Re: [PATCH] lazy freeing of memory through MADV_FREE

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

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.

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