[benchmark] THP performance testcase

[Ingo Molnar <mingo@kernel.org>]

( Changed the subject line to make it stand out better on lkml. Mail with 
  link & results quoted below. )

* Alex Thorlton <athorlton@sgi.com> wrote:

> [...] Here's a pointer to the test I wrote:
> 
> ftp://shell.sgi.com/collect/appsx_test/pthread_test.tar.gz
> 
> Everything to compile the test should be there (just run make in the 
> thp_pthread directory).  To run the test use something like:
> 
> time ./thp_pthread -C 0 -m 0 -c <max_cores> -b <memory>
> 
> I ran:
> 
> time ./thp_pthread -C 0 -m 0 -c 128 -b 128g
> 
> On a 256 core machine, with ~500gb of memory and got these results:
> 
> THP off:
> 
> real	0m57.797s
> user	46m22.156s
> sys	6m14.220s
> 
> THP on:
> 
> real	1m36.906s
> user	0m2.612s
> sys	143m13.764s
> 
> I snagged some code from another test we use, so I can't vouch for the
> usefulness/accuracy of all the output (actually, I know some of it is
> wrong).  I've mainly been looking at the total run time.
> 
> Don't want to bloat this e-mail up with too many test results, but I
> found this one really interesting.  Same machine, using all the cores,
> with the same amount of memory.  This means that each cpu is actually
> doing *less* work, since the chunk we reserve gets divided up evenly
> amongst the cpus:
> 
> time ./thp_pthread -C 0 -m 0 -c 256 -b 128g
> 
> THP off:
> 
> real	1m1.028s
> user	104m58.448s
> sys	8m52.908s
> 
> THP on:
> 
> real	2m26.072s
> user	60m39.404s
> sys	337m10.072s
> 
> Seems that the test scales really well in the THP off case, but, once
> again, with THP on, we really see the performance start to degrade.
> 
> I'm planning to start investigating possible ways to split up THPs, if
> we detect that that majority of the references to a THP are off-node.
> I've heard some horror stories about migrating pages in this situation
> (i.e., process switches cpu and then all the pages follow it), but I
> think we might be able to get some better results if we can cleverly
> determine an appropriate time to split up pages.  I've heard a bit of
> talk about doing something similar to this from a few people, but
> haven't seen any code/test results.  If anybody has any input on that
> topic, it would be greatly appreciated.