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

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Date:	Tue, 23 Nov 2010 18:05:20 +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 6:02 PM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:
>> 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.
>
> invalidate_mapping_pages() are using pagevec_lookup() and pagevec_lookup()
> are using radix tree lookup. Then, Even if rsync always use [0, inf) pair, I don't think
> it makes much slowdown.
>

I mean fdatasync causes slowdown, not fadvise.
if you fadvise(don't need) without fdatasync, you could loss the data.
-- 
Kind regards,
Minchan Kim
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