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- 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.
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