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From: Linus Walleij <linus.walleij@linaro.org>
Date: Fri, 28 Oct 2016 22:38:36 +0200
Message-ID: <CACRpkdYNfOzfnA-9a87PzSiA_YkLv4fq9y2cCtCZDK7CScEnFw@mail.gmail.com>
Subject: Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler
To: Jens Axboe <axboe@kernel.dk>
Cc: Ulf Hansson <ulf.hansson@linaro.org>,
        Paolo Valente <paolo.valente@linaro.org>,
        Christoph Hellwig <hch@infradead.org>, Arnd Bergmann <arnd@arndb.de>,
        Bart Van Assche <bart.vanassche@sandisk.com>, Jan Kara <jack@suse.cz>,
        Tejun Heo <tj@kernel.org>, linux-block@vger.kernel.org,
        Linux-Kernal <linux-kernel@vger.kernel.org>,
        Mark Brown <broonie@kernel.org>, Hannes Reinecke <hare@suse.de>,
        Grant Likely <grant.likely@secretlab.ca>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
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On Fri, Oct 28, 2016 at 4:22 PM, Jens Axboe <axboe@kernel.dk> wrote:
> On 10/28/2016 03:32 AM, Linus Walleij wrote:
>>
>> This is without using Bartlomiej's clever hack to pretend we have
>> 2 elements in the HW queue though. His early tests indicate that
>> it doesn't help much: the performance regression we see is due to
>> lack of block scheduling.
>
> A simple dd test, I don't see how that can be slower due to lack of
> scheduling. There's nothing to schedule there, just issue them in order?

Yeah I guess you're right, I guess it could be in part to not having
activated front- and back-end merges properly as Christoph pointed
out, I'll look closer at this.

> So that would probably be where I would start looking. A blktrace of the
> in-kernel code and the blk-mq enabled code would perhaps be
> enlightening. I don't think it's worth looking at the more complex test
> cases until the dd test case is at least as fast as the non-mq version.

Yeah.

> Was that with CFQ, btw, or what scheduler did it run?

CFQ, just plain defconfig.

> It'd be nice to NOT have to rely on that fake QD=2 setup, since it will
> mess with the IO scheduling as well.

I agree.

>> I try to find a way forward with this, and also massage the MMC/SD
>> code to be more MQ friendly to begin with (like only pick requests
>> when we get a request notification and stop pulling NULL requests
>> off the queue) but it's really a messy piece of code.
>
> Yeah, it does look pretty messy... I'd be happy to help out with that,
> and particularly in figuring out why the direct conversion is slower for
> a basic 'dd' test case.

I'm looking into it.

Yours,
Linus Walleij