APC is showing 100% fragmentation

Question

APC is showing 100% fragmentation. Is this bad?

Does it mean that it's not helping at all? What paths do I go down to improve situation?

Thanks in advance.

Answer 1

In my experience, yes. I had a system where APC was showing 100% fragmentation, and performance was bad. I increased APC's memory limit (to 200 MB in my case -- but we had a lot of code) enough to give it some slack room. Fragmentation dropped to zero, and IIRC, CPU usage on the server dropped by 50%.

Also, make sure you're using the apc.php script that comes with APC to monitor fragmentation/utilization. We've even written a nagios check to watch APC, 'cause we have enough traffic that apache locks up entirely when APC fills up.

Moral of the story: give APC enough memory, and monitor utilization.

Answer 2

[...] Fragmentation is what hurts performance, not the size of memory per se. But it also seems that fragmentation happens when memory is low [...]

Note also that there seems to be a bug with apc.php's graph: http://pecl.php.net/bugs/bug.php?id=13146

Answer 3

Fragmentation means that apc often throws out items from it's cache and adds new ones and has trouble finding large enough continous blocks.

There are two main ways to improve performance then

• Give more memory to APC. Ideally APC can store your complete scripts in memory.
• use apc.filter in php.ini to filter out files which aren't requested often or change often.

Also using apc_store() with a short time to live is bad, as is overwriting using apc_store() often.