I have multiple instances of APC running on my server with similar configurations (albeit with different cache sizes. However, one of the instances is performing extremely poorly, and I have no idea why (100% cache fragmentation, high miss rate).

The runtime settings I'm using are as follows (pretty much out of the box):

apc.cache_by_default        1
apc.canonicalize            1
apc.coredump_unmap          0
apc.enable_cli              0
apc.enabled                 1
apc.file_md5                0
apc.file_update_protection  2
apc.filters 
apc.gc_ttl                  3600
apc.include_once_override   0
apc.lazy_classes            0
apc.lazy_functions          0
apc.max_file_size           1M
apc.mmap_file_mask  
apc.num_files_hint          1000
apc.preload_path    
apc.report_autofilter       0
apc.rfc1867                 0
apc.rfc1867_freq            0
apc.rfc1867_name            APC_UPLOAD_PROGRESS
apc.rfc1867_prefix          upload_
apc.rfc1867_ttl             3600
apc.shm_segments            1
apc.shm_size                10M
apc.slam_defense            1
apc.stat                    1
apc.stat_ctime              0
apc.ttl                     0
apc.use_request_time        1
apc.user_entries_hint       4096
apc.user_ttl                0
apc.write_lock              1

APC is version 3.1.6, PHP is 5.3.3-1ubuntu9.5. I've tried restarting Apache multiple times, so this isn't a freak instance.

The instance with problems is simply running Wordpress with a few plugins installed. All other instances (~4) on the server are running perfectly fine with almost 100% hit rates and 0% fragmentation; for example this instance is holding a website built using the Symfony framework.

Any help would be much appreciated; I haven't had much experience with APC and was hoping for it to be an out-of-the-box speed boost ;).