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Some documentation to provide help with tunables. Signed-off-by: Vivek Goyal <vgoyal@redhat.com> Acked-by: Jeff Moyer <jmoyer@redhat.com> Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
46 lines
2.2 KiB
Plaintext
46 lines
2.2 KiB
Plaintext
CFQ ioscheduler tunables
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========================
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slice_idle
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----------
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This specifies how long CFQ should idle for next request on certain cfq queues
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(for sequential workloads) and service trees (for random workloads) before
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queue is expired and CFQ selects next queue to dispatch from.
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By default slice_idle is a non-zero value. That means by default we idle on
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queues/service trees. This can be very helpful on highly seeky media like
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single spindle SATA/SAS disks where we can cut down on overall number of
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seeks and see improved throughput.
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Setting slice_idle to 0 will remove all the idling on queues/service tree
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level and one should see an overall improved throughput on faster storage
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devices like multiple SATA/SAS disks in hardware RAID configuration. The down
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side is that isolation provided from WRITES also goes down and notion of
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IO priority becomes weaker.
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So depending on storage and workload, it might be useful to set slice_idle=0.
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In general I think for SATA/SAS disks and software RAID of SATA/SAS disks
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keeping slice_idle enabled should be useful. For any configurations where
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there are multiple spindles behind single LUN (Host based hardware RAID
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controller or for storage arrays), setting slice_idle=0 might end up in better
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throughput and acceptable latencies.
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CFQ IOPS Mode for group scheduling
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===================================
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Basic CFQ design is to provide priority based time slices. Higher priority
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process gets bigger time slice and lower priority process gets smaller time
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slice. Measuring time becomes harder if storage is fast and supports NCQ and
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it would be better to dispatch multiple requests from multiple cfq queues in
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request queue at a time. In such scenario, it is not possible to measure time
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consumed by single queue accurately.
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What is possible though is to measure number of requests dispatched from a
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single queue and also allow dispatch from multiple cfq queue at the same time.
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This effectively becomes the fairness in terms of IOPS (IO operations per
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second).
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If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches
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to IOPS mode and starts providing fairness in terms of number of requests
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dispatched. Note that this mode switching takes effect only for group
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scheduling. For non-cgroup users nothing should change.
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