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    SubjectRe: Delaying writes to disk when there's no need
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    On Mon, 31 Mar 2003, Andrew Morton wrote:
    > Daniel Pittman <daniel@rimspace.net> wrote:
    >>
    >> Capturing a real-time video stream from an IEEE1394 DV stream means
    >> writing a stead 3.5MB per second for two on two and a half hours.
    >>
    >> Linux isn't great at this, using the default writeout policy, even as
    >> recent as 2.5.64. The writer goes OK for a while but, eventually,
    >> blocks on writeout for long enough to drop a frame -- more than
    >> 8/25ths of a second.
    >>
    >>
    >> This can be resolved by tuning the default delay before write-out
    >> start to 5 seconds, down from 30, or by running sync every second, or
    >> by doing fsync tricks.
    >
    > Interesting.
    >
    > Yes, I expect that you could fix that up by altering
    > dirty_background_ratio and dirty_expire_centisecs.

    Those are, in fact, the precise knobs I turned. Well, those and the XFS
    pagebuf layer equivalents.

    > The problem with fsync() is that it waits on the writeout. You don't
    > want that to happen - you just want to tell the kernel "I won't be
    > overwriting or deleting this data". Make the kernel queue up and start
    > the IO but not wait on its completion.

    Yes, that would be good, because then I wouldn't need to write an IPC
    thing and fork or thread, so that the second thread can be busy blocking
    on the writeout for me.

    > It is quite appropriate to do this in fadvise(FADV_DONTNEED) - as a
    > lower-latency fsync(). The app would need to call it once per second
    > or so.
    >
    > It would also throw away any written-back pagecache inside your
    > (start, len) which is exactly what your applications wants to happen,
    > so the app should be calling fadvise _anyway_.
    >
    > What do you think?

    I will apply the patch and test later today. This, however, looks like
    a *really* good thing to me.

    Daniel

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