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Date: Tue, 20 Apr 2010 22:50:09 +0200 From: Corrado Zoccolo <czoccolo@...il.com> To: Miklos Szeredi <mszeredi@...e.cz> Cc: Jens Axboe <jens.axboe@...cle.com>, linux-kernel <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>, Suresh Jayaraman <sjayaraman@...e.de> Subject: Re: CFQ read performance regression On Mon, Apr 19, 2010 at 1:46 PM, Miklos Szeredi <mszeredi@...e.cz> wrote: > Hi Corrado, > > On Sat, 2010-04-17 at 14:46 +0200, Corrado Zoccolo wrote: >> I don't think this is related to CFQ. I've made a graph of the >> accessed (read) sectors (see attached). >> You can see that the green cloud (2.6.16) is much more concentrated, >> while the red one (2.6.32) is split in two, and you can better >> recognize the different lines. >> This means that the FS put more distance between the blocks of the >> files written by the tio threads, and the read time is therefore >> impacted, since the disk head has to perform longer seeks. On the >> other hand, if you read those files sequentially with a single thread, >> the performance may be better with the new layout, so YMMV. When >> testing 2.6.32 and up, you should consider testing also with >> low_latency setting disabled, since tuning for latency can negatively >> affect throughput. Hi Miklos, can you give more information about the setup? How much memory do you have, what is the disk configuration (is this a hw raid?) and so on. > > low_latency is set to zero in all tests. > > The layout difference doesn't explain why setting the scheduler to > "noop" consistently speeds up read throughput in 8-thread tiobench to > almost twice. This fact alone pretty clearly indicates that the I/O > scheduler is the culprit. From the attached btt output, I see that a lot of time is spent waiting to allocate new request structures. > S2G 0.022460311 6.581680621 23.144763751 15 Since noop doesn't attach fancy data to each request, it can save those allocations, thus resulting in no sleeps. The delays in allocation, though, may not be completely imputable to the I/O scheduler, and working in constrained memory conditions will negatively affect it. > There are other indications, see the attached btt output for both > traces. From there it appears that 2.6.16 does more and longer seeks, > yet it's getting an overall better performance. I see less seeks for 2.6.16, but longer on average. It seems that 2.6.16 allows more requests from the same process to be streamed to disk before switching to an other process. Since the timeslice is the same, it might be that we are limiting the number of requests per queue due to memory congestion. > I've also tested with plain "dd" instead of tiobench where the > filesystem layout stayed exactly the same between tests. Still the > speed difference is there. Does dropping caches before the read test change the situation? Thanks, Corrado > > Thanks, > Miklos > > ************************************************************************ > btt output for 2.6.16: > ==================== All Devices ==================== > > ALL MIN AVG MAX N > --------------- ------------- ------------- ------------- ----------- > > Q2Q 0.000000047 0.000854417 1.003550405 67465 > Q2G 0.000000458 0.000001211 0.000123527 46062 > G2I 0.000000123 0.000001815 0.000494517 46074 > Q2M 0.000000186 0.000001798 0.000010296 21404 > I2D 0.000000162 0.000158001 0.040794333 46062 > M2D 0.000000878 0.000133130 0.040585566 21404 > D2C 0.000053870 0.023778266 0.234154543 67466 > Q2C 0.000056746 0.023931014 0.234176000 67466 > > ==================== Device Overhead ==================== > > DEV | Q2G G2I Q2M I2D D2C > ---------- | --------- --------- --------- --------- --------- > ( 8, 64) | 0.0035% 0.0052% 0.0024% 0.4508% 99.3617% > ---------- | --------- --------- --------- --------- --------- > Overall | 0.0035% 0.0052% 0.0024% 0.4508% 99.3617% > > ==================== Device Merge Information ==================== > > DEV | #Q #D Ratio | BLKmin BLKavg BLKmax Total > ---------- | -------- -------- ------- | -------- -------- -------- -------- > ( 8, 64) | 67466 46062 1.5 | 8 597 1024 27543688 > > ==================== Device Q2Q Seek Information ==================== > > DEV | NSEEKS MEAN MEDIAN | MODE > ---------- | --------------- --------------- --------------- | --------------- > ( 8, 64) | 67466 866834.0 0 | 