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

[Davidlohr Bueso]

On Thu, 2013-07-18 at 18:07 +0900, 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.

Well, that's the total wait time. I can see your concern, but no, things
aren't *that* bad. The average amount of time spent waiting for the lock
would be 76888911.01/10678 = 7200us

> 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.
> 

I cannot really say. This is proprietary software. AFAICT if Oracle is
anything like Posgres, than probably no.


> And do you use 16KB-size hugepage?

No, 2Mb pages.

> 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.

Thanks,
Davidlohr


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