APC usage and hit chart

Alternative PHP Cache (APC) on Debian 6

APC usage and hit chartAfter spending some time reviewing way to increase WordPress performance in ways beyond simple file caching I decided to give APC (Alternative PHP Cache) and extended try. I use mod_fastcgid on my server, as opposed to mod_fastcgi. Web searches indicated that mod_fastcgi would be a better choice with APC since it could share the same cache between processes. With mod_fastcgid each process has its own cache, which would also mean more memory usage. I have a relatively small server, a 512 MB VPS. But in looking at free memory it appeared I’d be OK as long as I was careful and had some upper limits. Also, I had has some issues getting mod_fastcgi to run in the past so I didn’t want to try that again.

Why APC

APC is an opcode cache. Opcode caches will cache the compiled php script in memory so it does not have to be interpreted each time it is called. WordPress is built with PHP so this should help performance. (Technically “compiled” is probably technically wrong but does get the point across.) Other opcode caches are XCache and eAccelerator.

eAccelerator hasn’t been updated in a couple years so I was hesitant to try it. APC is by the same guys who develop PHP and it’s supposed to be rolled into PHP 6. So I decided to give it a try first. I may give XCache a try after running APC awhile. Although, if I’m honest, I probably won’t bother if APC works OK.  They can’t co-exist so I can only install one at a time.

The one reason not to use APC is memory. I’ll be improving cpu utilization and improving performance at the expense of memory.

Throughout this article the terms cache and caching refer to the opcode caching provided by APC.

Installation

APC is in the Debian 6 repositories so it can be installed from there. I use aptitude, so I run:

aptitude install php-apc

Once it’s installed I reload Apache and APC is running. No admin interface is running so I extract the apc.php file by running:

gzip -dc /usr/share/doc/php-apc/apc.php.gz > /path/to/my/mngmt/website/apc.php

The /path/to/my/mngmt/website is a website I have that’s a catch-all for various server related pages. Access to it is restricted and since it does include some version information and files paths you may want to restrict access. I can open this page and get statistics about APC usage.

When I opened apc.php and did a version check I saw that I was several versions behind. I install 3.1.3p2 from 2009 and the latest stable version was 3.1.9 fro May 2011 and there was a beta version from April 2012. APC was working and limited testing didn’t show any problems but I decided to update.

Update Through PECL

PECL is the PHP Community Extension Library so its installer will be installed along with packages it needs to compile APC. Finally APC will be installed.

sudo aptitude install php-pear php5-dev apache2-dev

Once these are installed I can install (upgrade) apc

sudo pecl install apc

Reloading Apache finishes things off. Because I had first installed apc through aptitude I didn’t have to do any manual edits.

Configuring APC

Configuring APC is where the real work is needed. I did several iterations testing different configurations. At this point I was testing for performance, I was looking to see what worked and what didn’t.

The apc.ini shows the settings I was most interested in and what I’ve currently settled on. Memory is the biggest issue with APC. As I mentioned, each FCGID process creates it’s own cache. I’ve configured my server for a maximum of three processes so I can control memory usage. The easy way to determine how much memory to allocate is to divide my free memory (without caching enabled) by 3 (the max number of FGCID process I’d have) and leave a little for OS buffers and breathing room. For me this worked out to about 100 MB (I had about 350 MB free. But that calculation is a limit, which doesn’t mean it’s a good size.

I’m going to jump ahead and show the configuration I settled on.My /etc/php5/conf.d/apc.ini contains the following entries:

extension=apc.so
apc.enabled=1
apc.cache_by_default=0
apc.filters=”+(theosquest.com|stats.rrn.co)/.*”
apc.shm_segments = 1
apc.shm_size=90M
apc.slam_defense=0
apc.stat=1
apc.ttl=0
apc.user_ttl=0

The first two lines tell PHP to load the APC extension and enable caching.

I started off with a 256 MB cache and let it cache all my sites. This was much too large so I’d have to monitor usage in case things started swapping to disk. Cache usage seemed to level off at around 90 MB after a few hours. I did go through a load the home page of all my sites (many are test sites or receive little traffic). This gave me an rough (and lowball) idea of what I’d need to cache everything. So this was right at my 90 MB estimate. In practice a 90 MB cache size resulted in a lot of cache fragmentation and cache reloads which isn’t an optimal situation.

I took the approach of reducing my FCGID processes to 2 and increasing the cache to 128 MB. This provided better cache performance but shortly after midnight I received a series of “[warn] [client 69.164.211.225] mod_fcgid: can’t apply process slot for /home/rayn/public_html/fcgi-bin.d/php5-default/php-fcgi-wrapper” errors. That IP address is my server so some daily maintenance scripts needed more PHP processes than were available. Later in the morning one visitor experienced a timeout error. It may have been unrelated but timeout errors were previously unheard of. So I switched back to three FCGID processes.

