Re: CFQ read performance regression
From: Miklos Szeredi
Date: Mon Apr 19 2010 - 07:46:10 EST
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.
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.
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'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.
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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