NAME
Apache::SizeLimit - Because size does matter.
SYNOPSIS
<Perl>
Apache::SizeLimit->set_max_process_size(150_000); # Max size in KB
Apache::SizeLimit->set_min_shared_size(10_000); # Min share in KB
Apache::SizeLimit->set_max_unshared_size(120_000); # Max unshared size in KB
</Perl>
PerlCleanupHandler Apache::SizeLimit
DESCRIPTION
******************************** NOIICE *******************
This version is only for httpd 1.x and mod_perl 1.x
series.
Future versions of this module may support both.
Currently, Apache2::SizeLimit is bundled with
mod_perl 2.x for that series.
******************************** NOTICE *******************
This module allows you to kill off Apache httpd processes if they grow
too large. You can make the decision to kill a process based on its
overall size, by setting a minimum limit on shared memory, or a maximum
on unshared memory.
You can set limits for each of these sizes, and if any limit is
exceeded, the process will be killed.
You can also limit the frequency that these sizes are checked so that
this module only checks every N requests.
This module is highly platform dependent, please read the "PER-PLATFORM
BEHAVIOR" section for details. It is possible that this module simply
does not support your platform.
API
You can set set the size limits from a Perl module or script loaded by
Apache by calling the appropriate class method on "Apache::SizeLimit":
* Apache::SizeLimit->set_max_process_size($size)
This sets the maximum size of the process, including both shared and
unshared memory.
* Apache::SizeLimit->set_max_unshared_size($size)
This sets the maximum amount of *unshared* memory the process can
use.
* Apache::SizeLimit->set_min_shared_size($size)
This sets the minimum amount of shared memory the process must have.
The two methods related to shared memory size are effectively a no-op if
the module cannot determine the shared memory size for your platform.
See "PER-PLATFORM BEHAVIOR" for more details.
Running the handler()
There are several ways to make this module actually run the code to kill
a process.
The simplest is to make "Apache::SizeLimit" a "PerlCleanupHandler" in
your Apache config:
PerlCleanupHandler Apache::SizeLimit
This will ensure that "Apache::SizeLimit->handler()" is run for all
requests.
If you want to combine this module with a cleanup handler of your own,
make sure that "Apache::SizeLimit" is the last handler run:
PerlCleanupHandler Apache::SizeLimit My::CleanupHandler
Remember, mod_perl will run stacked handlers from right to left, as
they're defined in your configuration.
If you have some cleanup code you need to run, but stacked handlers
aren't appropriate for your setup, you can also explicitly call the
"Apache::SizeLimit->handler()" function from your own cleanup handler:
package My::CleanupHandler
sub handler {
my $r = shift;
# Causes File::Temp to remove any temp dirs created during the
# request
File::Temp::cleanup();
return Apache::SizeLimit->handler($r);
}
* Apache::SizeLimit->add_cleanup_handler($r)
You can call this method inside a request to run
"Apache::SizeLimit"'s "handler()" method for just that request. It's
safe to call this method repeatedly -- the cleanup will only be run
once per request.
Checking Every N Requests
Since checking the process size can take a few system calls on some
platforms (e.g. linux), you may not want to check the process size for
every request.
* Apache::SizeLimit->set_check_interval($interval)
Calling this causes "Apache::SizeLimit" to only check the process
size every $interval requests. If you want this to affect all
processes, make sure to call this during server startup.
SHARED MEMORY OPTIONS
In addition to simply checking the total size of a process, this module
can factor in how much of the memory used by the process is actually
being shared by copy-on-write. If you don't understand how memory is
shared in this way, take a look at the mod_perl docs at
http://perl.apache.org/docs/.
You can take advantage of the shared memory information by setting a
minimum shared size and/or a maximum unshared size. Experience on one
heavily trafficked mod_perl site showed that setting maximum unshared
size and leaving the others unset is the most effective policy. This is
because it only kills off processes that are truly using too much
physical RAM, allowing most processes to live longer and reducing the
process churn rate.
PER-PLATFORM BEHAVIOR
This module is highly platform dependent, since finding the size of a
process is different for each OS, and some platforms may not be
supported. In particular, the limits on minimum shared memory and
maximum shared memory are currently only supported on Linux and BSD. If
you can contribute support for another OS, patches are very welcome.
Currently supported OSes:
linux
For linux we read the process size out of /proc/self/statm. If you are
worried about performance, you can consider using
"Apache::SizeLimit->set_check_interval()" to reduce how often this read
happens.
As of linux 2.6, /proc/self/statm does not report the amount of memory
shared by the copy-on-write mechanism as shared memory. This means that
decisions made based on shared memory as reported by that interface are
inherently wrong.
However, as of the 2.6.14 release of the kernel, there is
/proc/self/smaps entry for each process. /proc/self/smaps reports
various sizes for each memory segment of a process and allows us to
count the amount of shared memory correctly.
If "Apache::SizeLimit" detects a kernel that supports /proc/self/smaps
and the "Linux::Smaps" module is installed it will use that module
instead of /proc/self/statm.
