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Documentation / block / cfq-iosched.txt




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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:39 EST.

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(synchronous 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	
12	CFQ ioscheduler tunables
13	========================
14	
15	slice_idle
16	----------
17	This specifies how long CFQ should idle for next request on certain cfq queues
18	(for sequential workloads) and service trees (for random workloads) before
19	queue is expired and CFQ selects next queue to dispatch from.
20	
21	By default slice_idle is a non-zero value. That means by default we idle on
22	queues/service trees. This can be very helpful on highly seeky media like
23	single spindle SATA/SAS disks where we can cut down on overall number of
24	seeks and see improved throughput.
25	
26	Setting slice_idle to 0 will remove all the idling on queues/service tree
27	level and one should see an overall improved throughput on faster storage
28	devices like multiple SATA/SAS disks in hardware RAID configuration. The down
29	side is that isolation provided from WRITES also goes down and notion of
30	IO priority becomes weaker.
31	
32	So depending on storage and workload, it might be useful to set slice_idle=0.
33	In general I think for SATA/SAS disks and software RAID of SATA/SAS disks
34	keeping slice_idle enabled should be useful. For any configurations where
35	there are multiple spindles behind single LUN (Host based hardware RAID
36	controller or for storage arrays), setting slice_idle=0 might end up in better
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	group_idle
70	-----------
71	This parameter forces idling at the CFQ group level instead of CFQ
72	queue level. This was introduced after a bottleneck was observed
73	in higher end storage due to idle on sequential queue and allow dispatch
74	from a single queue. The idea with this parameter is that it can be run with
75	slice_idle=0 and group_idle=8, so that idling does not happen on individual
76	queues in the group but happens overall on the group and thus still keeps the
77	IO controller working.
78	Not idling on individual queues in the group will dispatch requests from
79	multiple queues in the group at the same time and achieve higher throughput
80	on higher end storage.
81	
82	Default value for this parameter is 8ms.
83	
84	low_latency
85	-----------
86	This parameter is used to enable/disable the low latency mode of the CFQ
87	scheduler. If enabled, CFQ tries to recompute the slice time for each process
88	based on the target_latency set for the system. This favors fairness over
89	throughput. Disabling low latency (setting it to 0) ignores target latency,
90	allowing each process in the system to get a full time slice.
91	
92	By default low latency mode is enabled.
93	
94	target_latency
95	--------------
96	This parameter is used to calculate the time slice for a process if cfq's
97	latency mode is enabled. It will ensure that sync requests have an estimated
98	latency. But if sequential workload is higher(e.g. sequential read),
99	then to meet the latency constraints, throughput may decrease because of less
100	time for each process to issue I/O request before the cfq queue is switched.
101	
102	Though this can be overcome by disabling the latency_mode, it may increase
103	the read latency for some applications. This parameter allows for changing
104	target_latency through the sysfs interface which can provide the balanced
105	throughput and read latency.
106	
107	Default value for target_latency is 300ms.
108	
109	slice_async
110	-----------
111	This parameter is same as of slice_sync but for asynchronous queue. The
112	default value is 40ms.
113	
114	slice_async_rq
115	--------------
116	This parameter is used to limit the dispatching of asynchronous request to
117	device request queue in queue's slice time. The maximum number of request that
118	are allowed to be dispatched also depends upon the io priority. Default value
119	for this is 2.
120	
121	slice_sync
122	----------
123	When a queue is selected for execution, the queues IO requests are only
124	executed for a certain amount of time(time_slice) before switching to another
125	queue. This parameter is used to calculate the time slice of synchronous
126	queue.
127	
128	time_slice is computed using the below equation:-
129	time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the
130	time_slice of synchronous queue, increase the value of slice_sync. Default
131	value is 100ms.
132	
133	quantum
134	-------
135	This specifies the number of request dispatched to the device queue. In a
136	queue's time slice, a request will not be dispatched if the number of request
137	in the device exceeds this parameter. This parameter is used for synchronous
138	request.
139	
140	In case of storage with several disk, this setting can limit the parallel
141	processing of request. Therefore, increasing the value can improve the
142	performance although this can cause the latency of some I/O to increase due
143	to more number of requests.
144	
145	CFQ Group scheduling
146	====================
147	
148	CFQ supports blkio cgroup and has "blkio." prefixed files in each
149	blkio cgroup directory. It is weight-based and there are four knobs
150	for configuration - weight[_device] and leaf_weight[_device].
151	Internal cgroup nodes (the ones with children) can also have tasks in
152	them, so the former two configure how much proportion the cgroup as a
153	whole is entitled to at its parent's level while the latter two
154	configure how much proportion the tasks in the cgroup have compared to
155	its direct children.
156	
157	Another way to think about it is assuming that each internal node has
158	an implicit leaf child node which hosts all the tasks whose weight is
159	configured by leaf_weight[_device]. Let's assume a blkio hierarchy
160	composed of five cgroups - root, A, B, AA and AB - with the following
161	weights where the names represent the hierarchy.
162	
163	        weight leaf_weight
164	 root :  125    125
165	 A    :  500    750
166	 B    :  250    500
167	 AA   :  500    500
168	 AB   : 1000    500
169	
170	root never has a parent making its weight is meaningless. For backward
171	compatibility, weight is always kept in sync with leaf_weight. B, AA
172	and AB have no child and thus its tasks have no children cgroup to
173	compete with. They always get 100% of what the cgroup won at the
174	parent level. Considering only the weights which matter, the hierarchy
175	looks like the following.
