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

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

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'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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