linux-kernel - Re: fadvise DONTNEED implementation (or lack thereof)

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Message-ID: <AANLkTim59Qx6TsvXnTBL5Lg6JorbGaqx3KsdBDWO04X9@mail.gmail.com>
Date:	Mon, 15 Nov 2010 15:07:57 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Ben Gamari <bgamari.foss@...il.com>, linux-kernel@...r.kernel.org,
	rsync@...ts.samba.org, linux-mm@...ck.org,
	Peter Zijlstra <peterz@...radead.org>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: fadvise DONTNEED implementation (or lack thereof)

On Sun, Nov 14, 2010 at 2:09 PM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:
>> On Tue,  9 Nov 2010 16:28:02 +0900 (JST), KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
>> > So, I don't think application developers will use fadvise() aggressively
>> > because we don't have a cross platform agreement of a fadvice behavior.
>> >
>> I strongly disagree. For a long time I have been trying to resolve
>> interactivity issues caused by my rsync-based backup script. Many kernel
>> developers have said that there is nothing the kernel can do without
>> more information from user-space (e.g. cgroups, madvise). While cgroups
>> help, the fix is round-about at best and requires configuration where
>> really none should be necessary. The easiest solution for everyone
>> involved would be for rsync to use FADV_DONTNEED. The behavior doesn't
>> need to be perfectly consistent between platforms for the flag to be
>> useful so long as each implementation does something sane to help
>> use-once access patterns.
>>
>> People seem to mention frequently that there are no users of
>> FADV_DONTNEED and therefore we don't need to implement it. It seems like
>> this is ignoring an obvious catch-22. Currently rsync has no fadvise
>> support at all, since using[1] the implemented hints to get the desired
>> effect is far too complicated^M^M^M^Mhacky to be considered
>> merge-worthy. Considering the number of Google hits returned for
>> fadvise, I wouldn't be surprised if there were countless other projects
>> with this same difficulty. We want to be able to tell the kernel about
>> our useage patterns, but the kernel won't listen.
>
> Because we have an alternative solution already. please try memcgroup :)

I think memcg could be a solution of them but fundamental solution is
that we have to cure it in VM itself.
I feel it's absolutely absurd to enable and use memcg for amending it.

I wonder what's the problem in Peter's patch 'drop behind'.
http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg179576.html

Could anyone tell me why it can't accept upstream?

-- 
Kind regards,
Minchan Kim
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