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    SubjectRe: [PATCH v6 0/4] ext4: Coordinate data-only flush requests sent by fsync
    On Tue, Nov 30, 2010 at 12:26:37PM +1100, Neil Brown wrote:
    > On Mon, 29 Nov 2010 19:48:52 -0500 Ric Wheeler <rwheeler@redhat.com> wrote:
    >
    > > On 11/29/2010 07:39 PM, Neil Brown wrote:
    > > > On Mon, 29 Nov 2010 14:05:36 -0800 "Darrick J. Wong"<djwong@us.ibm.com>
    > > > wrote:
    > > >
    > > >> On certain types of hardware, issuing a write cache flush takes a considerable
    > > >> amount of time. Typically, these are simple storage systems with write cache
    > > >> enabled and no battery to save that cache after a power failure. When we
    > > >> encounter a system with many I/O threads that write data and then call fsync
    > > >> after more transactions accumulate, ext4_sync_file performs a data-only flush,
    > > >> the performance of which is suboptimal because each of those threads issues its
    > > >> own flush command to the drive instead of trying to coordinate the flush,
    > > >> thereby wasting execution time.
    > > >>
    > > >> Instead of each fsync call initiating its own flush, there's now a flag to
    > > >> indicate if (0) no flushes are ongoing, (1) we're delaying a short time to
    > > >> collect other fsync threads, or (2) we're actually in-progress on a flush.
    > > >>
    > > >> So, if someone calls ext4_sync_file and no flushes are in progress, the flag
    > > >> shifts from 0->1 and the thread delays for a short time to see if there are any
    > > >> other threads that are close behind in ext4_sync_file. After that wait, the
    > > >> state transitions to 2 and the flush is issued. Once that's done, the state
    > > >> goes back to 0 and a completion is signalled.
    > > > I haven't seen any of the preceding discussion do I might be missing
    > > > something important, but this seems needlessly complex and intrusive.
    > > > In particular, I don't like adding code to md to propagate these timings up
    > > > to the fs, and I don't the arbitrary '2ms' number.
    > > >
    > > > Would it not be sufficient to simply gather flushes while a flush is pending.
    > > > i.e
    > > > - if no flush is pending, set the 'flush pending' flag, submit a flush,
    > > > then clear the flag.
    > > > - if a flush is pending, then wait for it to complete, and then submit a
    > > > single flush on behalf of all pending flushes.
    > > >
    > > > That way when flush is fast, you do a flush every time, and when it is slow
    > > > you gather multiple flushes together.
    > > > I think it would issues a few more flushes than your scheme, but it would be
    > > > a much neater solution. Have you tried that and found it to be insufficient?
    > > >
    > > > Thanks,
    > > > NeilBrown
    > > >
    > >
    > > The problem with that is that you can introduce a wait for the next flush longer
    > > than it would take to complete the flush. Having the wait adjust itself
    > > according to the speed of the device is much better I think....
    > >
    >
    > Hi Ric,
    >
    > You certainly can introduce a wait longer than the flush would take, but
    > the proposed code does that too.
    >
    > The proposed code waits "average flush time", then submits a flush for
    > all threads that are waiting.
    > My suggestion submits a flush (Which on average takes 'average flush time')
    > and then submits a flush for all threads that started waiting during that
    > time.
    >
    > So we do get two flushes rather than one, but also the flush starts
    > straight away, so will presumably finish sooner.
    >
    > I do have the wait 'adjust itself according to the speed of the device' by
    > making flushes wait for one flush to complete.
    >
    >
    > I'm guessing that the situation where this is an issue is when you have a
    > nearly constant stream of flush requests - is that right?

    Yes.

    --D

    > In that case:
    > - the current code would submit lots of over-lapping flush requests,
    > reducing the opportunity for optimising multiple requests in the
    > device,
    > - my proposal would submit a steady sequence of flushes with minimal
    > gaps
    > - the code from Darrick would submit a steady sequence of flush requests
    > which would be separated by pauses of 'average flush time'.
    > This would increase latency, but might increase throughput too, I
    > don't know.
    >
    > So it seems to me that the performance differences between my suggesting
    > and Darrick's proposal are not obvious. So some testing would be
    > interesting.
    >
    > I was going to suggest changes to Darrick's code to demonstrate my idea, but
    > I think that code is actually wrong, so it wouldn't be a good base to start.
    >
    > In particular, the usage of a continuation seems racy.
    > As soon as one thread sets EXT4_FLUSH_IDLE, another thread could call
    > INIT_COMPLETION, before some other threads has had a chance to wake up and
    > test the completion status in wait_for_completion.
    > The effect is that the other thread would wait for an extra time+flush which
    > it shouldn't have to. So it isn't a serious race, but it looks wrong.
    >
    > NeilBrown
    >


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