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Date: Fri, 28 Oct 2016 08:22:06 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Linus Walleij <linus.walleij@...aro.org>
Cc: 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 10/28/2016 03:32 AM, Linus Walleij wrote:
> On Fri, Oct 28, 2016 at 12:27 AM, Linus Walleij
> <linus.walleij@...aro.org> wrote:
>> On Thu, Oct 27, 2016 at 11:08 PM, Jens Axboe <axboe@...nel.dk> wrote:
>>
>>> blk-mq has evolved to support a variety of devices, there's nothing
>>> special about mmc that can't work well within that framework.
>>
>> There is. Read mmc_queue_thread() in drivers/mmc/card/queue.c
>
> 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.
>
>
> BEFORE switching to BLK-MQ (clean v4.9-rc2):
>
> time dd if=/dev/mmcblk0 of=/dev/null bs=1M count=1024
> 1024+0 records in
> 1024+0 records out
> 1073741824 bytes (1.0GB) copied, 47.781464 seconds, 21.4MB/s
> real 0m 47.79s
> user 0m 0.02s
> sys 0m 9.35s
>
> mount /dev/mmcblk0p1 /mnt/
> cd /mnt/
> time find . > /dev/null
> real 0m 3.60s
> user 0m 0.25s
> sys 0m 1.58s
>
> mount /dev/mmcblk0p1 /mnt/
> iozone -az -i0 -i1 -i2 -s 20m -I -f /mnt/foo.test
> (kBytes/second)
> random random
> kB reclen write rewrite read reread read write
> 20480 4 2112 2157 6052 6060 6025 40
> 20480 8 4820 5074 9163 9121 9125 81
> 20480 16 5755 5242 12317 12320 12280 165
> 20480 32 6176 6261 14981 14987 14962 336
> 20480 64 6547 5875 16826 16828 16810 692
> 20480 128 6762 6828 17899 17896 17896 1408
> 20480 256 6802 6871 16960 17513 18373 3048
> 20480 512 7220 7252 18675 18746 18741 7228
> 20480 1024 7222 7304 18436 17858 18246 7322
> 20480 2048 7316 7398 18744 18751 18526 7419
> 20480 4096 7520 7636 20774 20995 20703 7609
> 20480 8192 7519 7704 21850 21489 21467 7663
> 20480 16384 7395 7782 22399 22210 22215 7781
>
>
> AFTER switching to BLK-MQ:
>
> time dd if=/dev/mmcblk0 of=/dev/null bs=1M count=1024
> 1024+0 records in
> 1024+0 records out
> 1073741824 bytes (1.0GB) copied, 60.551117 seconds, 16.9MB/s
> real 1m 0.56s
> user 0m 0.02s
> sys 0m 9.81s
>
> mount /dev/mmcblk0p1 /mnt/
> cd /mnt/
> time find . > /dev/null
> real 0m 4.42s
> user 0m 0.24s
> sys 0m 1.81s
>
> mount /dev/mmcblk0p1 /mnt/
> iozone -az -i0 -i1 -i2 -s 20m -I -f /mnt/foo.test
> (kBytes/second)
> random random
> kB reclen write rewrite read reread read write
> 20480 4 2086 2201 6024 6036 6006 40
> 20480 8 4812 5036 8014 9121 9090 82
> 20480 16 5432 5633 12267 9776 12212 168
> 20480 32 6180 6233 14870 14891 14852 340
> 20480 64 6382 5454 16744 16771 16746 702
> 20480 128 6761 6776 17816 17846 17836 1394
> 20480 256 6828 6842 17789 17895 17094 3084
> 20480 512 7158 7222 17957 17681 17698 7232
> 20480 1024 7215 7274 18642 17679 18031 7300
> 20480 2048 7229 7269 17943 18642 17732 7358
> 20480 4096 7212 7360 18272 18157 18889 7371
> 20480 8192 7008 7271 18632 18707 18225 7282
> 20480 16384 6889 7211 18243 18429 18018 7246
>
>
> A simple dd readtest of 1 GB is always consistently 10+
> seconds slower with MQ. find in the rootfs is a second slower.
> iozone results are consistently lower throughput or the same.
>
> 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?
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.
Was that with CFQ, btw, or what scheduler did it run?
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 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.
--
Jens Axboe
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