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    SubjectRe: [PATCH 08/18] io-controller: idle for sometime on sync queue before expiring it
    On Tue, May 05, 2009 at 03:58:35PM -0400, Vivek Goyal wrote:
    > o When a sync queue expires, in many cases it might be empty and then
    > it will be deleted from the active tree. This will lead to a scenario
    > where out of two competing queues, only one is on the tree and when a
    > new queue is selected, vtime jump takes place and we don't see services
    > provided in proportion to weight.
    > o In general this is a fundamental problem with fairness of sync queues
    > where queues are not continuously backlogged. Looks like idling is
    > only solution to make sure such kind of queues can get some decent amount
    > of disk bandwidth in the face of competion from continusouly backlogged
    > queues. But excessive idling has potential to reduce performance on SSD
    > and disks with commnad queuing.
    > o This patch experiments with waiting for next request to come before a
    > queue is expired after it has consumed its time slice. This can ensure
    > more accurate fairness numbers in some cases.
    > o Introduced a tunable "fairness". If set, io-controller will put more
    > focus on getting fairness right than getting throughput right.
    > Signed-off-by: Vivek Goyal <>
    > ---

    Following is a fix which should go here. This patch helps me get much
    better fairness numbers for sync queues.

    o Fix a window where a queue can be expired without doing busy wait for
    next request. This fix allows better fairness number for sync queues.

    Signed-off-by: Vivek Goyal <>
    block/elevator-fq.c | 18 ++++++++++++++++--
    1 file changed, 16 insertions(+), 2 deletions(-)

    Index: linux14/block/elevator-fq.c
    --- linux14.orig/block/elevator-fq.c 2009-05-13 10:55:44.000000000 -0400
    +++ linux14/block/elevator-fq.c 2009-05-13 10:55:50.000000000 -0400
    @@ -3368,8 +3368,22 @@ void *elv_fq_select_ioq(struct request_q
    * The active queue has run out of time, expire it and select new.
    - if (elv_ioq_slice_used(ioq) && !elv_ioq_must_dispatch(ioq))
    - goto expire;
    + if (elv_ioq_slice_used(ioq) && !elv_ioq_must_dispatch(ioq)) {
    + /*
    + * Queue has used up its slice. Wait busy is not on otherwise
    + * we wouldn't have been here. There is a chance that after
    + * slice expiry no request from the queue completed hence
    + * wait busy timer could not be turned on. If that's the case
    + * don't expire the queue yet. Next request completion from
    + * the queue will arm the wait busy timer.
    + */
    + if (efqd->fairness && !ioq->nr_queued
    + && elv_ioq_nr_dispatched(ioq)) {
    + ioq = NULL;
    + goto keep_queue;
    + } else
    + goto expire;
    + }

    * If we have a RT cfqq waiting, then we pre-empt the current non-rt

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      Last update: 2009-05-13 17:05    [W:0.026 / U:25.292 seconds]
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