At this point it was obvious I had to cache less than everything. There’s only two sites that have enough traffic to benefit from caching so I decided limiting caching to them. I handled this through the next two lines. apc.cache_by_default=0 turns off caching for all sites. It now needs to be turned on explicitly. It’s turned on through the apc.filters line. The + means the filter is to add the matching files to caching. (By default the filter removes matches from caching.) The syntax is a POSIX regular expression and can be comma delimited. I had trouble getting a comma delimited list to work although all indications are that it should. So I ended up combining them into this one expression. Here’s a regular expression cheat sheet. I cache this site and one subdomain on a second site.

Caching doesn’t have to be enabled for a site in order to use it with a WordPress plugin that supports APC. For example, W3 Total Cache will use the cache even if it’s not enabled for the site.

I specified apc.shm_segments = 1 even though 1 is the default setting. I don’t want later version to change the default and start eating more memory. I specify apc.shm_size=90 to create a 90MB cache per FCGID process.

I also specified apc.stat=1 even though that’s the default. There’s debate about whether setting this to 0 will improve performance. A setting of 1 means that APC will check each script on request to see if it’s been modified. If it’s been modified it will be re-cached. A setting of zero disables this feature, The official documentation recommends setting it to 0 on a production server. A setting of 0 can cause problems as PHP changes (such as WordPress plugin updates) would go unnoticed. Once things are solid I may change it to 0 to see if there’s an improvement or any problems. My initial feeling is any improvement would be unnoticeable for my site and wouldn’t be worth the hassle of needing to manually flush the cache when I make changes. I’m also concerned it will cause problems with some plugins.

To make sure memory usage doesn’t get out of control I’ve also limited my FCGID processes to only three and I don’t allow them to spawn any child processes. I also don’t terminate to FCGID processes since this would clear the cache. This could be a potential memory problem so I’ll have to monitor it.  The following settings are specified in my /etc/apache2/conf.d/php-fcgid.conf.

DefaultInitEnv PHP_FCGI_CHILDREN 0
DefaultInitEnv PHP_FCGI_MAX_REQUESTS 0
FcgidMaxProcesses 2

This should keep memory usage below the physical memory in my server. PHP_FCGI_MAX_REQUESTS is set to zero based on this in the Apache mod_fcgid documentation:

By default, PHP FastCGI processes exit after handling 500 requests, and they may exit after this module has already connected to the application and sent the next request. When that occurs, an error will be logged and 500 Internal Server Error will be returned to the client. This PHP behavior can be disabled by setting PHP_FCGI_MAX_REQUESTS to 0, but that can be a problem if the PHP application leaks resources. Alternatively, PHP_FCGI_MAX_REQUESTS can be set to a much higher value than the default to reduce the frequency of this problem. FcgidMaxRequestsPerProcess can be set to a value less than or equal to PHP_FCGI_MAX_REQUESTS to resolve the problem.

I’ll have to monitor this. It’s possible the FCGID memory could keep growing until physical memory is filled.

PHP_FCGI_CHILDREN is set to 0 to prevent children from being spawned. From the Apache documentation:

PHP child process management (PHP_FCGI_CHILDREN) should always be disabled with mod_fcgid, which will only route one request at a time to application processes it has spawned; thus, any child processes created by PHP will not be used effectively. (Additionally, the PHP child processes may not be terminated properly.) By default, and with the environment variable setting PHP_FCGI_CHILDREN=0, PHP child process management is disabled.

Additional Notes

The screenshot below (click to open full size) shows my status (using apc.php) a couple hours after the caching started. The screenshot was done when the cache was still configured for 256 MB. I am running a unique cache for each fdcgid process so refreshing the page moves between the instances (but randomly). The size of the three caches stay pretty close to one another but they are clearly unique caches.

APC Info Page

When running top each of my three fcgid processes levels off at about 25% memory usage. So if my math is right this is 75% of 512 MB. Yet free (and top) show 350 MB still free. If each was allocating the full 256 MB to its own cache then it would be well over 100%, assuming free and top saw the shared memory it was using. So I’m assuming free doesn’t see the shared memory and reports is as available. In my limited memory this makes sense since the shared memory is still available to any application. The actual usage of my three caches never grew about the available free memory.

APC does require some work to configure and optimize it. To start with I’d enabled it for all my sites and PHP. This resulted in a lot of cache fragmentation and a few cache reloads after a few hours.

Benchmarks and timings on a test server are fine for comparisons, but I’ll need to run a configuration for awhile to see how it does, then start tweaking it. Right now my server seems healthy. Using APC did improve performance over no caching of any type, but I’m not sure of it’s benefit once page caching is used, especially if using a preloaded cache.

Right now my main concern is that I have too few FCGID processes, only two. I used some load testing tools and it didn’t cause any errors. I do have page caching enabled to PHP shouldn’t be needed when serving a page from the cache. I’ll be monitoring my error logs.

Anyone else with experience using APC with WordPress?

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