Re: [RFC PATCH] cfq-iosched: limit slice_idle when many busy queues are in idle window

[Tomoki Sekiyama <tomoki.sekiyama@hds.com>]

On 7/30/13 10:09 PM, Shaohua Li wrote:
> On Tue, Jul 30, 2013 at 03:30:33PM -0400, Tomoki Sekiyama wrote:
>> Hi,
>>
>> When some application launches several hundreds of processes that issue
>> only a few small sync I/O requests, CFQ may cause heavy latencies
>> (10+ seconds at the worst case), although the request rate is low enough for
>> the disk to handle it without waiting. This is because CFQ waits for
>> slice_idle (default:8ms) every time before processing each request, until
>> their thinktimes are evaluated.
>>
>> This scenario can be reproduced using fio with parameters below:
>>   fio -filename=/tmp/test -rw=randread -size=5G -runtime=15 -name=file1 \
>>       -bs=4k -numjobs=500 -thinktime=1000000
>> In this case, 500 processes issue a random read request every second.
> 
> For this workload CFQ should perfectly detect it's a seek queue and disable
> idle. I suppose the reason is CFQ hasn't enough data/time to disable idle yet,
> since your thinktime is long and runtime is short.

Right, CFQ will learn the patten, but it takes too long time to reach stable
performance when a lot of I/O processes are launched.

> I thought the real problem here is cfq_init_cfqq() shouldn't set idle_window
> when initializing a queue. We should enable idle window after we detect the
> queue is worthy idle.

Do you think the patch below is appropriate? Or should we check whether
busy_idle_queues in my original patch is high enough and only then
disable default idle_window in cfq_init_cfqq()?

> Thanks,
> Shaohua

Thanks,
Tomoki Sekiyama

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index d5cd313..abbe28f 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3514,11 +3514,8 @@ static void cfq_init_cfqq(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 
 	cfq_mark_cfqq_prio_changed(cfqq);
 
-	if (is_sync) {
-		if (!cfq_class_idle(cfqq))
-			cfq_mark_cfqq_idle_window(cfqq);
+	if (is_sync)
 		cfq_mark_cfqq_sync(cfqq);
-	}
 	cfqq->pid = pid;
 }