1CFQ ioscheduler tunables 2======================== 3 4slice_idle 5---------- 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 29=================================== 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. 46