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    SubjectRe: [PATCH] O_STREAMING - flag for optimal streaming I/O
    > Robert Love wrote:
    > >
    > > ...
    > >
    > > Andrew, any experience on one vs. the other?
    > I'd say that if you were designing a new application which
    > streams large amount of data then yes, you would design it
    > to use O_DIRECT. You would instantiate a separate IO worker
    > thread and a message passing mechanism so that thread would
    > pump your data for you, and would peform your readahead, etc.
    > If your filesystem supports O_DIRECT, of course. Not all do.
    > The strength of O_STREAMING is that you can take an existing,
    > working, megahuge application and make it play better with the
    > VM by changing a single line of code. No big redesign needed.

    I think it will be a really big win on quite low speed machines streaming
    at eg set top box speeds, without much memory (eg your Tivo).
    O_DIRECT needs at least 4M reads to overcome lack of readahead,
    so is only really useful if you are doing really big stuff (eg
    uncompressed video). It is possible that O_STREAMING will be just as
    good as this however, becasue there are latency issues with O_DIRECT
    (you really need aio or multiple threads to sustain good readahead,
    one test of mine needed 8 threads for optimal performance). I have
    a bunch of things I could retest with O_STREAMING and large readahead.

    O_DIRECT is always going to win for random access stuff though. It is
    good to have a choice.

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