Re: [PATCH] lazy freeing of memory through MADV_FREE

[Rik van Riel <riel@redhat.com>]

Eric Dumazet wrote:
> Rik van Riel a A(C)crit :
>> 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
> 
> 545 tps versus 610 tps for one thread ? It seems quite bad, no ?
> 
> Could you please find an explanation for this ?

I have no idea why this happens.  Especially the last one,
going from a write lock to a read lock on the mmap_sem
should not make ANY difference whatsoever since we're
running single threaded!

>> 2    1032        1136            1196               1200
>> 4    1070        1128            2014               2024
>> 8    1000        1088            1665               2087
>> 16    779        1073            1310               1999

Performance with 2 database threads is way better though,
and performance with 4 or more threads more than doubles...

If you have an explanation on why single threaded performance
went down a little on my quad core system, please let me know.

Does performance suffer at all on a real UP system?

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