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Diffstat (limited to 'Documentation/block/cfq-iosched.txt')
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diff --git a/Documentation/block/cfq-iosched.txt b/Documentation/block/cfq-iosched.txt deleted file mode 100644 index 6d670f5..0000000 --- a/Documentation/block/cfq-iosched.txt +++ /dev/null @@ -1,116 +0,0 @@ -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. - -CFQ IO scheduler Idling Theory -=============================== -Idling on a queue is primarily about waiting for the next request to come -on same queue after completion of a request. In this process CFQ will not -dispatch requests from other cfq queues even if requests are pending there. - -The rationale behind idling is that it can cut down on number of seeks -on rotational media. For example, if a process is doing dependent -sequential reads (next read will come on only after completion of previous -one), then not dispatching request from other queue should help as we -did not move the disk head and kept on dispatching sequential IO from -one queue. - -CFQ has following service trees and various queues are put on these trees. - - sync-idle sync-noidle async - -All cfq queues doing synchronous sequential IO go on to sync-idle tree. -On this tree we idle on each queue individually. - -All synchronous non-sequential queues go on sync-noidle tree. Also any -request which are marked with REQ_NOIDLE go on this service tree. On this -tree we do not idle on individual queues instead idle on the whole group -of queues or the tree. So if there are 4 queues waiting for IO to dispatch -we will idle only once last queue has dispatched the IO and there is -no more IO on this service tree. - -All async writes go on async service tree. There is no idling on async -queues. - -CFQ has some optimizations for SSDs and if it detects a non-rotational -media which can support higher queue depth (multiple requests at in -flight at a time), then it cuts down on idling of individual queues and -all the queues move to sync-noidle tree and only tree idle remains. This -tree idling provides isolation with buffered write queues on async tree. - -FAQ -=== -Q1. Why to idle at all on queues marked with REQ_NOIDLE. - -A1. We only do tree idle (all queues on sync-noidle tree) on queues marked - with REQ_NOIDLE. This helps in providing isolation with all the sync-idle - queues. Otherwise in presence of many sequential readers, other - synchronous IO might not get fair share of disk. - - For example, if there are 10 sequential readers doing IO and they get - 100ms each. If a REQ_NOIDLE request comes in, it will be scheduled - roughly after 1 second. If after completion of REQ_NOIDLE request we - do not idle, and after a couple of milli seconds a another REQ_NOIDLE - request comes in, again it will be scheduled after 1second. Repeat it - and notice how a workload can lose its disk share and suffer due to - multiple sequential readers. - - fsync can generate dependent IO where bunch of data is written in the - context of fsync, and later some journaling data is written. Journaling - data comes in only after fsync has finished its IO (atleast for ext4 - that seemed to be the case). Now if one decides not to idle on fsync - thread due to REQ_NOIDLE, then next journaling write will not get - scheduled for another second. A process doing small fsync, will suffer - badly in presence of multiple sequential readers. - - Hence doing tree idling on threads using REQ_NOIDLE flag on requests - provides isolation from multiple sequential readers and at the same - time we do not idle on individual threads. - -Q2. When to specify REQ_NOIDLE -A2. I would think whenever one is doing synchronous write and not expecting - more writes to be dispatched from same context soon, should be able - to specify REQ_NOIDLE on writes and that probably should work well for - most of the cases. |