cfq-iosched: Documentation help for new tunables

CFQ ioscheduler tunables

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slice_idle

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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

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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 sysfs tunable

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/sys/block/<disk>/queue/iosched/group_isolation

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If group_isolation=1, it provides stronger isolation between groups at the

expense of throughput. By default group_isolation is 0. In general that

and one wants stronger isolation between groups, then set group_isolation=1

but this will come at cost of reduced throughput.

/sys/block/<disk>/queue/iosched/slice_idle

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On a faster hardware CFQ can be slow, especially with sequential workload.

This happens because CFQ idles on a single queue and single queue might not

drive deeper request queue depths to keep the storage busy. In such scenarios

one can try setting slice_idle=0 and that would switch CFQ to IOPS

(IO operations per second) mode on NCQ supporting hardware.

That means CFQ will not idle between cfq queues of a cfq group and hence be

able to driver higher queue depth and achieve better throughput. That also

means that cfq provides fairness among groups in terms of IOPS and not in

terms of disk time.

/sys/block/<disk>/queue/iosched/group_idle

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If one disables idling on individual cfq queues and cfq service trees by

setting slice_idle=0, group_idle kicks in. That means CFQ will still idle

on the group in an attempt to provide fairness among groups.

By default group_idle is same as slice_idle and does not do anything if

slice_idle is enabled.

One can experience an overall throughput drop if you have created multiple

groups and put applications in that group which are not driving enough

IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle

on individual groups and throughput should improve.

What works

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- Currently only sync IO queues are support. All the buffered writes are