Re: [PATCH] lazy freeing of memory through MADV_FREE

[Nick Piggin]

Rik van Riel wrote:
> Andrew Morton wrote:
>
> > On Fri, 20 Apr 2007 17:38:06 -0400
> > Rik van Riel <riel@xxxxxxxxxx> 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?

--
SUSE Labs, Novell Inc.
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