Re: Bad SSD performance with recent kernels

[Shaohua Li]

2012/1/30 Wu Fengguang <wfg@xxxxxxxxxxxxxxx>:
> 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.

Thanks,
Shaohua
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