Having run out of other ways to really improve performance, I added a disk-based caching infrastructure to DWiki and then put in two caches.
The real cache is the renderer cache, which stores the results of
selected renderers (currently just the wikitext renderers). Via some
glue it's also used to store the results of the filesystem walk that's
the expensive bit of blog::prevnext.
The Brute Force Cache is for dealing with Slashdotting style situations; it just caches complete requests for N seconds when the system seems to be under load. I also hijacked it as a convenient place to add extra caching for Atom feeds and to force this caching on software that doesn't do conditional GET.
(For more details, see Caching.)
This required a new storage pool class. Like the comment store, it uses a customized and restricted interface to write things (and a new interface to read them). The cache storage pool stores objects, not data blobs, using the cPickle module to make the swap back and forth. (This may be a mistake, but it's fast and easy.)
Since removing files in DWiki makes me nervous, I didn't bother to
implement any sort of cache cleaning; you get to do that by hand. The
cache has TTLs, and the renderer cache has validation layered on top
of the cache object store, but when they detect something invalid they
just ignore it. (On the other hand, the cache storage layer does use
temporary files and rename(), so in a sense it's already removing
files.)
In theory the cache interface is generic, so later I can hook up a memcached setup or something without having to change higher-level code.