Asked this question at serverfault, and got confirmation that W3TC is most likely causing the issue, but several of my more W3TC/Wordpress specific issues weren't able to be answered. Reposting here as this community had hopefully answer those better. I emboldened them below.*
AWS EC2 Small instance, Apache 2 running WordPress and W3TC. Within an hour, my APC fragmentation hits 100%.
My APC settings are:
apc.enabled = 1 apc.shm_segments = 1 apc.shm_size = 100M apc.optimization = 0 apc.num_files_hint = 512 apc.user_entries_hint = 1024 apc.ttl = 7200 apc.user_ttl = 7200 apc.gc_ttl = 3600 apc.cache_by_default = 1 apc.use_request_time = 1 apc.filters = "apc\.php$" apc.mmap_file_mask = "/tmp/apc.XXXXXX" apc.slam_defense = 0 apc.file_update_protection = 2 apc.enable_cli = 0 apc.max_file_size = 2M apc.stat = 1 apc.write_lock = 1 apc.report_autofilter = 0 apc.include_once_override = 0 apc.rfc1867 = 0 apc.rfc1867_prefix = "upload_" apc.rfc1867_name = "APC_UPLOAD_PROGRESS" apc.rfc1867_freq = 0 apc.localcache = 0 apc.localcache.size = 256M apc.coredump_unmap = 0 apc.stat_ctime = 0 apc.canonicalize = 1 apc.lazy_functions = 0 apc.lazy_classes = 0 /etc/php.d/apc.ini
Mostly cribed settings from here. The shm was meant to be whittled down from such a high value after some observation, but apparently such a large value isn't even high enough....
I found an similar question/answer here. I do have some virtual hosts setup, but they aren't being touched much at all. Having users logged into the admin panel of WP does make things worse, but that's certainly not the main culprit. The question asker seems to suggest that it turns out W3TC is probably causing the problem, which the plugin author seems to agree with, but there aren't any helpful details beyond that. Why is it causing the problem? Could this be fixed?
Do I just turn for now and turn off object caching with APC? Is there nothing I can do? Does APC turned on without being used for object caching actually help anything? Would memcache be an ok substitute just for object caching here? Finally, maybe I just shouldn't worry so much about the fragmentation?