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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 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 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 follow-
ing additional features are enabled (cf. "$^P" in perlvar):
o Perl inserts the contents of $ENV{PERL5DB} (or "BEGIN {require
'perl5db.pl'}" if not present) before the first line of your pro-
gram.
o Each array "@{"_<$filename"}" holds the lines of $filename for a
file compiled by Perl. The same is also true for "eval"ed strings
that contain subroutines, 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 subroutines, 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 contain subroutines, or
which are currently being executed. 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 existence of
$DB::postponed{subname} is checked. If this key exists, "DB::post-
poned(subname)" is called if the "DB::postponed" subroutine also
exists.
o A hash %DB::sub is maintained, whose keys are subroutine names and
whose values have the form "filename: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 vari-
ables $DB::trace, $DB::single, or $DB::signal is true. These vari-
ables are not "local"izable. This feature is disabled when execut-
ing inside "DB::DB()", including functions called from it unless
"$^D & (1<<30)" is true.
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 possi-
ble, 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 created with only
the line:
sub DB::DB {print ++$i; scalar <STDIN>}
This debugger prints a number which increments for each statement
encountered and waits for you to hit a newline before continuing to the
next statement.
The following debugger is actually useful:
{
package DB;
sub DB {}
sub sub {print ++$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 initialization.)
After the rc file is read, the debugger reads the PERLDB_OPTS environ-
ment variable and uses it to set debugger options. The contents of this
variable are treated as if they were the argument of an "o ..." debug-
ger 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 "@{"::_<current_file"}", which holds
the lines of the currently-selected file (compiled by Perl), either
explicitly chosen with the debugger's "f" command, or implicitly by
flow of execution.
Values in this array are magical in numeric context: they compare
equal to zero only if the line is not breakable.
o %DB::dbline, is an alias for "%{"::_<current_file"}", which con-
tains breakpoints and actions keyed by line number in the cur-
rently-selected file, either explicitly chosen with the debugger's
"f" command, or implicitly by flow of execution.
As previously noted, 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".
Debugger customization functions
Some functions are provided to simplify customization.
o See "Options" in perldebug for description of options parsed by
"DB::parse_options(string)" parses debugger options; see "Options"
in pperldebug for a description of options recognized.
o "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" (subrou-
tine name, or info about "eval"), "args" ("undef" or a reference to
an array), "file", and "line".
o "DB::print_trace(FH, skip[, count[, short]])" prints formatted info
about caller frames. The last two functions may be convenient as
arguments to "<", "<<" commands.
Note that any variables and functions that are not documented in this
manpages (or in perldebug) 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 informa-
tion. 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" environment 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 <d>(14)
14: CURLYX[0] {1,32767}(28)
16: OPEN1(18)
18: EXACT <e>(20)
20: STAR(23)
21: EXACT <f>(0)
23: EXACT <g>(25)
25: CLOSE1(27)
27: WHILEM[1/1](0)
28: NOTHING(29)
29: EXACT <h>(31)
31: ANYOF[ij](42)
42: EXACT <k>(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 $` $& $' support.
The first line shows the pre-compiled form of the regex. The second
shows the size of the compiled form (in arbitrary units, usually 4-byte
words) and the total number of bytes allocated for the offset/length
table, usually 4+"size"*8. The next line shows the label id of the
first node that does a match.
The
anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
stclass `ANYOF[bc]' minlen 7
line (split into two lines above) contains optimizer information. In
the example shown, the optimizer found that the match should contain a
substring "de" at offset 1, plus substring "gh" at some offset between
3 and infinity. Moreover, when checking for these substrings (to aban-
don impossible matches quickly), Perl will check for the substring "gh"
before checking for the substring "de". The optimizer may also use the
knowledge that the match starts (at the "first" id) with a character
class, and no string shorter than 7 characters can possibly match.
The fields of interest which may appear in this line are
"anchored" STRING "at" POS
"floating" STRING "at" POS1..POS2
See above.
"matching floating/anchored"
Which substring to check first.
"minlen"
The minimal length of the match.
"stclass" TYPE
Type of first matching node.
"noscan"
Don't scan for the found substrings.
"isall"
Means that the optimizer information is all that the regular
expression contains, and thus one does not need to enter the regex
engine at all.
"GPOS"
Set if the pattern contains "\G".
"plus"
Set if the pattern starts with a repeated char (as in "x+y").
"implicit"
Set if the pattern starts with ".*".
"with eval"
Set if the pattern contain eval-groups, such as "(?{ code })" and
"(??{ code })".
"anchored(TYPE)"
If the pattern may match only at a handful of places, (with "TYPE"
being "BOL", "MBOL", or "GPOS". See the table below.
If a substring is known to match at end-of-line only, it may be fol-
lowed by "$", as in "floating `k'$".
The optimizer-specific information is used to avoid entering (a slow)
regex engine on strings that will not definitely 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 offset/length ta-
ble, 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 "off-
set[length]". Entries are numbered starting 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 <d>") 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 character---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 <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 currently 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 algo-
rithm for memory allocation, multiply 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 hap-
pens.
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 suggest 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 memory 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 "-DDEBUGGING". See the instructions for
how to do this in the INSTALL podpage at the top level of the Perl
source tree.
Using $ENV{PERL_DEBUG_MSTATS}
If your perl is using Perl's malloc() and was compiled with the neces-
sary switches (this is the default), then it will print memory usage
statistics after compiling your code when "$ENV{PERL_DEBUG_MSTATS} >
1", and before termination 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 currently 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 "SMALLEST" 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 present,
the memory footprints of the buckets are between the memory foot-
prints of two buckets "above".
For example, suppose under the previous example, the memory foot-
prints 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 continuity 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 buck-
ets. 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 similar to
sub getcwd;
WARNING: The discussion below supposes 32-bit architecture. In newer
releases of Perl, memory usage of the constructs discussed here is
greatly improved, but the story discussed below is a real-life story.
This story is mercilessly 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 statis-
tics 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 subsystems 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 program, you should set a
C breakpoint in Perl_warn(), continue until this point is reached, and
then set a C breakpoint 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 stringification 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 cre-
ated 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 freed, but are kept as additional arenas for "SV" alloca-
tions.
054 Creates a "HEK" for the name of the glob for the subroutine. This
name is a key in a stash.
Big allocations with this Id correspond to allocations of new are-
nas 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 categories of memory alloca-
tions, and statistics of allocations of different sizes for these cate-
gories.
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 allo-
cate memory, such allocations are not counted.
SEE ALSO
perldebug, perlguts, perlrun re, and Devel::DProf.
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