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72f924f6 VG |
1 | Block IO Controller |
2 | =================== | |
3 | Overview | |
4 | ======== | |
5 | cgroup subsys "blkio" implements the block io controller. There seems to be | |
6 | a need of various kinds of IO control policies (like proportional BW, max BW) | |
7 | both at leaf nodes as well as at intermediate nodes in a storage hierarchy. | |
8 | Plan is to use the same cgroup based management interface for blkio controller | |
9 | and based on user options switch IO policies in the background. | |
10 | ||
11 | In the first phase, this patchset implements proportional weight time based | |
12 | division of disk policy. It is implemented in CFQ. Hence this policy takes | |
13 | effect only on leaf nodes when CFQ is being used. | |
14 | ||
15 | HOWTO | |
16 | ===== | |
17 | You can do a very simple testing of running two dd threads in two different | |
18 | cgroups. Here is what you can do. | |
19 | ||
afc24d49 VG |
20 | - Enable Block IO controller |
21 | CONFIG_BLK_CGROUP=y | |
22 | ||
72f924f6 VG |
23 | - Enable group scheduling in CFQ |
24 | CONFIG_CFQ_GROUP_IOSCHED=y | |
25 | ||
26 | - Compile and boot into kernel and mount IO controller (blkio). | |
27 | ||
28 | mount -t cgroup -o blkio none /cgroup | |
29 | ||
30 | - Create two cgroups | |
31 | mkdir -p /cgroup/test1/ /cgroup/test2 | |
32 | ||
33 | - Set weights of group test1 and test2 | |
34 | echo 1000 > /cgroup/test1/blkio.weight | |
35 | echo 500 > /cgroup/test2/blkio.weight | |
36 | ||
37 | - Create two same size files (say 512MB each) on same disk (file1, file2) and | |
38 | launch two dd threads in different cgroup to read those files. | |
39 | ||
40 | sync | |
41 | echo 3 > /proc/sys/vm/drop_caches | |
42 | ||
43 | dd if=/mnt/sdb/zerofile1 of=/dev/null & | |
44 | echo $! > /cgroup/test1/tasks | |
45 | cat /cgroup/test1/tasks | |
46 | ||
47 | dd if=/mnt/sdb/zerofile2 of=/dev/null & | |
48 | echo $! > /cgroup/test2/tasks | |
49 | cat /cgroup/test2/tasks | |
50 | ||
51 | - At macro level, first dd should finish first. To get more precise data, keep | |
52 | on looking at (with the help of script), at blkio.disk_time and | |
53 | blkio.disk_sectors files of both test1 and test2 groups. This will tell how | |
54 | much disk time (in milli seconds), each group got and how many secotors each | |
55 | group dispatched to the disk. We provide fairness in terms of disk time, so | |
56 | ideally io.disk_time of cgroups should be in proportion to the weight. | |
57 | ||
58 | Various user visible config options | |
59 | =================================== | |
72f924f6 | 60 | CONFIG_BLK_CGROUP |
afc24d49 | 61 | - Block IO controller. |
72f924f6 VG |
62 | |
63 | CONFIG_DEBUG_BLK_CGROUP | |
afc24d49 VG |
64 | - Debug help. Right now some additional stats file show up in cgroup |
65 | if this option is enabled. | |
66 | ||
67 | CONFIG_CFQ_GROUP_IOSCHED | |
68 | - Enables group scheduling in CFQ. Currently only 1 level of group | |
69 | creation is allowed. | |
72f924f6 VG |
70 | |
71 | Details of cgroup files | |
72 | ======================= | |
73 | - blkio.weight | |
da69da18 GJ |
74 | - Specifies per cgroup weight. This is default weight of the group |
75 | on all the devices until and unless overridden by per device rule. | |
76 | (See blkio.weight_device). | |
72f924f6 VG |
77 | Currently allowed range of weights is from 100 to 1000. |
78 | ||
da69da18 GJ |
79 | - blkio.weight_device |
80 | - One can specify per cgroup per device rules using this interface. | |
81 | These rules override the default value of group weight as specified | |
82 | by blkio.weight. | |
83 | ||
84 | Following is the format. | |
85 | ||
86 | #echo dev_maj:dev_minor weight > /path/to/cgroup/blkio.weight_device | |
87 | Configure weight=300 on /dev/sdb (8:16) in this cgroup | |
88 | # echo 8:16 300 > blkio.weight_device | |
89 | # cat blkio.weight_device | |
90 | dev weight | |
91 | 8:16 300 | |
92 | ||
93 | Configure weight=500 on /dev/sda (8:0) in this cgroup | |
94 | # echo 8:0 500 > blkio.weight_device | |
95 | # cat blkio.weight_device | |
96 | dev weight | |
97 | 8:0 500 | |
98 | 8:16 300 | |
99 | ||
100 | Remove specific weight for /dev/sda in this cgroup | |
101 | # echo 8:0 0 > blkio.weight_device | |
102 | # cat blkio.weight_device | |
103 | dev weight | |
104 | 8:16 300 | |
105 | ||
72f924f6 VG |
106 | - blkio.time |
107 | - disk time allocated to cgroup per device in milliseconds. First | |
108 | two fields specify the major and minor number of the device and | |
109 | third field specifies the disk time allocated to group in | |
110 | milliseconds. | |
111 | ||
112 | - blkio.sectors | |
113 | - number of sectors transferred to/from disk by the group. First | |
114 | two fields specify the major and minor number of the device and | |
115 | third field specifies the number of sectors transferred by the | |
116 | group to/from the device. | |
