Messages in this thread | | | Subject | Re: Bad SSD performance with recent kernels | From | Shaohua Li <> | Date | Mon, 30 Jan 2012 16:12:22 +0800 |
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On Mon, 2012-01-30 at 08:36 +0100, Herbert Poetzl wrote: > 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@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 > > > 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 ... what's your /sys/block/sdx/queue/max_sectors_kb? if you make it smaller, does the performance increase? In my system, a smaller max_sectors_kb makes bs=2M and bs=128k have similar performance, which makes me think it's CPU doesn't catch up quickly after a request finishes.
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