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

[Paolo Valente <paolo.valente@linaro.org>]

> Il giorno 06 ott 2018, alle ore 05:12, Bart Van Assche =
<bvanassche@acm.org> ha scritto:
>=20
> 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.
>=20
> Hi Jan,
>=20
> 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.
>=20
> Bart.