Re: [PATCH 00/13] mm: preemptibility -v2

[Peter Zijlstra]

On Fri, 2010-04-09 at 14:14 +1000, Nick Piggin wrote:
> On Thu, Apr 08, 2010 at 09:17:37PM +0200, Peter Zijlstra wrote:
> > Hi,
> > 
> > This (still incomplete) patch-set makes part of the mm a lot more preemptible.
> > It converts i_mmap_lock and anon_vma->lock to mutexes.  On the way there it
> > also makes mmu_gather preemptible.
> > 
> > The main motivation was making mm_take_all_locks() preemptible, since it
> > appears people are nesting hundreds of spinlocks there.
> > 
> > The side-effects are that we can finally make mmu_gather preemptible, something
> > which lots of people have wanted to do for a long time.
> 
> What's the straight-line performance impact of all this? And how about
> concurrency, I wonder. mutexes of course are double the atomics, and
> you've added a refcount which is two more again for those paths using
> it.
> 
> Page faults are very important. We unfortunately have some databases
> doing a significant amount of mmap/munmap activity too. 

You think this would affect the mmap/munmap times in any significant
way? It seems to me those are relatively heavy ops to begin with.

> I'd like to
> see microbenchmark numbers for each of those (both anon and file backed
> for page faults).

OK, I'll dig out that fault test used in the whole mmap_sem/rwsem thread
a while back and modify it to also do file backed faults.

> kbuild does quite a few pages faults, that would be an easy thing to
> test. Not sure what reasonable kinds of cases exercise parallelism.
> 
> 
> > What kind of performance tests would people have me run on this to satisfy
> > their need for numbers? I've done a kernel build on x86_64 and if anything that
> > was slightly faster with these patches, but it was well within the noise
> > levels so it might be heat noise I'm looking at ;-)
> 
> Is it because you're reducing the number of TLB flushes, or what
> (kbuild isn't multi threaded so on x86 TLB flushes should be really
> fast anyway).

I'll try and get some perf stat runs to get some insight into this. But
the numbers were:

 time make O=defconfig -j48 bzImage (5x, cache hot)

without:  avg: 39.2018s +- 0.3407
with:     avg: 38.9886s +- 0.1814



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