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    Subject[PATCH]block: make auto block plug flush threshold per-disk based

    Subject: block: make auto block plug flush threshold per-disk based

    We do auto block plug flush to reduce latency, the threshold is 16
    requests. This works well if the task is accessing one or two drives.
    The problem is if the task is accessing a raid 0 device and the raid
    disk number is big, say 8 or 16, 16/8 = 2 or 16/16=1, we will have
    heavy lock contention.

    This patch makes the threshold per-disk based. The latency should be
    still ok accessing one or two drives. The setup with application
    accessing a lot of drives in the meantime uaually is big machine,
    avoiding lock contention is more important, because any contention
    will actually increase latency.

    Signed-off-by: Shaohua Li <shli@fusionio.com>
    ---
    block/blk-core.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    Index: linux/block/blk-core.c
    ===================================================================
    --- linux.orig/block/blk-core.c 2012-04-06 10:51:29.766932123 -0600
    +++ linux/block/blk-core.c 2012-04-06 10:56:46.386929357 -0600
    @@ -1277,7 +1277,8 @@
    list_for_each_entry_reverse(rq, &plug->list, queuelist) {
    int el_ret;

    - (*request_count)++;
    + if (rq->q == q)
    + (*request_count)++;

    if (rq->q != q || !blk_rq_merge_ok(rq, bio))
    continue;

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