Messages in this thread | | | Date | Mon, 30 Jan 2012 11:17:38 +0800 | Subject | Re: Bad SSD performance with recent kernels | From | Shaohua Li <> |
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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.
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