UMA cache back pressure

Hi.

I've created patch, based on earlier work of avg_at_, 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