linux-kernel - Re: [PATCH v2 6/6] mm, hugetlb: improve page-fault scalability

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Date:	Mon, 3 Feb 2014 15:41:38 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	akpm@...ux-foundation.org, riel@...hat.com, mgorman@...e.de,
	mhocko@...e.cz, aneesh.kumar@...ux.vnet.ibm.com,
	kamezawa.hiroyu@...fujitsu.com, hughd@...gle.com,
	david@...son.dropbear.id.au, liwanp@...ux.vnet.ibm.com,
	n-horiguchi@...jp.nec.com, dhillf@...il.com, rientjes@...gle.com,
	aswin@...com, scott.norton@...com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/6] mm, hugetlb: improve page-fault scalability

On Fri, Jan 31, 2014 at 09:36:46AM -0800, Davidlohr Bueso wrote:
> From: Davidlohr Bueso <davidlohr@...com>
> 
> The kernel can currently only handle a single hugetlb page fault at a time.
> This is due to a single mutex that serializes the entire path. This lock
> protects from spurious OOM errors under conditions of low of low availability
> of free hugepages. This problem is specific to hugepages, because it is
> normal to want to use every single hugepage in the system - with normal pages
> we simply assume there will always be a few spare pages which can be used
> temporarily until the race is resolved.
> 
> Address this problem by using a table of mutexes, allowing a better chance of
> parallelization, where each hugepage is individually serialized. The hash key
> is selected depending on the mapping type. For shared ones it consists of the
> address space and file offset being faulted; while for private ones the mm and
> virtual address are used. The size of the table is selected based on a compromise
> of collisions and memory footprint of a series of database workloads.

Hello,

Thanks for doing this patchset. :)
Just one question!
Why do we need a separate hash key depending on the mapping type?

Thanks.
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