Re: [PATCH] hugepage: allow parallelization of the hugepage fault path

[Hush Bensen <hush.bensen@gmail.com>]

On 07/18/2013 05:07 PM, Joonsoo Kim wrote:
> On Wed, Jul 17, 2013 at 12:50:25PM -0700, Davidlohr Bueso wrote:
>
>> From: Davidlohr Bueso <davidlohr.bueso@hp.com>
>>
>> - Cleaned up and forward ported to Linus' latest.
>> - Cache aligned mutexes.
>> - Keep non SMP systems using a single mutex.
>>
>> It was found that this mutex can become quite contended
>> during the early phases of large databases which make use of huge pages - for instance
>> startup and initial runs. One clear example is a 1.5Gb Oracle database, where lockstat
>> reports that this mutex can be one of the top 5 most contended locks in the kernel during
>> the first few minutes:
>>
>>      	     hugetlb_instantiation_mutex:   10678     10678
>>               ---------------------------
>>               hugetlb_instantiation_mutex    10678  [<ffffffff8115e14e>] hugetlb_fault+0x9e/0x340
>>               ---------------------------
>>               hugetlb_instantiation_mutex    10678  [<ffffffff8115e14e>] hugetlb_fault+0x9e/0x340
>>
>> contentions:          10678
>> acquisitions:         99476
>> waittime-total: 76888911.01 us
> Hello,
> I have a question :)
>
> So, each contention takes 7.6 ms in your result.
> Do you map this area with VM_NORESERVE?
> If we map with VM_RESERVE, when page fault, we just dequeue a huge page from a queue and clear
> a page and then map it to a page table. So I guess, it shouldn't take so long.

I don't think there is clear page operation after dequeue huge page, 
actually it's even not done during hugetlb_reserve_pages, do you know 
why? There is just clear operation in hugetlb_no_page.

> I'm wondering why it takes so long.
>
> And do you use 16KB-size hugepage?
> If so, region handling could takes some times. If you access the area as random order,
> the number of region can be more than 90000. I guess, this can be one reason to too long
> waittime.
>
> Thanks.
>
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