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SubjectRe: Performance degradation with FFSB between 2.6.20 and 2.6.21-rc7
On Thu, Apr 19 2007, Jens Axboe wrote:
> On Thu, Apr 19 2007, Valerie Clement wrote:
> > Jens Axboe wrote:
> > >Please tell me how you are running ffsb, and also please include a
> > >dmessg from a booted system.
> > >
> > Hi,
> > our mails crossed! please see my response to Andrew.
> > You could reproduce the problem with dd command as suggested, it's more
> > easy.
> > I'm sending you the dmesg info. For my tests I used the scsci sdc device.
>
> Thanks, it does. Can you try one thing for me? If you run the test on
> sdc, try doing:
>
> # echo 64 > /sys/block/sdc/queue/iosched/quantum
>
> and repeat the test.

And, then try this one as well (and don't tweak quantum for that
kernel):

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index b6491c0..9e37971 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -986,9 +986,9 @@ __cfq_dispatch_requests(struct cfq_data *cfqd, struct cfq_queue *cfqq,
* expire an async queue immediately if it has used up its slice. idle
* queue always expire after 1 dispatch round.
*/
- if ((!cfq_cfqq_sync(cfqq) &&
+ if (cfqd->busy_queues > 1 && ((!cfq_cfqq_sync(cfqq) &&
cfqd->dispatch_slice >= cfq_prio_to_maxrq(cfqd, cfqq)) ||
- cfq_class_idle(cfqq)) {
+ cfq_class_idle(cfqq))) {
cfqq->slice_end = jiffies + 1;
cfq_slice_expired(cfqd, 0, 0);
}
@@ -1051,19 +1051,21 @@ cfq_dispatch_requests(request_queue_t *q, int force)
while ((cfqq = cfq_select_queue(cfqd)) != NULL) {
int max_dispatch;

- /*
- * Don't repeat dispatch from the previous queue.
- */
- if (prev_cfqq == cfqq)
- break;
+ if (cfqd->busy_queues > 1) {
+ /*
+ * Don't repeat dispatch from the previous queue.
+ */
+ if (prev_cfqq == cfqq)
+ break;

- /*
- * So we have dispatched before in this round, if the
- * next queue has idling enabled (must be sync), don't
- * allow it service until the previous have continued.
- */
- if (cfqd->rq_in_driver && cfq_cfqq_idle_window(cfqq))
- break;
+ /*
+ * So we have dispatched before in this round, if the
+ * next queue has idling enabled (must be sync), don't
+ * allow it service until the previous have continued.
+ */
+ if (cfqd->rq_in_driver && cfq_cfqq_idle_window(cfqq))
+ break;
+ }

cfq_clear_cfqq_must_dispatch(cfqq);
cfq_clear_cfqq_wait_request(cfqq);
@@ -1370,7 +1372,9 @@ retry:
atomic_set(&cfqq->ref, 0);
cfqq->cfqd = cfqd;

- cfq_mark_cfqq_idle_window(cfqq);
+ if (key != CFQ_KEY_ASYNC)
+ cfq_mark_cfqq_idle_window(cfqq);
+
cfq_mark_cfqq_prio_changed(cfqq);
cfq_mark_cfqq_queue_new(cfqq);
cfq_init_prio_data(cfqq);
--
Jens Axboe

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