Messages in this thread |  | | | Date | Sun, 22 Apr 2007 12:50:34 +1000 | | From | Nick Piggin <> | | Subject | Re: [PATCH] lazy freeing of memory through MADV_FREE |
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Nick Piggin wrote: > 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?
Hmm, without the MADV_FREE patch, I wonder if it isn't doing something silly like read-faulting in a ZERO_PAGE then write faulting a new page straight afterwards.. I'll have to try a few tests.
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