I am setting up APC (v 3.1.9) on a high-traffic WordPress installation on CentOS 6.0 64 bit.

I have figured out many of the quirks with APC, but something is still not quite right. No matter what settings I change, APC never actually caches more than 32MB. I'm trying to bump it up to 256 MB. 32MB is a default amount for apc.shm_size, so I am wondering if it's stuck there somehow.

I have run the following

echo '2147483648' > /proc/sys/kernel/shmmax

to increase my system's shared memory to 2G (half of my 4G box). Then ran

ipcs -lm

which returns

------ Shared Memory Limits --------
max number of segments = 4096
max seg size (kbytes) = 2097152
max total shared memory (kbytes) = 8388608
min seg size (bytes) = 1

Also made a change in

/etc/sysctl.conf

then ran

sysctl -p

to make the settings stick on the server. Rebooted, too, for good measure.

In my APC settings, I have mmap enabled (which happens by default in recent versions of APC). php.ini looks like:

apc.stat=0
apc.shm_size="256M"
apc.max_file_size="10M"
apc.mmap_file_mask="/tmp/apc.XXXXXX"
apc.ttl="7200"

I am aware that mmap mode will ignore references to apc.shm_segments, so I have left it out with default 1.

phpinfo() indicates the following about APC:

Version     3.1.9
APC Debugging   Disabled
MMAP Support    Enabled
MMAP File Mask  /tmp/apc.bPS7rB
Locking type    pthread mutex Locks
Serialization Support   php
Revision    $Revision: 308812 $
Build Date  Oct 11 2011 22:55:02

Directive   Local Value 
apc.cache_by_default    On  
apc.canonicalize    O
apc.coredump_unmap  Off 
apc.enable_cli  Off 
apc.enabled On  On
apc.file_md5    Off 
apc.file_update_protection  2   
apc.filters no value    
apc.gc_ttl  3600    
apc.include_once_override   Off 
apc.lazy_classes    Off 
apc.lazy_functions  Off 
apc.max_file_size   10M 
apc.mmap_file_mask  /tmp/apc.bPS7rB 
apc.num_files_hint  1000    
apc.preload_path    no value    
apc.report_autofilter   Off 
apc.rfc1867 Off 
apc.rfc1867_freq    0   
apc.rfc1867_name    APC_UPLOAD_PROGRESS
apc.rfc1867_prefix  upload_ 
apc.rfc1867_ttl 3600    
apc.serializer  default 
apc.shm_segments    1   
apc.shm_size    256M    
apc.slam_defense    On  
apc.stat    Off 
apc.stat_ctime  Off 
apc.ttl 7200    
apc.use_request_time    On  
apc.user_entries_hint   4096    
apc.user_ttl    0   
apc.write_lock  On  

apc.php reveals the following graph, no matter how long the server runs (cache size fluctuates and hovers at just under 32MB.

See image http://i.stack.imgur.com/2bwMa.png

You can see that the cache is trying to allocate 256MB, but the brown piece of the pie keeps getting recycled at 32MB. This is confirmed as refreshing the apc.php page shows cached file counts that move up and down (implying that the cache is not holding onto all of its files).

Does anyone have an idea of how to get APC to use more than 32 MB for its cache size??

**Note that the identical behavior occurs for eaccelerator, xcache, and APC. I read here:

http://www.litespeedtech.com/support/forum/archive/index.php/t-5072.html

that suEXEC could cause this problem.