I wasn't sure if I should put a comment in about __DIE__ handlers and their use with evals, it seemed like that might be too general perl.
Index: src/docs/1.0/guide/troubleshooting.pod
===================================================================
RCS file: /home/cvspublic/modperl-docs/src/docs/1.0/guide/troubleshooting.pod,v
retrieving revision 1.28
diff -u -r1.28 troubleshooting.pod
--- src/docs/1.0/guide/troubleshooting.pod 5 May 2004 03:29:38 -0000 1.28
+++ src/docs/1.0/guide/troubleshooting.pod 6 May 2004 22:40:07 -0000
@@ -589,27 +589,45 @@
If something goes really wrong with your code, Perl may die with an
"Out of memory!" message and/or "Callback called exit". Common causes of this
-are never-ending loops, deep recursion, or calling an
-undefined subroutine. Here's one way to catch the problem: See Perl's
-INSTALL document for this item:
+are never-ending loops, deep recursion, or calling an undefined subroutine.
- =item -DPERL_EMERGENCY_SBRK
+If you are using perl 5.005 or later, and perl is compiled to use it's own
+malloc rutines, you can trap out of memory errors by setting aside an extra
+memory pool in the special variable $^M. By default perl uses the operating
+system malloc for many popular systems, so unless you build perl with
+'usemymalloc=y' you probably wont be able to use $^M. To check if your mod_perl
+was compiled to use perl's internal malloc(), stick the following code in a
+handler and see if usemymalloc is set to 'y'
+
+ use Config;
+
+ print Config::myconfig();
+
+Here is an explanation of $^M from perlvar(i):
+
+ $^M By default, running out of memory is an untrap-
+ pable, fatal error. However, if suitably built,
+ Perl can use the contents of $^M as an emergency
+ memory pool after die()ing. Suppose that your
+ Perl were compiled with -DPERL_EMERGENCY_SBRK and
+ used Perl's malloc. Then
+
+ $^M = 'a' x (1 << 16);
+
+ would allocate a 64K buffer for use in an emer-
+ gency. See the INSTALL file in the Perl distribu-
+ tion for information on how to enable this option.
+ To discourage casual use of this advanced feature,
+ there is no English long name for this variable.
- If PERL_EMERGENCY_SBRK is defined, running out of memory need not be a
- fatal error: a memory pool can allocated by assigning to the special
- variable $^M. See perlvar(1) for more details.
-
-If you compile with that option and add 'C<use Apache::Debug level
+If your perl installation supports $^M and you add 'C<use Apache::Debug level
=E<gt> 4;>' to your Perl script, it will allocate the C<$^M> emergency
pool and the C<$SIG{__DIE__}> handler will call C<Carp::confess>,
giving you a stack trace which should reveal where the problem is.
See the C<Apache::Resource> module for ways to control httpd
processes.
-Note that Perl 5.005 and later have C<PERL_EMERGENCY_SBRK> turned on -by default. - -The other trick is to have a startup script initialize +Another trick is to have a startup script initialize C<Carp::confess>, like so:
use Carp (); @@ -617,6 +635,24 @@
this way, when the real problem happens, C<Carp::confess> doesn't eat
memory in the emergency pool (C<$^M>).
+
+Some other mod_perl users have reported that this works well for them:
+
+ ## Allocate 64K as an emergency memory pool for use in out of memory situation
+ $^M = 0x00 x 65536;
+
+ ## Little trick to initialize this routine here so that in the case of OOM,
+ ## compiling this routine doesn't eat memory from the emergency memory pool $^M
+ use CGI::Carp ();
+ eval { CGI::Carp::confess('init') };
+
+ ## Importing CGI::Carp sets $main::SIG{__DIE__} = \&CGI::Carp::die;
+ ## Override that to additionally give a stack backtrace
+ $main::SIG{__DIE__} = \&CGI::Carp::confess;
+
+Discussion of $^M has come up on PerlMonks, and there is speculation that $^M is a
+forgotten feature that's not well supported. See
+http://perlmonks.org/index.pl?node_id=287850 for more information.
=head2 server reached MaxClients setting, consider raising the MaxClients setting
regards,
Brian
On May 6, 2004, at 1:19 PM, Stas Bekman wrote:
Brian Hirt wrote:I too followed the advice too, but it did nothing but lead my down the wrong path. The advice should be updated.
My point is that $^M does absolutely nothing unless you use perl's malloc, which isn't true for most common perl installations these days. compiling with PERL_EMERGENCY_SBRK doesn't help either because it's the default if you usemymalloc, and useless if you don't You MUST compile perl with "-Dusemymalloc=y". A simple grep in the perl hints directory shows that many popular systems such as linux, freebsd and openbsd default to the system malloc which disables the functionality of $^M.
I'd simply like to see the documentation updated, and I'm happy to do it. I know it would have saved me hours and hours of headaches. The documentation as it stands now is misleading.
of course if you use perl's malloc, the advice helps.
Doc patches are always welcome here. Please patch against the source pod.
http://perl.apache.org/download/docs.html#Download
I'd still like to know why mod_perl can get into an infinite loop writtitng "Callback called exit". In perl.c, when that happens my_exit_jump(); is called which should presumably exit the process, but somehow that doesn't happen and some sort of infinite loop occurs outside of my code that fills the log of with gigibytes of 'Callback called exit' messages.
Normally that happens when perl gets its calls stack messed up and it starts to loop. I know I hit that myself while developing mp2 when I was trying to write my own version of die/exit/etc, which I quickly gave up. It is possible that there is a bug in perl, which gets triggered only in certain situations. If you can give p5p a reproducable case, I'm sure it'll be fixed.
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