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