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Vivek Goyal6d6ac1c2010-08-23 12:25:29 +02001CFQ ioscheduler tunables
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4slice_idle
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6This specifies how long CFQ should idle for next request on certain cfq queues
7(for sequential workloads) and service trees (for random workloads) before
8queue is expired and CFQ selects next queue to dispatch from.
9
10By default slice_idle is a non-zero value. That means by default we idle on
11queues/service trees. This can be very helpful on highly seeky media like
12single spindle SATA/SAS disks where we can cut down on overall number of
13seeks and see improved throughput.
14
15Setting slice_idle to 0 will remove all the idling on queues/service tree
16level and one should see an overall improved throughput on faster storage
17devices like multiple SATA/SAS disks in hardware RAID configuration. The down
18side is that isolation provided from WRITES also goes down and notion of
19IO priority becomes weaker.
20
21So depending on storage and workload, it might be useful to set slice_idle=0.
22In general I think for SATA/SAS disks and software RAID of SATA/SAS disks
23keeping slice_idle enabled should be useful. For any configurations where
24there are multiple spindles behind single LUN (Host based hardware RAID
25controller or for storage arrays), setting slice_idle=0 might end up in better
26throughput and acceptable latencies.
27
28CFQ IOPS Mode for group scheduling
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30Basic CFQ design is to provide priority based time slices. Higher priority
31process gets bigger time slice and lower priority process gets smaller time
32slice. Measuring time becomes harder if storage is fast and supports NCQ and
33it would be better to dispatch multiple requests from multiple cfq queues in
34request queue at a time. In such scenario, it is not possible to measure time
35consumed by single queue accurately.
36
37What is possible though is to measure number of requests dispatched from a
38single queue and also allow dispatch from multiple cfq queue at the same time.
39This effectively becomes the fairness in terms of IOPS (IO operations per
40second).
41
42If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches
43to IOPS mode and starts providing fairness in terms of number of requests
44dispatched. Note that this mode switching takes effect only for group
45scheduling. For non-cgroup users nothing should change.