perldebguts - Guts of Perl debugging
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.
Perl has special debugging hooks at compile-time and runtime
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 following
additional features are enabled (cf. "$^P" in perlvar):
o Perl inserts the contents of $ENV{PERL5DB} (or "BEGIN
{require 'perl5db.pl'}" if not present) before the
first line of your program.
o Each array "@{"_<$filename"}" holds the lines of
$filename for a file compiled by Perl. The same is
also true for "eval"ed strings that contain 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_condition
$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::postponed(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 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 possible,
even though &DB::sub exists.
Writing Your Own Debugger [Toc] [Back]
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, " $sub0; &$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 environment variable and uses it to set debugger
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 "@{"::_<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 contains breakpoints and actions keyed by line
number in the currently-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_condition $action".
Debugger customization functions
Some functions are provided to simplify customization.
o See "Options" in perldebug for description of options
parsed by "DB::parse_options(string)" parses debugger
options; see "Options" in pperldebug for a description
of options recognized.
o "DB::dump_trace(skip[,count])" skips the specified
number of frames and returns a list containing information
about the calling frames (all of them, if
"count" is missing). Each entry is reference to a
hash with keys "context" (either ".", "$", or "@"),
"sub" (subroutine 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 [Toc] [Back] 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" 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 $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
scalar context return from Exporter::import: ''
In all cases shown above, the line indentation shows the
call tree. If bit 2 of "frame" is set, a line is printed
on exit from a subroutine as well. If bit 4 is set, the
arguments are printed along with the caller info. If bit
8 is set, the arguments are printed even if they are tied
or references. If bit 16 is set, the return value is
printed, too.
When a package is compiled, a line like this
Package lib/Carp.pm.
is printed with proper indentation.
Debugging regular expressions [Toc] [Back] There are two ways to enable debugging output for regular
expressions.
If your perl is compiled with "-DDEBUGGING", you may use
the -Dr flag on the command line.
Otherwise, one can "use re 'debug'", which has effects at
compile time and run time. It is not lexically scoped.
Compile-time output [Toc] [Back]
The debugging output at compile time looks like this:
Compiling REx `[bc]d(ef*g)+h[ij]k$'
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 $` $& $' support.
The first line shows the pre-compiled form of the regex.
The second shows the size of the compiled form (in arbitrary
units, usually 4-byte words) and the total number of
bytes allocated for the offset/length table, usually
4+"size"*8. The next line shows the label id of the first
node that does a match.
The
anchored `de' at 1 floating `gh' at 3..2147483647
(checking floating)
stclass `ANYOF[bc]' minlen 7
line (split into two lines above) contains optimizer
information. In the example shown, the optimizer found
that the match should contain a substring "de" at offset
1, plus substring "gh" at some offset between 3 and infinity.
Moreover, when checking for these substrings (to
abandon impossible matches quickly), Perl will check for
the substring "gh" before checking for the substring "de".
The optimizer may also use the knowledge that the match
starts (at the "first" id) with a character class, and no
string shorter than 7 characters can possibly match.
The fields of interest which may appear in this line are
"anchored" STRING "at" POS
"floating" STRING "at" POS1..POS2
See above.
"matching floating/anchored"
Which substring to check first.
"minlen"
The minimal length of the match.
"stclass" TYPE
Type of first matching node.
"noscan"
Don't scan for the found substrings.
"isall"
Means that the optimizer information is all that the
regular expression contains, and thus one does not
need to enter the regex engine at all.
"GPOS"
Set if the pattern contains "G".
"plus"
Set if the pattern starts with a repeated char (as in
"x+y").
"implicit"
Set if the pattern starts with ".*".
"with eval"
Set if the pattern contain eval-groups, such as "(?{
code })" and "(??{ code })".
"anchored(TYPE)"
If the pattern may match only at a handful of places,
(with "TYPE" being "BOL", "MBOL", or "GPOS". See the
table below.
If a substring is known to match at end-of-line only, it
may be followed by "$", as in "floating `k'$".
The optimizer-specific information is used to avoid 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
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 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 precompiled
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 precompiled
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 [Toc] [Back]
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 [Toc] [Back] 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, 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 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 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.
The -DL command-line switch is obsolete since circa Perl
5.6.0 (it was available only if Perl was built with "-DDEBUGGING").
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 suggested
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 analyze
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 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 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 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 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 [Toc] [Back]
(Note that -DL is obsolete since circa 5.6.0, and even
before that Perl needed to be compiled with -DDEBUGGING.)
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 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 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 created though the subroutine
itself is not defined yet.
It also creates C arrays to keep data for the stash.
This is one HV, but it grows; thus, there are 4 big
allocations: the big chunks are not freed, but are
kept as additional arenas for "SV" allocations.
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 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 allocations
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 [Toc] [Back]
If an extension or external library does not use the Perl
API to allocate memory, such allocations are not counted.
perldebug, perlguts, perlrun re, and Devel::DProf.
perl v5.8.5 2002-11-06 20 [ Back ] |
