4931402a9d
There are always questions about why CFQ is idling on various conditions. Recent ones is Christoph asking again why to idle on REQ_NOIDLE. His assertion is that XFS is relying more and more on workqueues and is concerned that CFQ idling on IO from every workqueue will impact XFS badly. So he suggested that I add some more documentation about CFQ idling and that can provide more clarity on the topic and also gives an opprotunity to poke a hole in theory and lead to improvements. So here is my attempt at that. Any comments are welcome. Signed-off-by: Vivek Goyal <vgoyal@redhat.com> Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
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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