linux-kernel - Re: [RFC PATCH] asynchronous page fault.

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Message-ID: <7bf10ff685a06b28084a61dad0740015.squirrel@webmail-b.css.fujitsu.com>
Date:	Mon, 28 Dec 2009 19:08:58 +0900 (JST)
From:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
To:	"Peter Zijlstra" <peterz@...radead.org>
Cc:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>,
	cl@...ux-foundation.org
Subject: Re: [RFC PATCH] asynchronous page fault.

Peter Zijlstra wrote:
> On Mon, 2009-12-28 at 09:36 +0900, KAMEZAWA Hiroyuki wrote:
>> Hmm ? for single-thread apps ? This patch's purpose is not for lockless
>> lookup, it's just a part of work. My purpose is avoiding false-sharing.
>
> False sharing in the sense of the mmap_sem cacheline containing other
> variables?  How could that ever be a problem for a single threaded
> application?
>
No problem at all. I just couldn't catch what you mean.


> For multi-threaded apps the contention on that cacheline is the largest
> issue, and moving it to a vma cacheline doesn't seem like a big
> improvement.
>
I feel mmap_sem's cacheline ping-pong is more terrible than
simple atomic_inc().
__down_read() does
  write (spinlock)
  write (->sem_activity)
  write (unlock)


> You want something much finer grained than vmas, there's lots of apps
> working on a single (or very few) vma(s). Leaving you with pretty much
> the same cacheline contention. Only now its a different cacheline.
>
Ya, maybe. I hope I can find some magical one.
Using per-cpu counter here as Christoph did may be an idea...

Thanks,
-Kame



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