Reading /proc/self/smaps is expensive compared to /proc/self/statm. It
must look at each page table entry of a process. Further, on
multiprocessor systems the access is synchronized with spinlocks. Again,
you might consider using "Apache::SizeLimit->set_check_interval()".
Copy-on-write and Shared Memory
The following example shows the effect of copy-on-write:
<Perl>
require Apache::SizeLimit;
package X;
use strict;
use Apache::Constants qw(OK);
my $x = "a" x (1024*1024);
sub handler {
my $r = shift;
my ($size, $shared) = $Apache::SizeLimit->_check_size();
$x =~ tr/a/b/;
my ($size2, $shared2) = $Apache::SizeLimit->_check_size();
$r->content_type('text/plain');
$r->print("1: size=$size shared=$shared\n");
$r->print("2: size=$size2 shared=$shared2\n");
return OK;
}
</Perl>
<Location /X>
SetHandler modperl
PerlResponseHandler X
</Location>
The parent Apache process allocates memory for the string in $x. The
"tr"-command then overwrites all "a" with "b" if the handler is called
with an argument. This write is done in place, thus, the process size
doesn't change. Only $x is not shared anymore by means of copy-on-write
between the parent and the child.
If /proc/self/smaps is available curl shows:
r2@s93:~/work/mp2> curl http://localhost:8181/X
1: size=13452 shared=7456
2: size=13452 shared=6432
Shared memory has lost 1024 kB. The process' overall size remains
unchanged.
Without /proc/self/smaps it says:
r2@s93:~/work/mp2> curl http://localhost:8181/X
1: size=13052 shared=3628
2: size=13052 shared=3636
One can see the kernel lies about the shared memory. It simply doesn't
count copy-on-write pages as shared.
solaris 2.6 and above
For solaris we simply retrieve the size of /proc/self/as, which contains
the address-space image of the process, and convert to KB. Shared memory
calculations are not supported.
NOTE: This is only known to work for solaris 2.6 and above. Evidently
the /proc filesystem has changed between 2.5.1 and 2.6. Can anyone
confirm or deny?
BSD (and OSX)
Uses "BSD::Resource::getrusage()" to determine process size. This is
pretty efficient (a lot more efficient than reading it from the /proc fs
anyway).
According to recent tests on OSX (July, 2006), "BSD::Resource" simply
reports zero for process and shared size on that platform, so OSX is not
supported by "Apache::SizeLimit".
AIX?
Uses "BSD::Resource::getrusage()" to determine process size. Not sure if
the shared memory calculations will work or not. AIX users?
Win32
Uses "Win32::API" to access process memory information. "Win32::API" can
be installed under ActiveState perl using the supplied ppm utility.
Everything Else
If your platform is not supported, then please send a patch to check the
process size. The more portable/efficient/correct the solution the
better, of course.
ABOUT THIS MODULE
This module was written in response to questions on the mod_perl mailing
list on how to tell the httpd process to exit if it gets too big.
Actually, there are two big reasons your httpd children will grow.
First, your code could have a bug that causes the process to increase in
size very quickly. Second, you could just be doing operations that
require a lot of memory for each request. Since Perl does not give
memory back to the system after using it, the process size can grow
quite large.
This module will not really help you with the first problem. For that
you should probably look into "Apache::Resource" or some other means of
setting a limit on the data size of your program. BSD-ish systems have
"setrlimit()", which will kill your memory gobbling processes. However,
it is a little violent, terminating your process in mid-request.
This module attempts to solve the second situation, where your process
slowly grows over time. It checks memory usage after every request, and
if it exceeds a threshold, exits gracefully.
By using this module, you should be able to discontinue using the Apache
configuration directive MaxRequestsPerChild, although for some folks,
using both in combination does the job.
DEPRECATED APIS
Previous versions of this module documented three globals for defining
memory size limits:
* $Apache::SizeLimit::MAX_PROCESS_SIZE
* $Apache::SizeLimit::MIN_SHARE_SIZE
* $Apache::SizeLimit::MAX_UNSHARED_SIZE
* $Apache::SizeLimit::CHECK_EVERY_N_REQUESTS
* $Apache::SizeLimit::USE_SMAPS
Direct use of these globals is deprecated, but will continue to work for
the foreseeable future.
It also documented three functions for use from registry scripts:
* Apache::SizeLimit::setmax()
* Apache::SizeLimit::setmin()
* Apache::SizeLimit::setmax_unshared()
Besides setting the appropriate limit, these functions *also* add a
cleanup handler to the current request.
AUTHOR
Doug Bagley <doug+modperl@bagley.org>, channeling Procrustes.
Brian Moseley <ix@maz.org>: Solaris 2.6 support
Doug Steinwand and Perrin Harkins <perrin@elem.com>: added support for
shared memory and additional diagnostic info
Matt Phillips <mphillips@virage.com> and Mohamed Hendawi
<mhendawi@virage.com>: Win32 support
Dave Rolsky <autarch@urth.org>, maintenance and fixes outside of
mod_perl tree (0.9+).