176	
177	          root
178	       /    |   \
179	      A     B    leaf
180	     500   250   125
181	   /  |  \
182	  AA  AB  leaf
183	 500 1000 750
184	
185	If all cgroups have active IOs and competing with each other, disk
186	time will be distributed like the following.
187	
188	Distribution below root. The total active weight at this level is
189	A:500 + B:250 + C:125 = 875.
190	
191	 root-leaf :   125 /  875      =~ 14%
192	 A         :   500 /  875      =~ 57%
193	 B(-leaf)  :   250 /  875      =~ 28%
194	
195	A has children and further distributes its 57% among the children and
196	the implicit leaf node. The total active weight at this level is
197	AA:500 + AB:1000 + A-leaf:750 = 2250.
198	
199	 A-leaf    : ( 750 / 2250) * A =~ 19%
200	 AA(-leaf) : ( 500 / 2250) * A =~ 12%
201	 AB(-leaf) : (1000 / 2250) * A =~ 25%
202	
203	CFQ IOPS Mode for group scheduling
204	===================================
205	Basic CFQ design is to provide priority based time slices. Higher priority
206	process gets bigger time slice and lower priority process gets smaller time
207	slice. Measuring time becomes harder if storage is fast and supports NCQ and
208	it would be better to dispatch multiple requests from multiple cfq queues in
209	request queue at a time. In such scenario, it is not possible to measure time
210	consumed by single queue accurately.
211	
212	What is possible though is to measure number of requests dispatched from a
213	single queue and also allow dispatch from multiple cfq queue at the same time.
214	This effectively becomes the fairness in terms of IOPS (IO operations per
215	second).
216	
217	If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches
218	to IOPS mode and starts providing fairness in terms of number of requests
219	dispatched. Note that this mode switching takes effect only for group
220	scheduling. For non-cgroup users nothing should change.
221	
222	CFQ IO scheduler Idling Theory
223	===============================
224	Idling on a queue is primarily about waiting for the next request to come
225	on same queue after completion of a request. In this process CFQ will not
226	dispatch requests from other cfq queues even if requests are pending there.
227	
228	The rationale behind idling is that it can cut down on number of seeks
229	on rotational media. For example, if a process is doing dependent
230	sequential reads (next read will come on only after completion of previous
231	one), then not dispatching request from other queue should help as we
232	did not move the disk head and kept on dispatching sequential IO from
233	one queue.
234	
235	CFQ has following service trees and various queues are put on these trees.
236	
237		sync-idle	sync-noidle	async
238	
239	All cfq queues doing synchronous sequential IO go on to sync-idle tree.
240	On this tree we idle on each queue individually.
241	
242	All synchronous non-sequential queues go on sync-noidle tree. Also any
243	request which are marked with REQ_NOIDLE go on this service tree. On this
244	tree we do not idle on individual queues instead idle on the whole group
245	of queues or the tree. So if there are 4 queues waiting for IO to dispatch
246	we will idle only once last queue has dispatched the IO and there is
247	no more IO on this service tree.
248	
249	All async writes go on async service tree. There is no idling on async
250	queues.
251	
252	CFQ has some optimizations for SSDs and if it detects a non-rotational
253	media which can support higher queue depth (multiple requests at in
254	flight at a time), then it cuts down on idling of individual queues and
255	all the queues move to sync-noidle tree and only tree idle remains. This
256	tree idling provides isolation with buffered write queues on async tree.
257	
258	FAQ
259	===
260	Q1. Why to idle at all on queues marked with REQ_NOIDLE.
261	
262	A1. We only do tree idle (all queues on sync-noidle tree) on queues marked
263	    with REQ_NOIDLE. This helps in providing isolation with all the sync-idle
264	    queues. Otherwise in presence of many sequential readers, other
265	    synchronous IO might not get fair share of disk.
266	
267	    For example, if there are 10 sequential readers doing IO and they get
268	    100ms each. If a REQ_NOIDLE request comes in, it will be scheduled
269	    roughly after 1 second. If after completion of REQ_NOIDLE request we
270	    do not idle, and after a couple of milli seconds a another REQ_NOIDLE
271	    request comes in, again it will be scheduled after 1second. Repeat it
272	    and notice how a workload can lose its disk share and suffer due to
273	    multiple sequential readers.
274	
275	    fsync can generate dependent IO where bunch of data is written in the
276	    context of fsync, and later some journaling data is written. Journaling
277	    data comes in only after fsync has finished its IO (atleast for ext4
278	    that seemed to be the case). Now if one decides not to idle on fsync
279	    thread due to REQ_NOIDLE, then next journaling write will not get
280	    scheduled for another second. A process doing small fsync, will suffer
281	    badly in presence of multiple sequential readers.
282	
283	    Hence doing tree idling on threads using REQ_NOIDLE flag on requests
284	    provides isolation from multiple sequential readers and at the same
285	    time we do not idle on individual threads.
286	
287	Q2. When to specify REQ_NOIDLE
288	A2. I would think whenever one is doing synchronous write and not expecting
289	    more writes to be dispatched from same context soon, should be able
290	    to specify REQ_NOIDLE on writes and that probably should work well for
291	    most of the cases.
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