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    SubjectRe: Unexpected splice "always copy" behavior observed


    On Wed, 19 May 2010, Mathieu Desnoyers wrote:
    >
    > Good point. This discard flag might do the trick and let us keep things simple.
    > The major concern here is to keep the page cache disturbance relatively low.
    > Which of new page allocation or stealing back the page has the lowest overhead
    > would have to be determined with benchmarks.

    We could probably make it easier somehow to do the writeback and discard
    thing, but I have had _very_ good experiences with even a rather trivial
    file writer that basically used (iirc) 8MB windows, and the logic was very
    trivial:

    - before writing a new 8M window, do "start writeback"
    (SYNC_FILE_RANGE_WRITE) on the previous window, and do
    a wait (SYNC_FILE_RANGE_WAIT_AFTER) on the window before that.

    in fact, in its simplest form, you can do it like this (this is from my
    "overwrite disk images" program that I use on old disks):

    for (index = 0; index < max_index ;index++) {
    if (write(fd, buffer, BUFSIZE) != BUFSIZE)
    break;
    /* This won't block, but will start writeout asynchronously */
    sync_file_range(fd, index*BUFSIZE, BUFSIZE, SYNC_FILE_RANGE_WRITE);
    /* This does a blocking write-and-wait on any old ranges */
    if (index)
    sync_file_range(fd, (index-1)*BUFSIZE, BUFSIZE, SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE|SYNC_FILE_RANGE_WAIT_AFTER);
    }

    and even if you don't actually do a discard (maybe we should add a
    SYNC_FILE_RANGE_DISCARD bit, right now you'd need to do a separate
    fadvise(FADV_DONTNEED) to throw it out) the system behavior is pretty
    nice, because the heavy writer gets good IO performance _and_ leaves only
    easy-to-free pages around after itself.

    Linus


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