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

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Date:	Thu, 18 Jul 2013 18:07:19 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Davidlohr Bueso <davidlohr.bueso@...com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Gibson <david@...son.dropbear.id.au>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Michal Hocko <mhocko@...e.cz>,
	"AneeshKumarK.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hillf Danton <dhillf@...il.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Eric B Munson <emunson@...bm.net>,
	Anton Blanchard <anton@...ba.org>
Subject: Re: [PATCH] hugepage: allow parallelization of the hugepage fault
 path

On Wed, Jul 17, 2013 at 12:50:25PM -0700, Davidlohr Bueso wrote:

> From: Davidlohr Bueso <davidlohr.bueso@...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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