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From: Vivek Goyal on 19 May 2010 16:40
o This patch basically reverts following commit.

76280af cfq-iosched: idling on deep seeky sync queues

o Idling in CFQ is bad on high end storage. This is especially more true of
random reads. Idling works very well for SATA disks with single
spindle but harms a lot on powerful storage boxes.

So even if deep queues can be little unfair to other random workload with
shallow depths, treat deep queues as sync-noidle workload and not sync,
because with sync workload we dispatch IO from only one queue at a time
and idle and we don't drive enough queue depth to keep the array busy.

o I am running aio-stress (random reads) as follows.

aio-stress -s 2g -O -t 4 -r 64k aio5 aio6 aio7 aio8 -o 3

Following are results with various combinations.

deadline: 232.94 MB/s

without patch
-------------
cfq default 75.32 MB/s
cfq, quantum=64 134.58 MB/s

with patch
----------
cfq default 78.37 MB/s
cfq, quantum=64 213.94 MB

Note that with the patch applied, cfq really scales well if "quantum" is
increased and comes close to deadline performance.

o Point being that on powerful arrays one queue is not sufficient to keep
array busy. This is already a bottleneck for sequential workloads. Lets
not aggravate the problem by marking random read queues as sync and
giving them exclusive access and hence effectively serializing the
access to array.

Signed-off-by: Vivek Goyal <vgoyal(a)redhat.com>
---
block/cfq-iosched.c | 12 +-----------
1 files changed, 1 insertions(+), 11 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 5f127cf..3336bd7 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -313,7 +313,6 @@ enum cfqq_state_flags {
CFQ_CFQQ_FLAG_sync, /* synchronous queue */
CFQ_CFQQ_FLAG_coop, /* cfqq is shared */
CFQ_CFQQ_FLAG_split_coop, /* shared cfqq will be splitted */
- CFQ_CFQQ_FLAG_deep, /* sync cfqq experienced large depth */
CFQ_CFQQ_FLAG_wait_busy, /* Waiting for next request */
};

@@ -342,7 +341,6 @@ CFQ_CFQQ_FNS(slice_new);
CFQ_CFQQ_FNS(sync);
CFQ_CFQQ_FNS(coop);
CFQ_CFQQ_FNS(split_coop);
-CFQ_CFQQ_FNS(deep);
CFQ_CFQQ_FNS(wait_busy);
#undef CFQ_CFQQ_FNS

@@ -3036,11 +3034,8 @@ cfq_update_idle_window(struct cfq_data *cfqd, struct cfq_queue *cfqq,

enable_idle = old_idle = cfq_cfqq_idle_window(cfqq);

- if (cfqq->queued[0] + cfqq->queued[1] >= 4)
- cfq_mark_cfqq_deep(cfqq);
-
if (!atomic_read(&cic->ioc->nr_tasks) || !cfqd->cfq_slice_idle ||
- (!cfq_cfqq_deep(cfqq) && CFQQ_SEEKY(cfqq)))
+ CFQQ_SEEKY(cfqq))
enable_idle = 0;
else if (sample_valid(cic->ttime_samples)) {
if (cic->ttime_mean > cfqd->cfq_slice_idle)
@@ -3593,11 +3588,6 @@ static void cfq_idle_slice_timer(unsigned long data)
*/
if (!RB_EMPTY_ROOT(&cfqq->sort_list))
goto out_kick;
-
- /*
- * Queue depth flag is reset only when the idle didn't succeed
- */
- cfq_clear_cfqq_deep(cfqq);
}
expire:
cfq_slice_expired(cfqd, timed_out);
--
1.6.5.2

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Prev: Fix uninitialized spinlock of printk_ratelimited()
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