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

[Shaohua Li <shli@kernel.org>]

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.

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.

Thanks,
Shaohua