Documentation/block/cfq-iosched.txt - kernel/msm - Gitiles

 CFQ ioscheduler tunables
 ========================

 slice_idle
 ----------
 This specifies how long CFQ should idle for next request on certain cfq queues
 (for sequential workloads) and service trees (for random workloads) before
 queue is expired and CFQ selects next queue to dispatch from.

 By default slice_idle is a non-zero value. That means by default we idle on
 queues/service trees. This can be very helpful on highly seeky media like
 single spindle SATA/SAS disks where we can cut down on overall number of
 seeks and see improved throughput.

 Setting slice_idle to 0 will remove all the idling on queues/service tree
 level and one should see an overall improved throughput on faster storage
 devices like multiple SATA/SAS disks in hardware RAID configuration. The down
 side is that isolation provided from WRITES also goes down and notion of
 IO priority becomes weaker.

 So depending on storage and workload, it might be useful to set slice_idle=0.
 In general I think for SATA/SAS disks and software RAID of SATA/SAS disks
 keeping slice_idle enabled should be useful. For any configurations where
 there are multiple spindles behind single LUN (Host based hardware RAID
 controller or for storage arrays), setting slice_idle=0 might end up in better
 throughput and acceptable latencies.

 CFQ IOPS Mode for group scheduling
 ===================================
 Basic CFQ design is to provide priority based time slices. Higher priority
 process gets bigger time slice and lower priority process gets smaller time
 slice. Measuring time becomes harder if storage is fast and supports NCQ and
 it would be better to dispatch multiple requests from multiple cfq queues in
 request queue at a time. In such scenario, it is not possible to measure time
 consumed by single queue accurately.

 What is possible though is to measure number of requests dispatched from a
 single queue and also allow dispatch from multiple cfq queue at the same time.
 This effectively becomes the fairness in terms of IOPS (IO operations per
 second).

 If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches
 to IOPS mode and starts providing fairness in terms of number of requests
 dispatched. Note that this mode switching takes effect only for group
 scheduling. For non-cgroup users nothing should change.
	CFQ ioscheduler tunables
	========================

	slice_idle
	----------
	This specifies how long CFQ should idle for next request on certain cfq queues
	(for sequential workloads) and service trees (for random workloads) before
	queue is expired and CFQ selects next queue to dispatch from.

	By default slice_idle is a non-zero value. That means by default we idle on
	queues/service trees. This can be very helpful on highly seeky media like
	single spindle SATA/SAS disks where we can cut down on overall number of
	seeks and see improved throughput.

	Setting slice_idle to 0 will remove all the idling on queues/service tree
	level and one should see an overall improved throughput on faster storage
	devices like multiple SATA/SAS disks in hardware RAID configuration. The down
	side is that isolation provided from WRITES also goes down and notion of
	IO priority becomes weaker.

	So depending on storage and workload, it might be useful to set slice_idle=0.
	In general I think for SATA/SAS disks and software RAID of SATA/SAS disks
	keeping slice_idle enabled should be useful. For any configurations where
	there are multiple spindles behind single LUN (Host based hardware RAID
	controller or for storage arrays), setting slice_idle=0 might end up in better
	throughput and acceptable latencies.

	CFQ IOPS Mode for group scheduling
	===================================
	Basic CFQ design is to provide priority based time slices. Higher priority
	process gets bigger time slice and lower priority process gets smaller time
	slice. Measuring time becomes harder if storage is fast and supports NCQ and
	it would be better to dispatch multiple requests from multiple cfq queues in
	request queue at a time. In such scenario, it is not possible to measure time
	consumed by single queue accurately.

	What is possible though is to measure number of requests dispatched from a
	single queue and also allow dispatch from multiple cfq queue at the same time.
	This effectively becomes the fairness in terms of IOPS (IO operations per
	second).

	If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches
	to IOPS mode and starts providing fairness in terms of number of requests
	dispatched. Note that this mode switching takes effect only for group
	scheduling. For non-cgroup users nothing should change.