Re: [RFC 1/2] deactive invalidated pages

[KOSAKI Motohiro]

> On Tue, Nov 23, 2010 at 5:01 PM, KOSAKI Motohiro
> <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
> >> Hi KOSAKI,
> >>
> >> 2010/11/23 KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>:
> >> >> 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.



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