On my way to the office this morning, every website on our shared VPS started giving the same error (several times, not the typical memory_limit error which is fatal):

Warning: Unknown: Unable to allocate memory for pool. in Unknown on line 0

The shared server is a 64-bit OpenVZ container running cPanel. There are only ~6 VPSes on the host-- this is the largest one at only 4GB. The host itself has 24GB RAM. As the below graphs show, the memory usage on the host and VPS are both rather low. CPU Usage/Disk/Host all seem to be normal. RlimitMem was set to 583653034, yet the memory usage is about the same as it usually is.

Apache 2.2, PHP 5.2 (mod_php)

Restarting Apache has corrected the problem for now. However, I'd like to prevent it from happening again and I'm not sure what was limiting the memory. RlimitMem was set to 583653034, yet the memory usage is about the same as it usually is. There's seems to be plenty of memory: what caused this error?

VPS Memory Usage

Host Memory Usage

APC Information

 apc.ttl=0
 apc.shm_size=0
 apc.mmap_file_mask=(blank)

1 Segment(s) with 32.0 MBytes (mmap memory, pthread mutex locking)