117 | ||
84c124da DS |
118 | - blkio.io_service_bytes |
119 | - Number of bytes transferred to/from the disk by the group. These | |
120 | are further divided by the type of operation - read or write, sync | |
121 | or async. First two fields specify the major and minor number of the | |
122 | device, third field specifies the operation type and the fourth field | |
123 | specifies the number of bytes. | |
124 | ||
125 | - blkio.io_serviced | |
126 | - Number of IOs completed to/from the disk by the group. These | |
127 | are further divided by the type of operation - read or write, sync | |
128 | or async. First two fields specify the major and minor number of the | |
129 | device, third field specifies the operation type and the fourth field | |
130 | specifies the number of IOs. | |
131 | ||
132 | - blkio.io_service_time | |
133 | - Total amount of time between request dispatch and request completion | |
134 | for the IOs done by this cgroup. This is in nanoseconds to make it | |
135 | meaningful for flash devices too. For devices with queue depth of 1, | |
136 | this time represents the actual service time. When queue_depth > 1, | |
137 | that is no longer true as requests may be served out of order. This | |
138 | may cause the service time for a given IO to include the service time | |
139 | of multiple IOs when served out of order which may result in total | |
140 | io_service_time > actual time elapsed. This time is further divided by | |
141 | the type of operation - read or write, sync or async. First two fields | |
142 | specify the major and minor number of the device, third field | |
143 | specifies the operation type and the fourth field specifies the | |
144 | io_service_time in ns. | |
145 | ||
146 | - blkio.io_wait_time | |
147 | - Total amount of time the IOs for this cgroup spent waiting in the | |
148 | scheduler queues for service. This can be greater than the total time | |
149 | elapsed since it is cumulative io_wait_time for all IOs. It is not a | |
150 | measure of total time the cgroup spent waiting but rather a measure of | |
151 | the wait_time for its individual IOs. For devices with queue_depth > 1 | |
152 | this metric does not include the time spent waiting for service once | |
153 | the IO is dispatched to the device but till it actually gets serviced | |
154 | (there might be a time lag here due to re-ordering of requests by the | |
155 | device). This is in nanoseconds to make it meaningful for flash | |
156 | devices too. This time is further divided by the type of operation - | |
157 | read or write, sync or async. First two fields specify the major and | |
158 | minor number of the device, third field specifies the operation type | |
159 | and the fourth field specifies the io_wait_time in ns. | |
160 | ||
812d4026 DS |
161 | - blkio.io_merged |
162 | - Total number of bios/requests merged into requests belonging to this | |
163 | cgroup. This is further divided by the type of operation - read or | |
164 | write, sync or async. | |
165 | ||
cdc1184c DS |
166 | - blkio.io_queued |
167 | - Total number of requests queued up at any given instant for this | |
168 | cgroup. This is further divided by the type of operation - read or | |
169 | write, sync or async. | |
170 | ||
171 | - blkio.avg_queue_size | |
afc24d49 | 172 | - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. |
cdc1184c DS |
173 | The average queue size for this cgroup over the entire time of this |
174 | cgroup's existence. Queue size samples are taken each time one of the | |
175 | queues of this cgroup gets a timeslice. | |
176 | ||
812df48d | 177 | - blkio.group_wait_time |
afc24d49 | 178 | - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. |
812df48d DS |
179 | This is the amount of time the cgroup had to wait since it became busy |
180 | (i.e., went from 0 to 1 request queued) to get a timeslice for one of | |
181 | its queues. This is different from the io_wait_time which is the | |
182 | cumulative total of the amount of time spent by each IO in that cgroup | |
183 | waiting in the scheduler queue. This is in nanoseconds. If this is | |
184 | read when the cgroup is in a waiting (for timeslice) state, the stat | |
185 | will only report the group_wait_time accumulated till the last time it | |
186 | got a timeslice and will not include the current delta. | |
187 | ||
188 | - blkio.empty_time | |
afc24d49 | 189 | - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. |
812df48d DS |
190 | This is the amount of time a cgroup spends without any pending |
191 | requests when not being served, i.e., it does not include any time | |
192 | spent idling for one of the queues of the cgroup. This is in | |
193 | nanoseconds. If this is read when the cgroup is in an empty state, | |
194 | the stat will only report the empty_time accumulated till the last | |
195 | time it had a pending request and will not include the current delta. | |
