Suggest changing dirhash defaults for FreeBSD 9.2.

[Robert_Burmeister]

I have seen significant benefits from setting the UFS dirhash cache tuneables
to effective values,
and I believe all FreeBSD users will see at least a small benefit as well.

The ball for establishing production defaults appears to have been dropped
in 2008.
I am suggesting we pick it up and finish the job.
(But not necessarily in time for the impending release.)


> http://www.freebsd.org/projects/summerofcode-2008.html
> Project: Dynamic memory allocation for dirhash in UFS2
> Student: Nick Barkas
> Mentor: David Malone
> Summary:
> 
> Modified dirhash code in perforce is now able to free up memory used by
> older dirhashes when the VM system invokes vm_lowmem events. This will
> allow the default dirhash_maxmem value to be increased, improving
> performance on large directory lookups when there is memory to spare on
> they system. There are versions of the low memory event handling code for
> both -CURRENT and 7-STABLE. A number of tests have been run showing the
> new event handler seems to work properly.
> 
> I intend to do further testing and benchmarking to find the best default
> values to use for vfs.ufs.dirhash_reclaimage (the number of seconds a
> dirhash can sit unused before the dirhash low memeory event handler will
> unconditionally delete it) and the minimum percentage of memory that will
> be freed upon vm_lowmem events even if there are not enough hashes older
> than dirhash_reclaimage (currently this is hard coded to 10%). I would
> also like to add some code to choose a reasonable new default
> vfs.ufs.dirhash_maxmem value based upon the amount of memory in the
> system, set automatically at boot time and tunable via sysctl. Once these
> tweaks have been made I plan to ask for testing from more users to shake
> out any bugs or potential workloads where the new code may hurt overall
> performance.

If someone has more up to date information please post.




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