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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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