linux-kernel - Re: [RFC] mmaped copy too slow?

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Date:	Tue, 15 Jan 2008 12:20:10 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	kosaki.motohiro@...fujitsu.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC] mmaped copy too slow?

Hi Rik

> While being able to deal with used-once mappings in page reclaim
> could be a good idea, this would require us to be able to determine
> the difference between a page that was accessed once since it was
> faulted in and a page that got accessed several times.

it makes sense that read ahead hit assume used-once mapping, may be.
I will try it.

(may be, i can repost soon)

> Given that page faults have overhead too, it does not surprise me
> that read+write is faster than mmap+memcpy.
> 
> In threaded applications, page fault overhead will be worse still,
> since the TLBs need to be synchronized between CPUs (at least at
> reclaim time).

sure.
but current is unnecessary large performance difference.
I hope improvement it because copy by mmapd is very common operation.

> Maybe we should just advise people to use read+write, since it is
> faster than mmap+memcpy?

Time is solved to it :)
thanks!


- kosaki



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