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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>
wifi-calibration
Namjae Jeon 2013-04-09 14:57:06 +02:00 committed by Jens Axboe
parent 64f8de4da7
commit fdc6fdc52e
1 changed files with 44 additions and 3 deletions
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47
Documentation/block/cfq-iosched.txt
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@ -5,7 +5,7 @@ The main aim of CFQ scheduler is to provide a fair allocation of the disk
I/O bandwidth for all the processes which requests an I/O operation.
CFQ maintains the per process queue for the processes which request I/O
operation(syncronous requests). In case of asynchronous requests, all the
operation(synchronous requests). In case of asynchronous requests, all the
requests from all the processes are batched together according to their
process's I/O priority.
@ -66,6 +66,47 @@ This parameter is used to set the timeout of synchronous requests. Default
value of this is 124ms. In case to favor synchronous requests over asynchronous
one, this value should be decreased relative to fifo_expire_async.
group_idle
-----------
This parameter forces idling at the CFQ group level instead of CFQ
queue level. This was introduced after after a bottleneck was observed
in higher end storage due to idle on sequential queue and allow dispatch
from a single queue. The idea with this parameter is that it can be run with
slice_idle=0 and group_idle=8, so that idling does not happen on individual
queues in the group but happens overall on the group and thus still keeps the
IO controller working.
Not idling on individual queues in the group will dispatch requests from
multiple queues in the group at the same time and achieve higher throughput
on higher end storage.
Default value for this parameter is 8ms.
latency
-------
This parameter is used to enable/disable the latency mode of the CFQ
scheduler. If latency mode (called low_latency) is enabled, CFQ tries
to recompute the slice time for each process based on the target_latency set
for the system. This favors fairness over throughput. Disabling low
latency (setting it to 0) ignores target latency, allowing each process in the
system to get a full time slice.
By default low latency mode is enabled.
target_latency
--------------
This parameter is used to calculate the time slice for a process if cfq's
latency mode is enabled. It will ensure that sync requests have an estimated
latency. But if sequential workload is higher(e.g. sequential read),
then to meet the latency constraints, throughput may decrease because of less
time for each process to issue I/O request before the cfq queue is switched.
Though this can be overcome by disabling the latency_mode, it may increase
the read latency for some applications. This parameter allows for changing
target_latency through the sysfs interface which can provide the balanced
throughput and read latency.
Default value for target_latency is 300ms.
slice_async
-----------
This parameter is same as of slice_sync but for asynchronous queue. The
@ -98,8 +139,8 @@ in the device exceeds this parameter. This parameter is used for synchronous
request.
In case of storage with several disk, this setting can limit the parallel
processing of request. Therefore, increasing the value can imporve the
performace although this can cause the latency of some I/O to increase due
processing of request. Therefore, increasing the value can improve the
performance although this can cause the latency of some I/O to increase due
to more number of requests.
CFQ Group scheduling