Re: [RFC PATCH RT] rwsem: The return of multi-reader PI rwsems

[Clark Williams]

On Wed, 9 Apr 2014 15:19:22 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

 
> This patch is built on top of the two other patches that I posted
> earlier, which should not be as controversial.
> 
> If you have any benchmark on large machines I would be very happy if
> you could test this patch against the unpatched version of -rt.
> 
> Cheers,
> 
> -- Steve
>

Steven

I wrote a program named whack_mmap_sem which creates a large (4GB)
buffer, then creates 2 x ncpus threads that are affined across all the
available cpus. These threads then randomly write into the buffer,
which should cause page faults galore.

I then built the following kernel configs:

  vanilla-3.13.15  - no RT patches applied
  rt-3.12.15       - PREEMPT_RT patchset
  rt-3.12.15-fixes - PREEMPT_RT + rwsem fixes
  rt-3.12.15-multi - PREEMPT_RT + rwsem fixes + rwsem-multi patch

My test h/w was a Dell R520 with a 6-core Intel(R) Xeon(R) CPU E5-2430
0 @ 2.20GHz (hyperthreaded). So whack_mmap_sem created 24 threads
which all partied in the 4GB address range.

I ran whack_mmap_sem with the argument -w 100000 which means each
thread does 100k writes to random locations inside the buffer and then
did five runs per each kernel. At the end of the run whack_mmap_sem
prints out the time of the run in microseconds.

The means of each group of five test runs are:

  vanilla.log:  1210117
       rt.log:  17210953 (14.2 x slower than vanilla)
 rt-fixes.log:  10062027 (8.3 x slower than vanilla)
 rt-multi.log:  3179582  (2.x x slower than vanilla)


As expected, vanilla kicked RT's butt when hammering on the
mmap_sem. But somewhat unexpectedly, your fixups helped quite a bit
and the multi+fixups got RT back into being almost respectable.

Obviously these are just preliminary results on one piece of h/w but
it looks promising.

Clark

P.S. If you want the source to whack_mmap_sem you can grab it from 
http://people.redhat.com/williams/whack_mmap_sem. If it looks like its
worth keeping around I can add it to rt-tests.

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