[net-next-2.6.git] / Documentation / block / cfq-iosched.txt

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.
Commit	Line	Data
6d6ac1c1 VG	1	CFQ ioscheduler tunables
	2	========================
	3
	4	slice_idle
	5	----------
	6	This specifies how long CFQ should idle for next request on certain cfq queues
	7	(for sequential workloads) and service trees (for random workloads) before
	8	queue is expired and CFQ selects next queue to dispatch from.
	9
	10	By default slice_idle is a non-zero value. That means by default we idle on
	11	queues/service trees. This can be very helpful on highly seeky media like
	12	single spindle SATA/SAS disks where we can cut down on overall number of
	13	seeks and see improved throughput.
	14
	15	Setting slice_idle to 0 will remove all the idling on queues/service tree
	16	level and one should see an overall improved throughput on faster storage
	17	devices like multiple SATA/SAS disks in hardware RAID configuration. The down
	18	side is that isolation provided from WRITES also goes down and notion of
	19	IO priority becomes weaker.
	20
	21	So depending on storage and workload, it might be useful to set slice_idle=0.
	22	In general I think for SATA/SAS disks and software RAID of SATA/SAS disks
	23	keeping slice_idle enabled should be useful. For any configurations where
	24	there are multiple spindles behind single LUN (Host based hardware RAID
	25	controller or for storage arrays), setting slice_idle=0 might end up in better
	26	throughput and acceptable latencies.
	27
	28	CFQ IOPS Mode for group scheduling
	29	===================================
	30	Basic CFQ design is to provide priority based time slices. Higher priority
	31	process gets bigger time slice and lower priority process gets smaller time
	32	slice. Measuring time becomes harder if storage is fast and supports NCQ and
	33	it would be better to dispatch multiple requests from multiple cfq queues in
	34	request queue at a time. In such scenario, it is not possible to measure time
	35	consumed by single queue accurately.
	36
	37	What is possible though is to measure number of requests dispatched from a
	38	single queue and also allow dispatch from multiple cfq queue at the same time.
	39	This effectively becomes the fairness in terms of IOPS (IO operations per
	40	second).
	41
	42	If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches
	43	to IOPS mode and starts providing fairness in terms of number of requests
	44	dispatched. Note that this mode switching takes effect only for group
	45	scheduling. For non-cgroup users nothing should change.