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

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Message-Id: <20101114140920.E013.A69D9226@jp.fujitsu.com>
Date:	Sun, 14 Nov 2010 14:09:29 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Ben Gamari <bgamari.foss@...il.com>
Cc:	kosaki.motohiro@...fujitsu.com, linux-kernel@...r.kernel.org,
	rsync@...ts.samba.org, linux-mm@...ck.org
Subject: Re: fadvise DONTNEED implementation (or lack thereof)

> 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 :)



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