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Date: Mon, 28 Dec 2009 19:08:58 +0900 (JST)
Subject: Re: [RFC PATCH] asynchronous page fault.
From: "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>
To: "Peter Zijlstra" <peterz@infradead.org>
Cc: "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       "linux-mm@kvack.org" <linux-mm@kvack.org>,
       "minchan.kim@gmail.com" <minchan.kim@gmail.com>,
       cl@linux-foundation.org
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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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