[PATCH] Adaptive spinning for lockmgr

[Kris Kennaway]

Attilio Rao wrote:
> 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.

OK, I am mis-remembering then.  Ideally it would be tested in several 
representative workloads to see where it helps.  I can't promise whether 
I can do this though, for the same reason as you :(

Kris