Re: [PATCH 0/2] rwsem: performance enhancements for systems withmany cores

From: Davidlohr Bueso
Date: Fri Jun 21 2013 - 20:43:40 EST

On Fri, 2013-06-21 at 17:25 -0700, Michel Lespinasse wrote:
> On Fri, Jun 21, 2013 at 5:00 PM, Davidlohr Bueso <davidlohr.bueso@xxxxxx> wrote:
> > On Fri, 2013-06-21 at 16:51 -0700, Tim Chen wrote:
> >> In this patchset, we introduce two optimizations to read write semaphore.
> >> The first one reduces cache bouncing of the sem->count field
> >> by doing a pre-read of the sem->count and avoid cmpxchg if possible.
> >> The second patch introduces similar optimistic spining logic in
> >> the mutex code for the writer lock acquisition of rw-sem.
> >>
> >> Combining the two patches, in testing by Davidlohr Bueso on aim7 workloads
> >> on 8 socket 80 cores system, he saw improvements of
> >> alltests (+14.5%), custom (+17%), disk (+11%), high_systime
> >> (+5%), shared (+15%) and short (+4%), most of them after around 500
> >> users when i_mmap was implemented as rwsem.
> >>
> >> Feedbacks on the effectiveness of these tweaks on other workloads
> >> will be appreciated.
> >
> > Tim, I was really hoping to send all this in one big bundle. I was doing
> > some further testing (enabling hyperthreading and some Oracle runs),
> > fortunately everything looks ok and we are getting actual improvements
> > on large boxes.
> >
> > That said, how about I send you my i_mmap rwsem patchset for a v2 of
> > this patchset?
> I'm a bit confused about the state of these patchsets - it looks like
> I'm only copied into half of the conversations. Should I wait for a v2
> here, or should I hunt down for Alex's version of things, or... ?

Except for some internal patch logistics, you haven't been left out on
any conversations :)

My original plan was to send out, in one patchset:

- rwsem optimizations from Alex (patch 1/2 here, which should be
actually 4 patches) +
- rwsem optimistic spinning (patch 2/2 here) +
- i_mmap_mutex to rwsem conversion (5 more patches)

Now, I realize that the i_mmap stuff might not be welcomed in a
rwsem-specific optimizations patchset like this one, but I think it's
relevant to include everything in a single bundle as it really shows the
performance boosts and it's what I have been using and measuring the
original negative rwsem performance when compared to a mutex.

If folks don't agree, I can always send it as a separate patchset.


the rwsem spin on owner functionality (2/2) + 4 from Alex )which is
really patch 1/2 here + I haven't sent out any

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