if the pattern contain eval-groups, such as "(?{
code })" and "(??{ code })".
"anchored(TYPE)"
If the pattern may match only at a handful of places,
(with "TYPE" being "BOL", "MBOL", or "GPOS". See the
table below.
If a substring is known to match at end-of-line only, it
may be followed by "$", as in "floating `k'$".
The optimizer-specific information is used to avoid enter-
ing (a slow) regex engine on strings that will not defi-
nitely match. If the "isall" flag is set, a call to the
regex engine may be avoided even when the optimizer found
an appropriate place for the match.
Above the optimizer section is 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.
Following the optimizer information is a dump of the off-
set/length table, here split across several lines:
Offsets: [45]
1[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 5[1]
0[0] 12[1] 0[0] 6[1] 0[0] 7[1] 0[0] 9[1] 8[1] 0[0] 10[1] 0[0]
11[1] 0[0] 12[0] 12[0] 13[1] 0[0] 14[4] 0[0] 0[0] 0[0] 0[0]
0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 18[1] 0[0] 19[1] 20[0]
The first line here indicates that the offset/length table
contains 45 entries. Each entry is a pair of integers,
denoted by "offset[length]". Entries are numbered start-
ing with 1, so entry #1 here is "1[4]" and entry #12 is
"5[1]". "1[4]" indicates that the node labeled "1:" (the
"1: ANYOF[bc]") begins at character position 1 in the pre-
compiled form of the regex, and has a length of 4
characters. "5[1]" in position 12 indicates that the node
labeled "12:" (the "12: EXACT ") begins at character
position 5 in the pre-compiled form of the regex, and has
a length of 1 character. "12[1]" in position 14 indicates
that the node labeled "14:" (the "14: CURLYX[0]
{1,32767}") begins at character position 12 in the pre-
compiled form of the regex, and has a length of 1 charac-
ter---that is, it corresponds to the "+" symbol in the
precompiled regex.
"0[0]" items indicate that there is no corresponding node.
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 | 1: ANYOF
3 | 11: EXACT
4 | 13: CURLYX {1,32767}
4 | 26: WHILEM
0 out of 1..32767 cc=effff31c
4 | 15: OPEN1
4 | 17: EXACT
5 | 19: STAR
EXACT can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
6 | 22: EXACT
7 <__gh__> | 24: CLOSE1
7 <__gh__> | 26: WHILEM
1 out of 1..32767 cc=effff31c
Setting an EVAL scope, savestack=12
7 <__gh__> | 15: OPEN1
7 <__gh__> | 17: EXACT
restoring \1 to 4(4)..7
failed, try continuation...
7 <__gh__> | 27: NOTHING
7 <__gh__> | 28: EXACT
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 |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.
The -DL command-line switch is obsolete since circa Perl
5.6.0 (it was available only if Perl was built with "-DDE-
BUGGING"). The switch was used to track Perl's memory
allocations and possible memory leaks. These days the use
of malloc debugging tools like Purify or valgrind is sug-
gested instead.
One way to find out how much memory is being used by Perl
data structures is to install the Devel::Size module from
CPAN: it gives you the minimum number of bytes required to
store a particular data structure. Please be mindful of
the difference between the size() and total_size().
If Perl has been compiled using Perl's malloc you can ana-
lyze Perl memory usage by setting the
$ENV{PERL_DEBUG_MSTATS}.
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 an 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.
SEE ALSO
perldebug, perlguts, perlrun re, and Devel::DProf.
perl v5.8.8 2006-01-07 PERLDEBGUTS(1)