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

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Date:	Tue, 09 Jun 2015 11:42:29 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Romain Francoise <romain@...bokech.com>,
	Tahsin Erdogan <tahsin@...gle.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: Make CFQ default to IOPS mode on SSDs

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


View attachment "elv-register.patch" of type "text/x-patch" (2291 bytes)