Re: fadvise DONTNEED implementation (or lack thereof)

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2010-11-15 at 15:07 +0900, Minchan Kim wrote:
> On Sun, Nov 14, 2010 at 2:09 PM, KOSAKI Motohiro
> <kosaki.motohiro@jp.fujitsu.com> wrote:
> >> On Tue,  9 Nov 2010 16:28:02 +0900 (JST), KOSAKI Motohiro <kosaki.motohiro@jp.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 :)

Using memcgroup for this is utter crap, it just contains the trainwreck,
it doesn't solve it in any way.

> 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.

Agreed..

> 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?

Read the thread, its quite clear nobody got convinced it was a good idea
and wanted to fix the use-once policy, then Rik rewrote all of
page-reclaim.