Suggest changing dirhash defaults for FreeBSD 9.2.

[Robert_Burmeister]

>>>> I believe that increasing the following values by 10 would benefit
>>>> most FreeBSD users without disadvantage.
>>>>
>>>> vfs.ufs.dirhash_maxmem: 2097152 to 20971520
>>>>
>>>> vfs.ufs.dirhash_reclaimage: 5 to 50 or 60
>>> vfs.ufs.dirhash_maxmem is further autotuned based on available
>>> physical memory.
>>> See r214359 for details.
>>>
> 
> To what value does the algorithm tune in your case? On my 16 GB machine,
> it's ~~ 25 MB:
> 
> vfs.ufs.dirhash_maxmem: 26968064
> 
> The policy is to use fractions of the installed RAM (though in a
> roundabout way), so it should scale reasonably well to both systems with
> large and small memories.
> 
> I'll bump vfs.ufs.dirhash_reclaimage to 60, it's worth it.

I'm running 2 Gigs of RAM, so it sets to 2 Megs.
As compiling most of the system uses ~1.5 Gigs, I can certainly use the
cache.

Note that allocated kernel memory does not scale linearly with physical RAM,
so a policy to use fractions of kernel memory may be more appropriate.

~25 Megs is a good value, however, as the reclaim value is a portion of 
vfs.ufs.dirhash_maxmem, a much larger cache and it becomes pyric
as it can keep wiping out a small active cache.

(I tested vfs.ufs.dirhash_maxmem up to 60 megs.)




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