Re: [PATCH] lazy freeing of memory through MADV_FREE

Nick Piggin
2007-04-23T00:17:08Z

Rik van Riel wrote:
> Nick Piggin wrote:
>
>> Rik van Riel wrote:

>>> 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?
>
>
> No, that's just the glibc change, with a vanilla kernel.

OK. That would be interesting to see with the mmap_sem change,
because that should increase scalability.

> The third column is glibc change + mmap_sem patch.
>
> The fourth column has your patch in it, too.
>
>> 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?).
>
>
> Well, your patch causes the performance to drop from
> 596 transactions/second to 545. Your patch is the only
> difference between the third and the fourth column.

Yeah. That's funny, because it means either there is some
contention on the mmap_sem (or ptl) at 1 thread, or that my
patch alters the uncontended performance.

>> 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?
>
>
> I wonder if the increased parallelism simply caused
> more cache line bouncing, with bounces happening in
> some inner loop instead of an outer loop.
>
> Btw, it is quite possible that the MySQL sysbench
> thing gives different results on your system. It
> would be good to know what it does on a real SMP
> system, vs. a single quad-core chip :)
>
> Other architectures would be interesting to know,
> too.

I don't see why parallelism should come into it at 1 thread, unless
MySQL is parallelising individual transactions. Anyway, I'll try to do
some more digging.

--
SUSE Labs, Novell Inc.