Re: [PATCH] block: BFQ default for single queue devices

[Paolo Valente]

> Il giorno 06 ott 2018, alle ore 05:12, Bart Van Assche <bvanassche@xxxxxxx> ha scritto:
> 
> On 10/5/18 2:16 AM, Jan Kara wrote:
>> On Thu 04-10-18 15:42:52, Bart Van Assche wrote:
>>> What I think is missing is measurement results for BFQ on a system with
>>> multiple CPU sockets and against a fast storage medium. Eliminating
>>> the host lock from the SCSI core yielded a significant performance
>>> improvement for such storage devices. Since the BFQ scheduler locks and
>>> unlocks bfqd->lock for every dispatch operation it is very likely that BFQ
>>> will slow down I/O for fast storage devices, even if their driver only
>>> creates a single hardware queue.
>> Well, I'm not sure why that is missing. I don't think anyone proposed to
>> default to BFQ for such setup? Neither was anyone claiming that BFQ is
>> better in such situation... The proposal has been: Default to BFQ for slow
>> storage, leave it to deadline-mq otherwise.
> 
> Hi Jan,
> 
> How do you define slow storage? The proposal at the start of this thread was to make BFQ the default for all block devices that create a single hardware queue. That includes all SATA storage since scsi-mq only creates a single hardware queue when using the SATA protocol. The proposal to make BFQ the default for systems with a single hard disk probably makes sense but I am not sure that making BFQ the default for systems equipped with one or more (SATA) SSDs is also a good idea. Especially for multi-socket systems since BFQ reintroduces a queue-wide lock.

No, BFQ has no queue-wide lock.  The very first change made to BFQ for
porting it to blk-mq was to remove the queue lock.  Guided by Jens, I
replaced that lock with the exact, same scheduler lock used in
mq-deadline.

Thanks,
Paolo

> As you know no queue-wide locking happens during I/O in the scsi-mq core nor in the blk-mq core.
> 
> Bart.