How to tune cache sizes

OpenACS maintains many caches, which can be adjusted by different means depending on the version of OpenACS. Most important is the setting of the sizes via configuration files or package parameters (see e.g. [[https://github.com/openacs/openacs-core/blob/master/packages/acs-tcl/tcl/community-core-init.tcl#L1|acs-tcl/tcl/community-core-init.tcl]).

But what are good sizes? For managing the caches, using the NaviServer module nsstats is recommended. This module consists of a single file and is therefore easy installable,  but note that the newest features of the newest versions require often recent versions of NaviServer as well. To be on the safe side, use the nsstats version contained in the modules tar file for every release version of NaviServer (see e.g. NaviServer releases). The install script naviserver-openacs installs nsstats automatically under /admin.

The nsstats module provides among many other things an overview of the used caches with various usage statistics.

If one clicks on a single cache, a detail view is provided. The newest version of the detail view of a cache in nsstats contains now as well estimates for a good cache size. Useful cache entries are entries which were reused at least once. So, when a cache with 1MB size has e.g. a utilization of 50%, and only half of the used entries are reused, then effectively, the calculation suggests that 250KB + 10% are sufficient.

The only memory waste are actually the entries in the cache without any reuse, since the cache size is a max-size, the space which is not used does not cost memory.

The graphic shows, that only 5% of the size of the cache at openacs.org are actually useful, less than 2% has a reuse of 5 or better. Certainly, the reading are only useful after running the server for a while.

Another dimension for cache tuning is cache partitioning as supported in OpenACS 5.10, which is important for highly busy sites in case the cache lock times go up. More about this later.