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Subject: Re: [PATCH] block: BFQ default for single queue devices
From:   Paolo Valente <paolo.valente@linaro.org>
In-Reply-To: <20bfa679-3131-e0af-f69d-2fbec32fbced@acm.org>
Date:   Sat, 6 Oct 2018 08:46:36 +0200
Cc:     Jan Kara <jack@suse.cz>, Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Jens Axboe <axboe@kernel.dk>,
        Linus Walleij <linus.walleij@linaro.org>,
        linux-block <linux-block@vger.kernel.org>,
        linux-mmc <linux-mmc@vger.kernel.org>,
        linux-mtd@lists.infradead.org, Pavel Machek <pavel@ucw.cz>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        Richard Weinberger <richard@nod.at>,
        Artem Bityutskiy <dedekind1@gmail.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Andreas Herrmann <aherrmann@suse.com>,
        Mel Gorman <mgorman@suse.com>,
        Chunyan Zhang <zhang.chunyan@linaro.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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To:     Bart Van Assche <bvanassche@acm.org>
Sender: linux-kernel-owner@vger.kernel.org
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> 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.