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SubjectRe: [PATCH] cfq-iosched: reduce write depth only if sync was delayed
Date
Corrado Zoccolo <czoccolo@gmail.com> writes:

> Hi Jeff, I remember you saw large performance drop on your SAN for
> sequential writes with low_latency=1. Can you test if Shaohua's and
> this patch fix allow to recover some bandwidth? I think that enabling
> the queue depth ramp up only if a sync request was delayed should
> disable it for fast hardware like yours, so you should not be seeing
> the slowdown any more.

Average of 10 runs. Low latency set to 0:

Unit information
================
File size = megabytes
Blk Size = bytes
Rate = megabytes per second
CPU% = percentage of CPU used during the test
Latency = milliseconds
Lat% = percent of requests that took longer than X seconds
CPU Eff = Rate divided by CPU% - throughput per cpu load

Sequential Reads
File Blk Num Avg Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
2.6.32 8192 65536 8 90.08 94.51% 51.316 10268.25 0.00000 0.00000 95
2.6.32 8192 65536 16 99.36 199.8% 89.248 13883.81 0.00000 0.00000 50

Random Reads
File Blk Num Avg Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
2.6.32 8192 65536 8 81.16 85.90% 45.672 5963.19 0.00000 0.00000 94
2.6.32 8192 65536 16 116.32 230.3% 58.371 6098.36 0.00000 0.00000 51

Sequential Writes
File Blk Num Avg Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
2.6.32 8192 65536 8 112.17 1085.% 42.623 17114.54 0.00152 0.00000 10
2.6.32 8192 65536 16 111.26 2117.% 84.964 26480.60 0.03202 0.00000 5

Random Writes
File Blk Num Avg Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
2.6.32 8192 65536 8 162.48 887.1% 6.106 313.54 0.00000 0.00000 18
2.6.32 8192 65536 16 156.38 1767.% 14.077 1254.62 0.00000 0.00000 9


Average of 10 runs. Low latency set to 1:

Unit information
================
File size = megabytes
Blk Size = bytes
Rate = megabytes per second
CPU% = percentage of CPU used during the test
Latency = milliseconds
Lat% = percent of requests that took longer than X seconds
CPU Eff = Rate divided by CPU% - throughput per cpu load

Sequential Reads
File Blk Num Avg Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
2.6.32 8192 65536 8 92.23 100.1% 52.119 6920.18 0.00000 0.00000 92
2.6.32 8192 65536 16 97.88 217.0% 99.691 7453.18 0.00000 0.00000 45

Random Reads
File Blk Num Avg Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
2.6.32 8192 65536 8 98.70 107.5% 42.994 3409.08 0.00000 0.00000 92
2.6.32 8192 65536 16 140.41 323.9% 59.616 4525.46 0.00000 0.00000 43

Sequential Writes
File Blk Num Avg Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
2.6.32 8192 65536 8 112.33 1076.% 42.617 17072.28 0.00076 0.00000 10
2.6.32 8192 65536 16 111.84 2097.% 85.156 28221.77 0.02976 0.00000 5

Random Writes
File Blk Num Avg Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
2.6.32 8192 65536 8 159.33 870.5% 6.469 765.50 0.00000 0.00000 18
2.6.32 8192 65536 16 141.60 1632.% 15.364 2337.57 0.00000 0.00000 9


Cheers,
Jeff


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