[PATCH] cfq-iosched: allow groups preemption for sync-noidle workloads

From: Konstantin Khlebnikov
Date: Thu Jun 23 2011 - 12:22:18 EST

commit v2.6.32-102-g8682e1f "blkio: Provide some isolation between groups" break
fast switching between task and journal-thread for very common write-fsync workload.
cfq wait idle slice at each cfqq switch, if this task is from non-root blkio cgroup.

This patch move idling sync-noidle preempting check little bit upwards and update
new service_tree->count check for case with two different groups.
I do not quite understand what means these check for new_cfqq, but now it even works.

Without patch I got 49 iops and with this patch 798, for this trivial fio script:


Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx>
block/cfq-iosched.c | 14 +++++++-------
1 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 3c7b537..c71533e 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3318,19 +3318,19 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq))
return true;

- if (new_cfqq->cfqg != cfqq->cfqg)
- return false;
- if (cfq_slice_used(cfqq))
- return true;
/* Allow preemption only if we are idling on sync-noidle tree */
if (cfqd->serving_type == SYNC_NOIDLE_WORKLOAD &&
cfqq_type(new_cfqq) == SYNC_NOIDLE_WORKLOAD &&
- new_cfqq->service_tree->count == 2 &&
+ new_cfqq->service_tree->count == 1+(new_cfqq->cfqg == cfqq->cfqg) &&
return true;

+ if (new_cfqq->cfqg != cfqq->cfqg)
+ return false;
+ if (cfq_slice_used(cfqq))
+ return true;
* So both queues are sync. Let the new request get disk time if
* it's a metadata request and the current queue is doing regular IO.

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