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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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