Re: [PATCH] lazy freeing of memory through MADV_FREE

[Nick Piggin <nickpiggin@yahoo.com.au>]

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.

-- 
SUSE Labs, Novell Inc.