linux-kernel - Re: [RFC 1/2] deactive invalidated pages

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Date:	Tue, 23 Nov 2010 17:44:57 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...nel.dk>
Subject: Re: [RFC 1/2] deactive invalidated pages

On Tue, Nov 23, 2010 at 5:01 PM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:
>> Hi KOSAKI,
>>
>> 2010/11/23 KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>:
>> >> By Other approach, app developer uses POSIX_FADV_DONTNEED.
>> >> But it has a problem. If kernel meets page is writing
>> >> during invalidate_mapping_pages, it can't work.
>> >> It is very hard for application programmer to use it.
>> >> Because they always have to sync data before calling
>> >> fadivse(..POSIX_FADV_DONTNEED) to make sure the pages could
>> >> be discardable. At last, they can't use deferred write of kernel
>> >> so that they could see performance loss.
>> >> (http://insights.oetiker.ch/linux/fadvise.html)
>> >
>> > If rsync use the above url patch, we don't need your patch.
>> > fdatasync() + POSIX_FADV_DONTNEED should work fine.
>>
>> It works well. But it needs always fdatasync before calling fadvise.
>> For small file, it hurt performance since we can't use the deferred write.
>
> I doubt rsync need to call fdatasync. Why?
>
> If rsync continue to do following loop, some POSIX_FADV_DONTNEED
> may not drop some dirty pages. But they can be dropped at next loop's
> POSIX_FADV_DONTNEED. Then, It doesn't make serious issue.
>
> 1) read
> 2) write
> 3) POSIX_FADV_DONTNEED
> 4) goto 1

fadvise need pair (offset and len).
if the pair in next turn is different with one's previous turn, it
couldn't be dropped.

-- 
Kind regards,
Minchan Kim
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