K 10
svn:author
V 5
markj
K 8
svn:date
V 27
2020-12-06T22:45:50.906146Z
K 7
svn:log
V 1077
uma: Make uma_zone_set_maxcache() work better with small limits

The old implementation chose the largest bucket zone such that if the
per-CPU caches are fully populated, the total number of items cached is
no larger than the specified limit.  If no such zone existed, UMA would
not do any caching.

We can now use uz_bucket_size_max to set a precise limit on the number
of items in a zone's bucket, so the total size of per-CPU caches can be
bounded more easily.  Implement a new policy in uma_zone_set_maxcache():
choose a bucket size such that up to half of the limit can be cached in
per-CPU caches, with the rest going to the full bucket cache.  This
fixes a problem with the kstack_cache zone: the limit of 4 * mp_ncpus
items meant that the zone would not do any caching, defeating the whole
purpose of the zone.  That's because the smallest bucket size holds up
to 2 items and we may cache up to 3 full buckets per CPU, and
2 * 3 * mp_ncpus > 4 * mp_ncpus.

Reported by:	mjg
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D27168

END