I just know that it's not under
constant stress. In normal situation it uses 1-1.5GB of available RAM
and load average rarey reaches 5. It also runs smoothly for weeks until
we get these peaks, the load gets suddendly high, memory is exhausted,
the server starts swapping and it all freezes.
I haven't been able to pinpoint what causes these peaks, although they
seem to be related with the process of recreating the page cache.
Regarding the page cache (which is set to disk) I noticed that in the
past weeks it didn't seem to be correctly purged, as we reached 17k
subdirs under wp-content/w3tc/pgcache (first level, not counting their
subdirs) and a manual click on the empty page cache button only lead to
a timeout (and a manual rm -rf took several minutes) -- also updating
users details gave a never ending spinning working icon (I believe that
w3tc hooks page cache purge to user edit).
After changing some settings yesterday, the page cache seems to be
purged, but the server is completely unstable. The main problem is that
now I still haven't been able to revert to a stable situation, no
matter what settings I use in w3tc.
We have APC running on the server both for opcode caching and for manual
caching of most widgets (this is done by our custom template).
Let me know if you need more information