Re: [PATCH] lazy freeing of memory through MADV_FREE

Nick Piggin
2007-04-22T02:36:48Z

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?

--
SUSE Labs, Novell Inc.