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

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Message-ID: <1538692972.8223.7.camel@acm.org>
Date:   Thu, 04 Oct 2018 15:42:52 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Paolo Valente <paolo.valente@...aro.org>
Cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jens Axboe <axboe@...nel.dk>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-block <linux-block@...r.kernel.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        linux-mtd@...ts.infradead.org, Pavel Machek <pavel@....cz>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Richard Weinberger <richard@....at>,
        Artem Bityutskiy <dedekind1@...il.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Jan Kara <jack@...e.cz>, Andreas Herrmann <aherrmann@...e.com>,
        Mel Gorman <mgorman@...e.com>,
        Chunyan Zhang <zhang.chunyan@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] block: BFQ default for single queue devices

On Thu, 2018-10-04 at 22:39 +0200, Paolo Valente wrote:
> No, kernel build is, for evident reasons, one of the workloads I cared
> most about.  Actually, I tried to focus on all my main
> kernel-development tasks, such as also git checkout, git merge, git
> grep, ...
> 
> According to my test results, with BFQ these tasks are at least as
> fast as, or, in most system configurations, much faster than with the
> other schedulers.  Of course, at the same time the system also remains
> responsive with BFQ.
> 
> You can repeat these tests using one of my first scripts in the S
> suite: kern_dev_tasks_vs_rw.sh (usually, the older the tests, the more
> hypertrophied the names I gave :) ).
> 
> I stopped sharing also my kernel-build results years ago, because I
> went on obtaining the same, identical good results for years, and I'm
> aware that I tend to show and say too much stuff.

On my test setup building the kernel is slightly slower when using the BFQ
scheduler compared to using scheduler "none" (kernel 4.18.12, NVMe SSD,
single CPU with 6 cores, hyperthreading disabled). I am aware that the
proposal at the start of this thread was to make BFQ the default for devices
with a single hardware queue and not for devices like NVMe SSDs that support
multiple hardware queues.

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.

Bart.