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 <[email protected]> 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.

--
SUSE Labs, Novell Inc.
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