46 строки
2.2 KiB
Plaintext
46 строки
2.2 KiB
Plaintext
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.
|