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2792d871 NJ |
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 | ||
6d6ac1c1 VG |
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 | ||
2792d871 NJ |
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. | |
104 | ||
6d6ac1c1 VG |
105 | CFQ IOPS Mode for group scheduling |
106 | =================================== | |
107 | Basic CFQ design is to provide priority based time slices. Higher priority | |
108 | process gets bigger time slice and lower priority process gets smaller time | |
109 | slice. Measuring time becomes harder if storage is fast and supports NCQ and | |
110 | it would be better to dispatch multiple requests from multiple cfq queues in | |
111 | request queue at a time. In such scenario, it is not possible to measure time | |
112 | consumed by single queue accurately. | |
113 | ||
114 | What is possible though is to measure number of requests dispatched from a | |
115 | single queue and also allow dispatch from multiple cfq queue at the same time. | |
116 | This effectively becomes the fairness in terms of IOPS (IO operations per | |
117 | second). | |
118 | ||
119 | If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches | |
120 | to IOPS mode and starts providing fairness in terms of number of requests | |
121 | dispatched. Note that this mode switching takes effect only for group | |
122 | scheduling. For non-cgroup users nothing should change. | |
4931402a VG |
123 | |
124 | CFQ IO scheduler Idling Theory | |
125 | =============================== | |
126 | Idling on a queue is primarily about waiting for the next request to come | |
127 | on same queue after completion of a request. In this process CFQ will not | |
128 | dispatch requests from other cfq queues even if requests are pending there. | |
129 | ||
130 | The rationale behind idling is that it can cut down on number of seeks | |
131 | on rotational media. For example, if a process is doing dependent | |
132 | sequential reads (next read will come on only after completion of previous | |
133 | one), then not dispatching request from other queue should help as we | |
134 | did not move the disk head and kept on dispatching sequential IO from | |
135 | one queue. | |
136 | ||
137 | CFQ has following service trees and various queues are put on these trees. | |
138 | ||
139 | sync-idle sync-noidle async | |
140 | ||
141 | All cfq queues doing synchronous sequential IO go on to sync-idle tree. | |
142 | On this tree we idle on each queue individually. | |
143 | ||
144 | All synchronous non-sequential queues go on sync-noidle tree. Also any | |
145 | request which are marked with REQ_NOIDLE go on this service tree. On this | |
146 | tree we do not idle on individual queues instead idle on the whole group | |
147 | of queues or the tree. So if there are 4 queues waiting for IO to dispatch | |
148 | we will idle only once last queue has dispatched the IO and there is | |
149 | no more IO on this service tree. | |
150 | ||
151 | All async writes go on async service tree. There is no idling on async | |
152 | queues. | |
153 | ||
154 | CFQ has some optimizations for SSDs and if it detects a non-rotational | |
155 | media which can support higher queue depth (multiple requests at in | |
156 | flight at a time), then it cuts down on idling of individual queues and | |
157 | all the queues move to sync-noidle tree and only tree idle remains. This | |
158 | tree idling provides isolation with buffered write queues on async tree. | |
159 | ||
160 | FAQ | |
161 | === | |
162 | Q1. Why to idle at all on queues marked with REQ_NOIDLE. | |
163 | ||
164 | A1. We only do tree idle (all queues on sync-noidle tree) on queues marked | |
165 | with REQ_NOIDLE. This helps in providing isolation with all the sync-idle | |
166 | queues. Otherwise in presence of many sequential readers, other | |
167 | synchronous IO might not get fair share of disk. | |
168 | ||
169 | For example, if there are 10 sequential readers doing IO and they get | |
170 | 100ms each. If a REQ_NOIDLE request comes in, it will be scheduled | |
171 | roughly after 1 second. If after completion of REQ_NOIDLE request we | |
172 | do not idle, and after a couple of milli seconds a another REQ_NOIDLE | |
173 | request comes in, again it will be scheduled after 1second. Repeat it | |
174 | and notice how a workload can lose its disk share and suffer due to | |
175 | multiple sequential readers. | |
176 | ||
177 | fsync can generate dependent IO where bunch of data is written in the | |
178 | context of fsync, and later some journaling data is written. Journaling | |
179 | data comes in only after fsync has finished its IO (atleast for ext4 | |
180 | that seemed to be the case). Now if one decides not to idle on fsync | |
181 | thread due to REQ_NOIDLE, then next journaling write will not get | |
182 | scheduled for another second. A process doing small fsync, will suffer | |
183 | badly in presence of multiple sequential readers. | |
184 | ||
185 | Hence doing tree idling on threads using REQ_NOIDLE flag on requests | |
186 | provides isolation from multiple sequential readers and at the same | |
187 | time we do not idle on individual threads. | |
188 | ||
189 | Q2. When to specify REQ_NOIDLE | |
190 | A2. I would think whenever one is doing synchronous write and not expecting | |
191 | more writes to be dispatched from same context soon, should be able | |
192 | to specify REQ_NOIDLE on writes and that probably should work well for | |
193 | most of the cases. |