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    SubjectRe: fadvise DONTNEED implementation (or lack thereof)
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    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.




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