linux-kernel - Re: Bad SSD performance with recent kernels

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Date:	Mon, 30 Jan 2012 08:13:47 +0100
From:	Herbert Poetzl <herbert@...hfloor.at>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	Wu Fengguang <wfg@...ux.intel.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>
Subject: Re: Bad SSD performance with recent kernels

On Mon, Jan 30, 2012 at 11:17:38AM +0800, Shaohua Li wrote:
> 2012/1/30 Wu Fengguang <wfg@...ux.intel.com>:
>> On Sun, Jan 29, 2012 at 02:13:51PM +0100, Eric Dumazet wrote:
>>> Le dimanche 29 janvier 2012 à 19:16 +0800, Wu Fengguang a écrit :


>>>> Note that as long as buffered read(2) is used, it makes almost no
>>>> difference (well, at least for now) to do "dd bs=128k" or "dd bs=2MB":
>>>> the 128kb readahead size will be used underneath to submit read IO.


>>> Hmm...

>>> # echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=128k count=32768
>>> 32768+0 enregistrements lus
>>> 32768+0 enregistrements écrits
>>> 4294967296 octets (4,3 GB) copiés, 20,7718 s, 207 MB/s


>>> # echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=2M count=2048
>>> 2048+0 enregistrements lus
>>> 2048+0 enregistrements écrits
>>> 4294967296 octets (4,3 GB) copiés, 27,7824 s, 155 MB/s

>> Interesting. Here are my test results:

>> root@...-nex04 /home/wfg# echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=128k count=32768
>> 32768+0 records in
>> 32768+0 records out
>> 4294967296 bytes (4.3 GB) copied, 19.0121 s, 226 MB/s
>> root@...-nex04 /home/wfg# echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=2M count=2048
>> 2048+0 records in
>> 2048+0 records out
>> 4294967296 bytes (4.3 GB) copied, 19.0214 s, 226 MB/s

>> Maybe the /dev/sda performance bug on your machine is sensitive to timing?
> I got similar result:
> 128k: 224M/s
> 1M: 182M/s

> 1M block size is slow, I guess it's CPU related.

> And as for the big regression with newer kernel than 2.6.38,
> please check if idle=poll helps. CPU idle dramatically impacts
> disk performance and even latest cpuidle governor doesn't help
> for some CPUs.

here are the tests with idle=poll and after switching to 128k
(instead of 1M) blocksize (same amount of data transferred)

kernel    ------------ read /dev/sda -------------
          --- noop ---  - deadline -  ---- cfs ---
          [MB/s]  %CPU  [MB/s]  %CPU  [MB/s]  %CPU
--------------------------------------------------
3.2.2      45.82   3.7   44.85   3.6   45.04   3.4
3.2.2i     45.59   2.3   51.78   2.6   46.03   2.2
3.2.2i128 250.24  20.9  252.68  21.3  250.00  21.6

kernel    -- write ---  ------------------read -----------------
          --- noop ---  --- noop ---  - deadline -  ---- cfs ---
          [MB/s]  %CPU  [MB/s]  %CPU  [MB/s]  %CPU  [MB/s]  %CPU
----------------------------------------------------------------
3.2.2     270.95  42.6  162.36   9.9  162.63   9.9  162.65  10.1
3.2.2i    269.10  41.4  170.82   6.6  171.20   6.6  170.91   6.7
3.2.2i128 270.38  67.7  162.35  10.2  163.01  10.3  162.34  10.7

HTH,
Herbert

> Thanks,
> Shaohua
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