Documentation: cfq-iosched: update documentation help for cfq tunables
Add the documentation text for latency, target_latency & group_idle tunnable parameters in the block/cfq-iosched.txt. Also fix few typo(spelling) mistakes. Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com> Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com> Language somewhat modified by Jens. Signed-off-by: Jens Axboe <axboe@kernel.dk>
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@ -5,7 +5,7 @@ The main aim of CFQ scheduler is to provide a fair allocation of the disk
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I/O bandwidth for all the processes which requests an I/O operation.
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CFQ maintains the per process queue for the processes which request I/O
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operation(syncronous requests). In case of asynchronous requests, all the
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operation(synchronous requests). In case of asynchronous requests, all the
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requests from all the processes are batched together according to their
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process's I/O priority.
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@ -66,6 +66,47 @@ This parameter is used to set the timeout of synchronous requests. Default
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value of this is 124ms. In case to favor synchronous requests over asynchronous
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one, this value should be decreased relative to fifo_expire_async.
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group_idle
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-----------
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This parameter forces idling at the CFQ group level instead of CFQ
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queue level. This was introduced after after a bottleneck was observed
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in higher end storage due to idle on sequential queue and allow dispatch
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from a single queue. The idea with this parameter is that it can be run with
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slice_idle=0 and group_idle=8, so that idling does not happen on individual
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queues in the group but happens overall on the group and thus still keeps the
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IO controller working.
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Not idling on individual queues in the group will dispatch requests from
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multiple queues in the group at the same time and achieve higher throughput
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on higher end storage.
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Default value for this parameter is 8ms.
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latency
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-------
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This parameter is used to enable/disable the latency mode of the CFQ
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scheduler. If latency mode (called low_latency) is enabled, CFQ tries
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to recompute the slice time for each process based on the target_latency set
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for the system. This favors fairness over throughput. Disabling low
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latency (setting it to 0) ignores target latency, allowing each process in the
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system to get a full time slice.
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By default low latency mode is enabled.
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target_latency
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--------------
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This parameter is used to calculate the time slice for a process if cfq's
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latency mode is enabled. It will ensure that sync requests have an estimated
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latency. But if sequential workload is higher(e.g. sequential read),
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then to meet the latency constraints, throughput may decrease because of less
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time for each process to issue I/O request before the cfq queue is switched.
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Though this can be overcome by disabling the latency_mode, it may increase
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the read latency for some applications. This parameter allows for changing
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target_latency through the sysfs interface which can provide the balanced
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throughput and read latency.
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Default value for target_latency is 300ms.
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slice_async
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-----------
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This parameter is same as of slice_sync but for asynchronous queue. The
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@ -98,8 +139,8 @@ in the device exceeds this parameter. This parameter is used for synchronous
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request.
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In case of storage with several disk, this setting can limit the parallel
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processing of request. Therefore, increasing the value can imporve the
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performace although this can cause the latency of some I/O to increase due
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processing of request. Therefore, increasing the value can improve the
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performance although this can cause the latency of some I/O to increase due
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to more number of requests.
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CFQ Group scheduling
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