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    SubjectRe: [RFC 1/2] deactive invalidated pages
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    On Tue, Nov 23, 2010 at 6:02 PM, KOSAKI Motohiro
    <kosaki.motohiro@jp.fujitsu.com> wrote:
    >> On Tue, Nov 23, 2010 at 5:01 PM, KOSAKI Motohiro
    >> <kosaki.motohiro@jp.fujitsu.com> wrote:
    >> >> Hi KOSAKI,
    >> >>
    >> >> 2010/11/23 KOSAKI Motohiro <kosaki.motohiro@jp.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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