0(34558) > ---------- | --------------- --------------- --------------- | --------------- > Overall | NSEEKS MEAN MEDIAN | MODE > Average | 67466 866834.0 0 | 0(34558) > > ==================== Device D2D Seek Information ==================== > > DEV | NSEEKS MEAN MEDIAN | MODE > ---------- | --------------- --------------- --------------- | --------------- > ( 8, 64) | 46062 1265503.9 0 | 0(13242) > ---------- | --------------- --------------- --------------- | --------------- > Overall | NSEEKS MEAN MEDIAN | MODE > Average | 46062 1265503.9 0 | 0(13242) > > ==================== Plug Information ==================== > > DEV | # Plugs # Timer Us | % Time Q Plugged > ---------- | ---------- ---------- | ---------------- > ( 8, 64) | 29271( 533) | 3.878105328% > > DEV | IOs/Unp IOs/Unp(to) > ---------- | ---------- ---------- > ( 8, 64) | 19.2 19.7 > ---------- | ---------- ---------- > Overall | IOs/Unp IOs/Unp(to) > Average | 19.2 19.7 > > ==================== Active Requests At Q Information ==================== > > DEV | Avg Reqs @ Q > ---------- | ------------- > ( 8, 64) | 0.8 > > ==================== I/O Active Period Information ==================== > > DEV | # Live Avg. Act Avg. !Act % Live > ---------- | ---------- ------------- ------------- ------ > ( 8, 64) | 545 0.100012237 0.005766640 94.56 > ---------- | ---------- ------------- ------------- ------ > Total Sys | 545 0.100012237 0.005766640 94.56 > > ************************************************************************ > btt output for 2.6.32: > > ==================== All Devices ==================== > > ALL MIN AVG MAX N > --------------- ------------- ------------- ------------- ----------- > > Q2Q 0.000000279 0.001710581 1.803934429 69429 > Q2G 0.000000908 0.001798735 23.144764798 54940 > S2G 0.022460311 6.581680621 23.144763751 15 > G2I 0.000000628 0.000001576 0.000120409 54942 > Q2M 0.000000628 0.000001611 0.000013201 14490 > I2D 0.000000768 0.289812035 86.820205789 54940 > M2D 0.000005518 0.098208187 0.794441158 14490 > D2C 0.000173141 0.008056256 0.219516385 69430 > Q2C 0.000179077 0.259305605 86.820559403 69430 > > ==================== Device Overhead ==================== > > DEV | Q2G G2I Q2M I2D D2C > ---------- | --------- --------- --------- --------- --------- > ( 8, 64) | 0.5489% 0.0005% 0.0001% 88.4394% 3.1069% > ---------- | --------- --------- --------- --------- --------- > Overall | 0.5489% 0.0005% 0.0001% 88.4394% 3.1069% > > ==================== Device Merge Information ==================== > > DEV | #Q #D Ratio | BLKmin BLKavg BLKmax Total > ---------- | -------- -------- ------- | -------- -------- -------- -------- > ( 8, 64) | 69430 54955 1.3 | 8 520 2048 28614984 > > ==================== Device Q2Q Seek Information ==================== > > DEV | NSEEKS MEAN MEDIAN | MODE > ---------- | --------------- --------------- --------------- | --------------- > ( 8, 64) | 69430 546377.3 0 | 0(50235) > ---------- | --------------- --------------- --------------- | --------------- > Overall | NSEEKS MEAN MEDIAN | MODE > Average | 69430 546377.3 0 | 0(50235) > > ==================== Device D2D Seek Information ==================== > > DEV | NSEEKS MEAN MEDIAN | MODE > ---------- | --------------- --------------- --------------- | --------------- > ( 8, 64) | 54955 565286.3 0 | 0(37535) > ---------- | --------------- --------------- --------------- | --------------- > Overall | NSEEKS MEAN MEDIAN | MODE > Average | 54955 565286.3 0 | 0(37535) > > ==================== Plug Information ==================== > > DEV | # Plugs # Timer Us | % Time Q Plugged > ---------- | ---------- ---------- | ---------------- > ( 8, 64) | 2310( 0) | 0.049353257% > > DEV | IOs/Unp IOs/Unp(to) > ---------- | ---------- ---------- > ( 8, 64) | 7.3 0.0 > ---------- | ---------- ---------- > Overall | IOs/Unp IOs/Unp(to) > Average | 7.3 0.0 > > ==================== Active Requests At Q Information ==================== > > DEV | Avg Reqs @ Q > ---------- | ------------- > ( 8, 64) | 132.8 > > ==================== I/O Active Period Information ==================== > > DEV | # Live Avg. Act Avg. !Act % Live > ---------- | ---------- ------------- ------------- ------ > ( 8, 64) | 4835 0.023848998 0.000714665 97.09 > ---------- | ---------- ------------- ------------- ------ > Total Sys | 4835 0.023848998 0.000714665 97.09 > > > >
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