Hi.

I've created patch, based on earlier work of avg@, to add back pressure to UMA allocation caches. The problem of physical memory or KVA exhaustion existed there for many years and it is quite critical now for improving systems performance while keeping stability. Changes done in memory allocation last years improved situation. but haven't fixed completely. My patch solves remaining problems from two sides: a) reducing bucket sizes every time system detects low memory condition; and b) as last-resort mechanism for very low memory condition, it cycling over all CPUs to purge their per-CPU UMA caches. Benefit of this approach is in absence of any additional hard-coded limits on cache sizes -- they are self-tuned, based on load and memory pressure.


With this change I believe it should be safe enough to enable UMA allocation caches in ZFS via vfs.zfs.zio.use_uma tunable (at least for amd64). I did many tests on machine with 24 logical cores (and as result strong allocation cache effects), and can say that with 40GB RAM using UMA caches, allowed by this change, by two times increases results of SPEC NFS benchmark on ZFS pool of several SSDs. To test system stability I've run the same test with physical memory limited to just 2GB and system successfully survived that, and even showed results 1.5 times better then with just last resort measures of b). In both cases tools/umastat no longer shows unbound UMA cache growth, that makes me believe in viability of this approach for longer runs.

I would like to hear some comments about that:
http://people.freebsd.org/~mav/uma_pressure.patch

Thank you.

--
Alexander Motin
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