[PATCH] Adaptive spinning for lockmgr

[Attilio Rao]

2009/6/14 Kris Kennaway <kris at freebsd.org>:
> Attilio Rao wrote:
>>
>> This patch enables adaptive spinning for lockmgr:
>> http://www.freebsd.org/~attilio/adaptive_lockmgr.diff
>>
>> and it should presumably improve performance on disks/vfs/buffer cache
>> based benchmarks, so, if you want to try out and report any benchmarks
>> result, I'd love to see it.
>> Please note that there are some parameters to tune: for example, you
>> would like to not enable adaptive spinning to default while you just
>> want that for a class of locks (and in that case you want to apply the
>> reversed logic for what is living now) or you want to use different
>> values  for retries and loops. Interested developers can refer to such
>> 3 variables.
>> Peter Holm alredy tested that patch for about 24hours without any
>> regression to report.
>>
>> Also note that the patch is not 100% yet as long as it needs UPDATES
>> and manpages updates, but they will be added just in time before to
>> commit.
>> The modify is all there.
>
> I have a vague memory that we had tested a version of this in the past and
> found that it caused a performance loss in common cases?  Many lockmgr
> callers are not amenable to adaptive spinning because they have to wait on
> slow I/O.  Testing only with e.g. md backing might give results that are
> non-representative.

I don't think I ever implemented adaptive spinning in lockmgr so if
somebody else did I don't know. Said that, probabilly the best
approach would be to disable it by default ad use a LK_ADAPTIVESPIN
flag on a per instance basis.
Such conditions, though, need to be explored a bit and I have no time
to dedicate to this right now.

Thanks,
Attilio


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