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Date: Tue, 04 Feb 2014 07:25:15 -0800
From: Dave Hansen <dave.hansen@intel.com>
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Subject: Re: [PATCH v10 00/16] Volatile Ranges v10
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To: Minchan Kim <minchan@kernel.org>, Jason Evans <je@fb.com>
Cc: John Stultz <john.stultz@linaro.org>, Johannes Weiner <hannes@cmpxchg.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mgorman@suse.de>, Hugh Dickins <hughd@google.com>, Rik van Riel <riel@redhat.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Michel Lespinasse <walken@google.com>, Dhaval Giani <dhaval.giani@gmail.com>, "H. Peter Anvin" <hpa@zytor.com>, Android Kernel Team <kernel-team@android.com>, Robert Love <rlove@google.com>, Mel Gorman <mel@csn.ul.ie>, Dmitry Adamushko <dmitry.adamushko@gmail.com>, Dave Chinner <david@fromorbit.com>, Neil Brown <neilb@suse.de>, Andrea Righi <andrea@betterlinux.com>, Andrea Arcangeli <aarcange@redhat.com>, "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>, Mike Hommey <mh@glandium.org>, Taras Glek <tglek@mozilla.com>, Jan Kara <jack@suse.cz>, KOSAKI Motohiro <kosaki.motohiro@gmail.com>, Rob Clark <robdclark@gmail.com>, "pliard@google.com" <pliard@google.com>

On 02/03/2014 08:58 PM, Minchan Kim wrote:
> Of course, every thread could do madvise(MADV_FREE) in parallel because
> VM in Linux doesn't need write-side semaphore but read-side semaphore.
> Additionally, page faulting also needs read-side semaphore so
> page faulting, madvise(MADV_FREE) in threads could be done in parallel
> without any scalability issue if they don't overlap same virtual addresses
> within 4M range because they need a page table lock but it's very
> unlikely in allocator, IMO.

In practice, things holding mmap_sem for read don't scale well, either,
especially when their hold times are short.  It's _better_ than if they
took it for write, but still doesn't scale well.  Check out the red
(threads) line in "Anonymous memory page fault" for instance:

> https://www.sr71.net/~dave/intel/willitscale/systems/bigbox/3.13.0-slub-08988-gd891ea2-dirty/foo.html


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