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

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Message-ID: <CAAeU0aOTjcZa0FFUweSAMPz62Kj7WMqmJBiFoHiPX6f7tc5gQA@mail.gmail.com>
Date:	Tue, 9 Jun 2015 14:54:30 -0700
From:	Tahsin Erdogan <tahsin@...gle.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Romain Francoise <romain@...bokech.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: Make CFQ default to IOPS mode on SSDs

Thanks for catching this.

In my testing, I was switching to cfq through sysfs. Since disk
initialization happens earlier than manual switching, I didn't hit
this problem.

On Tue, Jun 9, 2015 at 10:42 AM, Jens Axboe <axboe@...nel.dk> wrote:
> On 06/09/2015 04:18 AM, Romain Francoise wrote:
>>
>> Hi,
>>
>> On Tue, May 19, 2015 at 01:55:21PM -0700, Tahsin Erdogan wrote:
>>>
>>> --- 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;
>>
>>
>> Did you test this patch with regular AHCI SSD devices? Applying it on
>> top of v4.1-rc7 makes no difference, slice_idle is still initialized to
>> 8 in my setup, while rotational is 0.
>>
>> Isn't the elevator initialized long before the non-rotational flag is
>> actually set on the device (which probably happens after it's probed on
>> the scsi bus)?
>
>
> You are absolutely correct. What happens is that the queue is allocated and
> initialized, and cfq checks the flag. But the flag is set later in the
> process, when we have finished probing the device checked if it's rotational
> or not.
>
> There are a few options to handle this. The attached might work, not tested
> at all. Basically it adds an io sched registration hook, that is called when
> we are adding the disk on the queue. Non-rotational detection should be done
> at that point.
>
> Does that work for you?
>
> --
> Jens Axboe
>
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