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-rw-r--r--Documentation/block/cfq-iosched.txt77
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1CFQ (Complete Fairness Queueing)
2===============================
3
4The main aim of CFQ scheduler is to provide a fair allocation of the disk
5I/O bandwidth for all the processes which requests an I/O operation.
6
7CFQ maintains the per process queue for the processes which request I/O
8operation(syncronous requests). In case of asynchronous requests, all the
9requests from all the processes are batched together according to their
10process's I/O priority.
11
1CFQ ioscheduler tunables 12CFQ ioscheduler tunables
2======================== 13========================
3 14
@@ -25,6 +36,72 @@ there are multiple spindles behind single LUN (Host based hardware RAID
25controller or for storage arrays), setting slice_idle=0 might end up in better 36controller or for storage arrays), setting slice_idle=0 might end up in better
26throughput and acceptable latencies. 37throughput and acceptable latencies.
27 38
39back_seek_max
40-------------
41This specifies, given in Kbytes, the maximum "distance" for backward seeking.
42The distance is the amount of space from the current head location to the
43sectors that are backward in terms of distance.
44
45This parameter allows the scheduler to anticipate requests in the "backward"
46direction and consider them as being the "next" if they are within this
47distance from the current head location.
48
49back_seek_penalty
50-----------------
51This parameter is used to compute the cost of backward seeking. If the
52backward distance of request is just 1/back_seek_penalty from a "front"
53request, then the seeking cost of two requests is considered equivalent.
54
55So scheduler will not bias toward one or the other request (otherwise scheduler
56will bias toward front request). Default value of back_seek_penalty is 2.
57
58fifo_expire_async
59-----------------
60This parameter is used to set the timeout of asynchronous requests. Default
61value of this is 248ms.
62
63fifo_expire_sync
64----------------
65This parameter is used to set the timeout of synchronous requests. Default
66value of this is 124ms. In case to favor synchronous requests over asynchronous
67one, this value should be decreased relative to fifo_expire_async.
68
69slice_async
70-----------
71This parameter is same as of slice_sync but for asynchronous queue. The
72default value is 40ms.
73
74slice_async_rq
75--------------
76This parameter is used to limit the dispatching of asynchronous request to
77device request queue in queue's slice time. The maximum number of request that
78are allowed to be dispatched also depends upon the io priority. Default value
79for this is 2.
80
81slice_sync
82----------
83When a queue is selected for execution, the queues IO requests are only
84executed for a certain amount of time(time_slice) before switching to another
85queue. This parameter is used to calculate the time slice of synchronous
86queue.
87
88time_slice is computed using the below equation:-
89time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the
90time_slice of synchronous queue, increase the value of slice_sync. Default
91value is 100ms.
92
93quantum
94-------
95This specifies the number of request dispatched to the device queue. In a
96queue's time slice, a request will not be dispatched if the number of request
97in the device exceeds this parameter. This parameter is used for synchronous
98request.
99
100In case of storage with several disk, this setting can limit the parallel
101processing of request. Therefore, increasing the value can imporve the
102performace although this can cause the latency of some I/O to increase due
103to more number of requests.
104
28CFQ IOPS Mode for group scheduling 105CFQ IOPS Mode for group scheduling
29=================================== 106===================================
30Basic CFQ design is to provide priority based time slices. Higher priority 107Basic CFQ design is to provide priority based time slices. Higher priority