Re: [PATCH] hugepage: allow parallelization of the hugepage faultpath

[Hush Bensen]

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@xxxxxx>
> >
> > - 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.
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@xxxxxxxxxx  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/