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    SubjectRe: trying to understand READ_META, READ_SYNC, WRITE_SYNC & co
    On Fri, Jun 25, 2010 at 01:03:20PM +0200, Christoph Hellwig wrote:
    > On Wed, Jun 23, 2010 at 09:44:20PM -0400, Vivek Goyal wrote:
    > > Let me explain the general idling logic and then see if it makes sense in case
    > > of WRITE_SYNC.
    > >
    > > Once a request has completed, if the cfq queue is empty, we have two choices.
    > > Either expire the cfq queue and move on to dispatch requests from a
    > > different queue or we idle on the queue hoping we will get more IO from
    > > same process/queue.
    >
    > queues are basically processes in this context?
    >
    > > Idling can help (on SATA disks with high seek cost), if
    > > our guess was right and soon we got another request from same process. We
    > > cut down on number of seeks hence increased throghput.
    >
    > I don't really understand the logic behind this. If we lots of I/O
    > that actually is close to each other we should generally submit it in
    > one batch. That is true for pagecache writeback, that is true for
    > metadata (at least in XFS..), and it's true for any sane application
    > doing O_DIRECT / O_SYNC style I/O.
    >
    > What workloads produde I/O that is local (not random) writes with small
    > delays between the I/O requests?

    Biggest thing is multiple small files operations like on the same
    directory. Best case I measured back when doing AS io scheduler
    versus deadline was about 100x improvement on a uncached kernel
    grep workload when competing with a streaming writeout (the writeout
    probably ended up going somewhat slower naturally, but it is fairer).



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