Suggest changing dirhash defaults for FreeBSD 9.2.

[Robert Burmeister]

On 8/27/2013 9:40 AM, Sergey Kandaurov wrote:
> On 27 August 2013 16:41, Robert Burmeister
> <Robert.Burmeister at utoledo.edu>  wrote:
>> I have been experimenting with dirhash settings, and have scoured the internet for other peoples' experience with it.
>> (I found the performance improvement in compiling has forestalled the need to add an SSD drive. ;-)
>>
>> 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.
>
Sorry, the documentation has not been updated since 2008.
https://wiki.freebsd.org/DirhashDynamicMemory

What about bumping up vfs.ufs.dirhash_reclaimage
> all hashes older than 5 seconds (tunable via a sysctl, 
> vfs.ufs.dirhash_reclaimage) will always be deleted when a vm_lowmem event 
> occurs, and if that doesn't free up at least 10% of the memory currently being 
> used by dirhash, more hashes will be destroyed from the head of the TAILQ 
> until we do free up at least 10% of the memory initially used.
As the memory scavenger will keep nibbling until at least 10% of the cache is free,
eradicating all entries older than 5 seconds for the first chop seems overly 
aggressive.