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third_party_mesa3d/docs
Dmitry Osipenko fd9f7b748e util/mesa-db: Introduce multipart mesa-db cache
Whenever a single file mesa-db cache hits max size limit, a half of cache
is evicted and the cache file is defragmented. The downside of this eviction
strategy is that it causes high disk IO usage during eviction if mesa-db
cache file size is large.

In order to mitigate this downside, we will split mesa-db into multiple
part such that only one part will be evicted at a time. Each part will be
an individual single file mesa-db cache, like a DB shard. The new multipart
mesa-db cache will merge the parts into a single virtual cache.

This patch introduces two new environment variables:

1. MESA_DISK_CACHE_DATABASE_NUM_PARTS:
Controls number of mesa-db cache file parts. By default 50 parts will be
created. The old pre-multipart mesa-db cache files will be auto-removed
if they exist, i.e. Mesa will switch to the new DB version automatically.

2. MESA_DISK_CACHE_DATABASE_EVICTION_SCORE_2X_PERIOD:
Controls the eviction score doubling time period. The evicted DB part
selection is based on cache entries size weighted by 'last_access_time' of
the entries. By default the cache eviction score is doubled for each month
of cache entry age, i.e. for two equally sized entries where one entry is
older by one month than the other, the older entry will have x2 eviction
score than the other entry. Database part with a highest total eviction
score is selected for eviction.

This patch brings x40 performance improvement of cache eviction time using
multipart cache vs a single file cache due to a smaller eviction portions
and more optimized eviction algorithm.

Acked-by: Timothy Arceri <tarceri@itsqueeze.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20256>
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