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SubjectRe: 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@linux.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@lkp-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@lkp-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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