Apache::SizeLimit(3) User Contributed Perl Documentation Apache::SizeLimit(3)
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+).
perl v5.8.8 2007-07-17 Apache::SizeLimit(3)
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