Re: [PATCH] block: Make CFQ default to IOPS mode on SSDs

[Tahsin Erdogan]

On Tue, May 19, 2015 at 1:55 PM, Tahsin Erdogan <tahsin@xxxxxxxxxx> wrote:
> CFQ idling causes reduced IOPS throughput on non-rotational disks.
> Since disk head seeking is not applicable to SSDs, it doesn't really
> help performance by anticipating future near-by IO requests.
>
> By turning off idling (and switching to IOPS mode), we allow other
> processes to dispatch IO requests down to the driver and so increase IO
> throughput.
>
> Following FIO benchmark results were taken on a cloud SSD offering with
> idling on and off:
>
> Idling     iops    avg-lat(ms)    stddev            bw
> ------------------------------------------------------
>     On     7054    90.107         38.697     28217KB/s
>    Off    29255    21.836         11.730    117022KB/s
>
> fio --name=temp --size=100G --time_based --ioengine=libaio \
>     --randrepeat=0 --direct=1 --invalidate=1 --verify=0 \
>     --verify_fatal=0 --rw=randread --blocksize=4k --group_reporting=1 \
>     --filename=/dev/sdb --runtime=10 --iodepth=64 --numjobs=10
>
> And the following is from a local SSD run:
>
> Idling     iops    avg-lat(ms)    stddev            bw
> ------------------------------------------------------
>     On    19320    33.043         14.068     77281KB/s
>    Off    21626    29.465         12.662     86507KB/s
>
> fio --name=temp --size=5G --time_based --ioengine=libaio \
>     --randrepeat=0 --direct=1 --invalidate=1 --verify=0 \
>     --verify_fatal=0 --rw=randread --blocksize=4k --group_reporting=1 \
>     --filename=/fio_data --runtime=10 --iodepth=64 --numjobs=10
>
> Reviewed-by: Nauman Rafique <nauman@xxxxxxxxxx>
> Signed-off-by: Tahsin Erdogan <tahsin@xxxxxxxxxx>
> ---
>  block/cfq-iosched.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 5da8e6e..402be01 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -4460,7 +4460,7 @@ static int cfq_init_queue(struct request_queue *q, struct elevator_type *e)
>         cfqd->cfq_slice[1] = cfq_slice_sync;
>         cfqd->cfq_target_latency = cfq_target_latency;
>         cfqd->cfq_slice_async_rq = cfq_slice_async_rq;
> -       cfqd->cfq_slice_idle = cfq_slice_idle;
> +       cfqd->cfq_slice_idle = blk_queue_nonrot(q) ? 0 : cfq_slice_idle;
>         cfqd->cfq_group_idle = cfq_group_idle;
>         cfqd->cfq_latency = 1;
>         cfqd->hw_tag = -1;
> --
> 2.2.0.rc0.207.ga3a616c
>

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