4.8 ffs_dirpref problem

Don Lewis truckman at FreeBSD.org
Thu Oct 30 19:00:47 PST 2003

On 30 Oct, Ken Marx wrote:
> Don Lewis wrote:
>> On 30 Oct, Ken Marx wrote:

>>>Happy to test further if you have tweaks to your patch or
>>>things you'd like us to test in particular. E.g., load,
>>>newfs, etc.
>> You might want to try your hash patch along my patch to see if
>> decreasing the maximum hash chain lengths makes a difference in system
>> time.
> Sorry - should hvae mentioned: Both tests included our hash patch.
> I just re-ran with the hash stuff back to original. Sorry to say
> there's no appreciable difference. Using your dirpref patch, still
> apprx 75 sys sec/1.5gb:
> 	tot:      1185.54 real        14.15 user       747.43 sys
> Prehaps the dirpref patch lowers the frequency of having to search so much,
> and hence exercises the hashtable less.

I'm pretty sure that is the situation, but I wasn't sure if there was
still enough hash table usage to make balancing it worthwhile.  It
sounds like there isn't.

It looks like there is still about a 10% increase in system time for my
modifications to dirpref versus reverting to the old algorithm.  It
would be nice to know where the extra time was being spent, but that
would require probably require some kernel profiling.

It might also be interesting to play with the cylinder group selection
criteria.  Should minbfree be 75% of avgbfree, 100% of avgbfree, or 125%
of avgbfree?  Probably not anything over 100%, since the algorithm would
fail if all the cylinder groups were evenly filled ...

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