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Subject: Re: [PATCH] block: BFQ default for single queue devices
To:     Jan Kara <jack@suse.cz>
Cc:     Paolo Valente <paolo.valente@linaro.org>,
        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>,
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        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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 <1538692972.8223.7.camel@acm.org> <20181005091626.GA9686@quack2.suse.cz>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <20bfa679-3131-e0af-f69d-2fbec32fbced@acm.org>
Date:   Fri, 5 Oct 2018 20:12:27 -0700
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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. As you know no queue-wide locking happens during I/O in the 
scsi-mq core nor in the blk-mq core.

Bart.