Documentation: update tunable options in block/cfq-iosched.txt

Update tunable options in block/cfq-iosched.txt. Signed-off-by: Namjae Jeon <linkinjeon@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation: update tunable options in block/cfq-iosched.txt
Update tunable options in block/cfq-iosched.txt. Signed-off-by: Namjae Jeon <linkinjeon@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Namjae Jeon · Jens Axboe
1 parent 865826644e
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Documentation/block/cfq-iosched.txt
+CFQ (Complete Fairness Queueing)
+===============================
+
+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
+requests from all the processes are batched together according to their
+process's I/O priority.
+
 CFQ ioscheduler tunables
 ========================
  
@@ -24,6 +35,72 @@
 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.
+
+back_seek_max
+-------------
+This specifies, given in Kbytes, the maximum "distance" for backward seeking.
+The distance is the amount of space from the current head location to the
+sectors that are backward in terms of distance.
+
+This parameter allows the scheduler to anticipate requests in the "backward"
+direction and consider them as being the "next" if they are within this
+distance from the current head location.
+
+back_seek_penalty
+-----------------
+This parameter is used to compute the cost of backward seeking. If the
+backward distance of request is just 1/back_seek_penalty from a "front"
+request, then the seeking cost of two requests is considered equivalent.
+
+So scheduler will not bias toward one or the other request (otherwise scheduler
+will bias toward front request). Default value of back_seek_penalty is 2.
+
+fifo_expire_async
+-----------------
+This parameter is used to set the timeout of asynchronous requests. Default
+value of this is 248ms.
+
+fifo_expire_sync
+----------------
+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.
+
+slice_async
+-----------
+This parameter is same as of slice_sync but for asynchronous queue. The
+default value is 40ms.
+
+slice_async_rq
+--------------
+This parameter is used to limit the dispatching of asynchronous request to
+device request queue in queue's slice time. The maximum number of request that
+are allowed to be dispatched also depends upon the io priority. Default value
+for this is 2.
+
+slice_sync
+----------
+When a queue is selected for execution, the queues IO requests are only
+executed for a certain amount of time(time_slice) before switching to another
+queue. This parameter is used to calculate the time slice of synchronous
+queue.
+
+time_slice is computed using the below equation:-
+time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the
+time_slice of synchronous queue, increase the value of slice_sync. Default
+value is 100ms.
+
+quantum
+-------
+This specifies the number of request dispatched to the device queue. In a
+queue's time slice, a request will not be dispatched if the number of request
+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
+to more number of requests.
  
 CFQ IOPS Mode for group scheduling
 ===================================
	1	+CFQ (Complete Fairness Queueing)
	2	+===============================
	3	+
	4	+The main aim of CFQ scheduler is to provide a fair allocation of the disk
	5	+I/O bandwidth for all the processes which requests an I/O operation.
	6	+
	7	+CFQ maintains the per process queue for the processes which request I/O
	8	+operation(syncronous requests). In case of asynchronous requests, all the
	9	+requests from all the processes are batched together according to their
	10	+process's I/O priority.
	11	+
1	12	CFQ ioscheduler tunables
2	13	========================
3	14
...	...	@@ -24,6 +35,72 @@
24	35	there are multiple spindles behind single LUN (Host based hardware RAID
25	36	controller or for storage arrays), setting slice_idle=0 might end up in better
26	37	throughput and acceptable latencies.
	38	+
	39	+back_seek_max
	40	+-------------
	41	+This specifies, given in Kbytes, the maximum "distance" for backward seeking.
	42	+The distance is the amount of space from the current head location to the
	43	+sectors that are backward in terms of distance.
	44	+
	45	+This parameter allows the scheduler to anticipate requests in the "backward"
	46	+direction and consider them as being the "next" if they are within this
	47	+distance from the current head location.
	48	+
	49	+back_seek_penalty
	50	+-----------------
	51	+This parameter is used to compute the cost of backward seeking. If the
	52	+backward distance of request is just 1/back_seek_penalty from a "front"
	53	+request, then the seeking cost of two requests is considered equivalent.
	54	+
	55	+So scheduler will not bias toward one or the other request (otherwise scheduler
	56	+will bias toward front request). Default value of back_seek_penalty is 2.
	57	+
	58	+fifo_expire_async
	59	+-----------------
	60	+This parameter is used to set the timeout of asynchronous requests. Default
	61	+value of this is 248ms.
	62	+
	63	+fifo_expire_sync
	64	+----------------
	65	+This parameter is used to set the timeout of synchronous requests. Default
	66	+value of this is 124ms. In case to favor synchronous requests over asynchronous
	67	+one, this value should be decreased relative to fifo_expire_async.
	68	+
	69	+slice_async
	70	+-----------
	71	+This parameter is same as of slice_sync but for asynchronous queue. The
	72	+default value is 40ms.
	73	+
	74	+slice_async_rq
	75	+--------------
	76	+This parameter is used to limit the dispatching of asynchronous request to
	77	+device request queue in queue's slice time. The maximum number of request that
	78	+are allowed to be dispatched also depends upon the io priority. Default value
	79	+for this is 2.
	80	+
	81	+slice_sync
	82	+----------
	83	+When a queue is selected for execution, the queues IO requests are only
	84	+executed for a certain amount of time(time_slice) before switching to another
	85	+queue. This parameter is used to calculate the time slice of synchronous
	86	+queue.
	87	+
	88	+time_slice is computed using the below equation:-
	89	+time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the
	90	+time_slice of synchronous queue, increase the value of slice_sync. Default
	91	+value is 100ms.
	92	+
	93	+quantum
	94	+-------
	95	+This specifies the number of request dispatched to the device queue. In a
	96	+queue's time slice, a request will not be dispatched if the number of request
	97	+in the device exceeds this parameter. This parameter is used for synchronous
	98	+request.
	99	+
	100	+In case of storage with several disk, this setting can limit the parallel
	101	+processing of request. Therefore, increasing the value can imporve the
	102	+performace although this can cause the latency of some I/O to increase due
	103	+to more number of requests.
27	104
28	105	CFQ IOPS Mode for group scheduling
29	106	===================================