117 lines
5.7 KiB
Plaintext
117 lines
5.7 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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CFQ IO scheduler Idling Theory
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===============================
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Idling on a queue is primarily about waiting for the next request to come
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on same queue after completion of a request. In this process CFQ will not
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dispatch requests from other cfq queues even if requests are pending there.
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The rationale behind idling is that it can cut down on number of seeks
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on rotational media. For example, if a process is doing dependent
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sequential reads (next read will come on only after completion of previous
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one), then not dispatching request from other queue should help as we
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did not move the disk head and kept on dispatching sequential IO from
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one queue.
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CFQ has following service trees and various queues are put on these trees.
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sync-idle sync-noidle async
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All cfq queues doing synchronous sequential IO go on to sync-idle tree.
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On this tree we idle on each queue individually.
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All synchronous non-sequential queues go on sync-noidle tree. Also any
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request which are marked with REQ_NOIDLE go on this service tree. On this
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tree we do not idle on individual queues instead idle on the whole group
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of queues or the tree. So if there are 4 queues waiting for IO to dispatch
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we will idle only once last queue has dispatched the IO and there is
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no more IO on this service tree.
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All async writes go on async service tree. There is no idling on async
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queues.
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CFQ has some optimizations for SSDs and if it detects a non-rotational
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media which can support higher queue depth (multiple requests at in
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flight at a time), then it cuts down on idling of individual queues and
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all the queues move to sync-noidle tree and only tree idle remains. This
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tree idling provides isolation with buffered write queues on async tree.
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FAQ
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===
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Q1. Why to idle at all on queues marked with REQ_NOIDLE.
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A1. We only do tree idle (all queues on sync-noidle tree) on queues marked
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with REQ_NOIDLE. This helps in providing isolation with all the sync-idle
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queues. Otherwise in presence of many sequential readers, other
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synchronous IO might not get fair share of disk.
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For example, if there are 10 sequential readers doing IO and they get
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100ms each. If a REQ_NOIDLE request comes in, it will be scheduled
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roughly after 1 second. If after completion of REQ_NOIDLE request we
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do not idle, and after a couple of milli seconds a another REQ_NOIDLE
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request comes in, again it will be scheduled after 1second. Repeat it
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and notice how a workload can lose its disk share and suffer due to
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multiple sequential readers.
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fsync can generate dependent IO where bunch of data is written in the
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context of fsync, and later some journaling data is written. Journaling
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data comes in only after fsync has finished its IO (atleast for ext4
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that seemed to be the case). Now if one decides not to idle on fsync
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thread due to REQ_NOIDLE, then next journaling write will not get
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scheduled for another second. A process doing small fsync, will suffer
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badly in presence of multiple sequential readers.
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Hence doing tree idling on threads using REQ_NOIDLE flag on requests
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provides isolation from multiple sequential readers and at the same
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time we do not idle on individual threads.
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Q2. When to specify REQ_NOIDLE
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A2. I would think whenever one is doing synchronous write and not expecting
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more writes to be dispatched from same context soon, should be able
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to specify REQ_NOIDLE on writes and that probably should work well for
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most of the cases.
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