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Date: Mon, 30 Jan 2012 18:31:34 +0800 From: Shaohua Li <shaohua.li@...el.com> To: Herbert Poetzl <herbert@...hfloor.at> 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 2012/1/30 Shaohua Li <shaohua.li@...el.com>: > 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@...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 ... > 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. Looks the 2.6.39 block plug introduces some latency here. deleting blk_start_plug/blk_finish_plug in generic_file_aio_read seems workaround the issue. The plug seems not good for sequential IO, because readahead code already has plug and has fine grained control. On the other hand, ondemand_readahead seems not handle the case that req_size is big well. -- 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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