linux-kernel - Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler

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Date:   Fri, 28 Oct 2016 08:29:08 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Ulf Hansson <ulf.hansson@...aro.org>,
        Paolo Valente <paolo.valente@...aro.org>,
        Christoph Hellwig <hch@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        linux-block@...r.kernel.org,
        Linux-Kernal <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Hannes Reinecke <hare@...e.de>,
        Grant Likely <grant.likely@...retlab.ca>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra
 scheduler

On Fri, Oct 28, 2016 at 11:32:21AM +0200, Linus Walleij wrote:
> So I'm not just complaining by the way, I'm trying to fix this. Also
> Bartlomiej from Samsung has done some stabs at switching MMC/SD
> to blk-mq. I just rebased my latest stab at a naïve switch to blk-mq
> to v4.9-rc2 with these results.
> 
> The patch to enable MQ looks like this:
> https://git.kernel.org/cgit/linux/kernel/git/linusw/linux-stericsson.git/commit/?h=mmc-mq&id=8f79b527e2e854071d8da019451da68d4753f71d
> 
> I run these tests directly after boot with cold caches. The results
> are consistent: I ran the same commands 10 times in a row.

A couple comments from a quick look over the patch:

In the changelog you complain:

". Lack of front- and back-end merging in the MQ block layer creating
several small requests instead of a few large ones."

In blk-mq merging is controller by the BLK_MQ_F_SHOULD_MERGE and
BLK_MQ_F_SG_MERGE flags.  You set the former, but not the latter.
BLK_MQ_F_SG_MERGE controls wether multiple physical contiguous pages get
merged into a single segment.  For a dd after a fresh boot that is
probably very common.  Except for the polarity of the merge flags the
basic merge functionality between the legacy and blk-mq path should be
the same, and if they aren't you've found a bug we need to address.

You also say that you disable the pipelining.  How much of a performance
gain did this feature give when added?  How much does just removing that
on it's own cost you?  While I think that features is rather messy and
should be avoided if possible I don't see how it's impossible to
implement in blk-mq.  If you just increase your queue depth and use
the old scheme you should get it - if you currently can't handle the
second command for some reason (i.e. the special request magic) you
can just return BLK_MQ_RQ_QUEUE_BUSY from the queue_rq function.