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Subject[RFC PATCH] cfq-iosced: Implement IOPS mode and group_idle tunable V3
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Hi,

This is V3 of the group_idle and CFQ IOPS mode implementation patchset. Since V2
I have cleaned up the code a bit to clarify the confusion lingering around in
what cases do we charge time slice and in what cases do we charge number of
requests.

What's the problem
------------------
On high end storage (I got on HP EVA storage array with 12 SATA disks in
RAID 5), CFQ's model of dispatching requests from a single queue at a
time (sequential readers/write sync writers etc), becomes a bottleneck.
Often we don't drive enough request queue depth to keep all the disks busy
and suffer a lot in terms of overall throughput.
All these problems primarily originate from two things. Idling on per
cfq queue and quantum (dispatching limited number of requests from a
single queue) and till then not allowing dispatch from other queues. Once
you set the slice_idle=0 and quantum to higher value, most of the CFQ's
problem on higher end storage disappear.

This problem also becomes visible in IO controller where one creates
multiple groups and gets the fairness but overall throughput is less. In
the following table, I am running increasing number of sequential readers
(1,2,4,8) in 8 groups of weight 100 to 800.

Kernel=2.6.35-rc5-iops+
GROUPMODE=1 NRGRP=8
DIR=/mnt/iostestmnt/fio DEV=/dev/dm-4
Workload=bsr iosched=cfq Filesz=512M bs=4K
group_isolation=1 slice_idle=8 group_idle=8 quantum=8
=========================================================================
AVERAGE[bsr] [bw in KB/s]
-------
job Set NR cgrp1 cgrp2 cgrp3 cgrp4 cgrp5 cgrp6 cgrp7 cgrp8 total
--- --- -- ---------------------------------------------------------------
bsr 3 1 6186 12752 16568 23068 28608 35785 42322 48409 213701
bsr 3 2 5396 10902 16959 23471 25099 30643 37168 42820 192461
bsr 3 4 4655 9463 14042 20537 24074 28499 34679 37895 173847
bsr 3 8 4418 8783 12625 19015 21933 26354 29830 36290 159249
Notice that overall throughput is just around 160MB/s with 8 sequential reader
in each group.

With this patch set, I have set slice_idle=0 and re-ran same test.

Kernel=2.6.35-rc5-iops+
GROUPMODE=1 NRGRP=8
DIR=/mnt/iostestmnt/fio DEV=/dev/dm-4
Workload=bsr iosched=cfq Filesz=512M bs=4K
group_isolation=1 slice_idle=0 group_idle=8 quantum=8
=========================================================================
AVERAGE[bsr] [bw in KB/s]
-------
job Set NR cgrp1 cgrp2 cgrp3 cgrp4 cgrp5 cgrp6 cgrp7 cgrp8 total
--- --- -- ---------------------------------------------------------------
bsr 3 1 6523 12399 18116 24752 30481 36144 42185 48894 219496
bsr 3 2 10072 20078 29614 38378 46354 52513 58315 64833 320159
bsr 3 4 11045 22340 33013 44330 52663 58254 63883 70990 356520
bsr 3 8 12362 25860 37920 47486 61415 47292 45581 70828 348747
Notice how overall throughput has shot upto 348MB/s while retaining the ability
to do the IO control.

So this is not the default mode. This new tunable group_idle, allows one to
set slice_idle=0 to disable some of the CFQ features and and use primarily
group service differentation feature.

If you have thoughts on other ways of solving the problem, I am all ears
to it.

Thanks
Vivek


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