Messages in this thread | | | Date | Sun, 22 Apr 2007 12:36:48 +1000 | From | Nick Piggin <> | Subject | Re: [PATCH] lazy freeing of memory through MADV_FREE |
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Rik van Riel wrote: > Andrew Morton wrote: > >> On Fri, 20 Apr 2007 17:38:06 -0400 >> Rik van Riel <riel@redhat.com> wrote: >> >>> Andrew Morton wrote: >>> >>>> I've also merged Nick's "mm: madvise avoid exclusive mmap_sem". >>>> >>>> - Nick's patch also will help this problem. It could be that your >>>> patch >>>> no longer offers a 2x speedup when combined with Nick's patch. >>>> >>>> It could well be that the combination of the two is even better, >>>> but it >>>> would be nice to firm that up a bit. >>> >>> I'll test that. >> >> >> Thanks. > > > Well, good news. > > It turns out that Nick's patch does not improve peak > performance much, but it does prevent the decline when > running with 16 threads on my quad core CPU! > > We _definately_ want both patches, there's a huge benefit > in having them both. > > Here are the transactions/seconds for each combination: > > vanilla new glibc madv_free kernel madv_free + mmap_sem > threads > > 1 610 609 596 545 > 2 1032 1136 1196 1200 > 4 1070 1128 2014 2024 > 8 1000 1088 1665 2087 > 16 779 1073 1310 1999
Is "new glibc" meaning MADV_DONTNEED + kernel with mmap_sem patch?
The strange thing with your madv_free kernel is that it doesn't help single-threaded performance at all. So that work to avoid zeroing the new page is not a win at all there (maybe due to the cache effects I was worried about?).
However MADV_FREE does improve scalability, which is interesting. The most likely reason I can see why that may be the case is that it avoids mmap_sem when faulting pages back in (I doubt it is due to avoiding the page allocator, but maybe?).
So where is the down_write coming from in this workload, I wonder? Heap management? What syscalls?
x86_64's rwsems are crap under heavy parallelism (even read-only), as I fixed in my recent generic rwsems patch. I don't expect MySQL to be such a mmap_sem microbenchmark, but I wonder how much this would help?
What if we ran the private futexes patch to further cut down mmap_sem contention?
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