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

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Message-ID: <20120130073621.GN29272@MAIL.13thfloor.at>
Date:	Mon, 30 Jan 2012 08:36:21 +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 03:22:38PM +0800, Shaohua Li wrote:
> On Mon, 2012-01-30 at 08:13 +0100, Herbert Poetzl wrote:
>> 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

> What's 3.2.2i and 3.2.2i128? 

3.2.2 ...... kernel with default options (bs=1M)
3.2.2i ..... kernel with idle=poll (bs=1M)
3.2.2i128 .. kernel with idle=poll (bs=128k)

> does idle=poll help?

doesn't look like, at least to me ...

HTC,
Herbert
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