196 | ||
197 | - blkio.idle_time | |
afc24d49 | 198 | - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. |
812df48d DS |
199 | This is the amount of time spent by the IO scheduler idling for a |
200 | given cgroup in anticipation of a better request than the exising ones | |
201 | from other queues/cgroups. This is in nanoseconds. If this is read | |
202 | when the cgroup is in an idling state, the stat will only report the | |
203 | idle_time accumulated till the last idle period and will not include | |
204 | the current delta. | |
205 | ||
72f924f6 | 206 | - blkio.dequeue |
afc24d49 | 207 | - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. This |
72f924f6 VG |
208 | gives the statistics about how many a times a group was dequeued |
209 | from service tree of the device. First two fields specify the major | |
210 | and minor number of the device and third field specifies the number | |
211 | of times a group was dequeued from a particular device. | |
212 | ||
84c124da DS |
213 | - blkio.reset_stats |
214 | - Writing an int to this file will result in resetting all the stats | |
215 | for that cgroup. | |
216 | ||
72f924f6 VG |
217 | CFQ sysfs tunable |
218 | ================= | |
219 | /sys/block/<disk>/queue/iosched/group_isolation | |
6d6ac1c1 | 220 | ----------------------------------------------- |
72f924f6 VG |
221 | |
222 | If group_isolation=1, it provides stronger isolation between groups at the | |
223 | expense of throughput. By default group_isolation is 0. In general that | |
224 | means that if group_isolation=0, expect fairness for sequential workload | |
225 | only. Set group_isolation=1 to see fairness for random IO workload also. | |
226 | ||
227 | Generally CFQ will put random seeky workload in sync-noidle category. CFQ | |
228 | will disable idling on these queues and it does a collective idling on group | |
229 | of such queues. Generally these are slow moving queues and if there is a | |
230 | sync-noidle service tree in each group, that group gets exclusive access to | |
231 | disk for certain period. That means it will bring the throughput down if | |
232 | group does not have enough IO to drive deeper queue depths and utilize disk | |
233 | capacity to the fullest in the slice allocated to it. But the flip side is | |
234 | that even a random reader should get better latencies and overall throughput | |
235 | if there are lots of sequential readers/sync-idle workload running in the | |
236 | system. | |
237 | ||
238 | If group_isolation=0, then CFQ automatically moves all the random seeky queues | |
239 | in the root group. That means there will be no service differentiation for | |
240 | that kind of workload. This leads to better throughput as we do collective | |
241 | idling on root sync-noidle tree. | |
242 | ||
243 | By default one should run with group_isolation=0. If that is not sufficient | |
244 | and one wants stronger isolation between groups, then set group_isolation=1 | |
245 | but this will come at cost of reduced throughput. | |
246 | ||
6d6ac1c1 VG |
247 | /sys/block/<disk>/queue/iosched/slice_idle |
248 | ------------------------------------------ | |
249 | On a faster hardware CFQ can be slow, especially with sequential workload. | |
250 | This happens because CFQ idles on a single queue and single queue might not | |
251 | drive deeper request queue depths to keep the storage busy. In such scenarios | |
252 | one can try setting slice_idle=0 and that would switch CFQ to IOPS | |
253 | (IO operations per second) mode on NCQ supporting hardware. | |
254 | ||
255 | That means CFQ will not idle between cfq queues of a cfq group and hence be | |
256 | able to driver higher queue depth and achieve better throughput. That also | |
257 | means that cfq provides fairness among groups in terms of IOPS and not in | |
258 | terms of disk time. | |
259 | ||
260 | /sys/block/<disk>/queue/iosched/group_idle | |
261 | ------------------------------------------ | |
262 | If one disables idling on individual cfq queues and cfq service trees by | |
263 | setting slice_idle=0, group_idle kicks in. That means CFQ will still idle | |
264 | on the group in an attempt to provide fairness among groups. | |
265 | ||
266 | By default group_idle is same as slice_idle and does not do anything if | |
267 | slice_idle is enabled. | |
268 | ||
269 | One can experience an overall throughput drop if you have created multiple | |
270 | groups and put applications in that group which are not driving enough | |
271 | IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle | |
272 | on individual groups and throughput should improve. | |
273 | ||
72f924f6 VG |
274 | What works |
275 | ========== | |
276 | - Currently only sync IO queues are support. All the buffered writes are | |
277 | still system wide and not per group. Hence we will not see service | |
278 | differentiation between buffered writes between groups. |