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

[Jens Axboe <axboe@kernel.dk>]

On 06/05/2015 04:58 PM, Tahsin Erdogan wrote:
> On Wed, May 27, 2015 at 1:14 PM, Tahsin Erdogan <tahsin@google.com> wrote:
>> On Tue, May 19, 2015 at 1:55 PM, Tahsin Erdogan <tahsin@google.com> 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@google.com>
>>> Signed-off-by: Tahsin Erdogan <tahsin@google.com>
>>> ---
>>>   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
>>>
>>
>> Ping...
>
> Trying once more..

This one worked :-). I agree, it's probably the sane thing to do, I'll 
apply this for 4.2.

-- 
Jens Axboe