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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 | ||
2786c4e5 VG |
11 | Currently two IO control policies are implemented. First one is proportional |
12 | weight time based division of disk policy. It is implemented in CFQ. Hence | |
13 | this policy takes effect only on leaf nodes when CFQ is being used. The second | |
14 | one is throttling policy which can be used to specify upper IO rate limits | |
15 | on devices. This policy is implemented in generic block layer and can be | |
16 | used on leaf nodes as well as higher level logical devices like device mapper. | |
72f924f6 VG |
17 | |
18 | HOWTO | |
19 | ===== | |
2786c4e5 VG |
20 | Proportional Weight division of bandwidth |
21 | ----------------------------------------- | |
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22 | You can do a very simple testing of running two dd threads in two different |
23 | cgroups. Here is what you can do. | |
24 | ||
afc24d49 VG |
25 | - Enable Block IO controller |
26 | CONFIG_BLK_CGROUP=y | |
27 | ||
72f924f6 VG |
28 | - Enable group scheduling in CFQ |
29 | CONFIG_CFQ_GROUP_IOSCHED=y | |
30 | ||
f6e07d38 JS |
31 | - Compile and boot into kernel and mount IO controller (blkio); see |
32 | cgroups.txt, Why are cgroups needed?. | |
72f924f6 | 33 | |
f6e07d38 JS |
34 | mount -t tmpfs cgroup_root /sys/fs/cgroup |
35 | mkdir /sys/fs/cgroup/blkio | |
36 | mount -t cgroup -o blkio none /sys/fs/cgroup/blkio | |
72f924f6 VG |
37 | |
38 | - Create two cgroups | |
f6e07d38 | 39 | mkdir -p /sys/fs/cgroup/blkio/test1/ /sys/fs/cgroup/blkio/test2 |
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40 | |
41 | - Set weights of group test1 and test2 | |
f6e07d38 JS |
42 | echo 1000 > /sys/fs/cgroup/blkio/test1/blkio.weight |
43 | echo 500 > /sys/fs/cgroup/blkio/test2/blkio.weight | |
72f924f6 VG |
44 | |
45 | - Create two same size files (say 512MB each) on same disk (file1, file2) and | |
46 | launch two dd threads in different cgroup to read those files. | |
47 | ||
48 | sync | |
49 | echo 3 > /proc/sys/vm/drop_caches | |
50 | ||
51 | dd if=/mnt/sdb/zerofile1 of=/dev/null & | |
f6e07d38 JS |
52 | echo $! > /sys/fs/cgroup/blkio/test1/tasks |
53 | cat /sys/fs/cgroup/blkio/test1/tasks | |
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54 | |
55 | dd if=/mnt/sdb/zerofile2 of=/dev/null & | |
f6e07d38 JS |
56 | echo $! > /sys/fs/cgroup/blkio/test2/tasks |
57 | cat /sys/fs/cgroup/blkio/test2/tasks | |
72f924f6 VG |
58 | |
59 | - At macro level, first dd should finish first. To get more precise data, keep | |
60 | on looking at (with the help of script), at blkio.disk_time and | |
61 | blkio.disk_sectors files of both test1 and test2 groups. This will tell how | |
55d01595 | 62 | much disk time (in milliseconds), each group got and how many sectors each |
72f924f6 VG |
63 | group dispatched to the disk. We provide fairness in terms of disk time, so |
64 | ideally io.disk_time of cgroups should be in proportion to the weight. | |
65 | ||
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66 | Throttling/Upper Limit policy |
67 | ----------------------------- | |
68 | - Enable Block IO controller | |
69 | CONFIG_BLK_CGROUP=y | |
70 | ||
71 | - Enable throttling in block layer | |
72 | CONFIG_BLK_DEV_THROTTLING=y | |
73 | ||
f6e07d38 JS |
74 | - Mount blkio controller (see cgroups.txt, Why are cgroups needed?) |
75 | mount -t cgroup -o blkio none /sys/fs/cgroup/blkio | |
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76 | |
77 | - Specify a bandwidth rate on particular device for root group. The format | |
52b233c8 | 78 | for policy is "<major>:<minor> <bytes_per_second>". |
2786c4e5 | 79 | |
9b61fc4c | 80 | echo "8:16 1048576" > /sys/fs/cgroup/blkio/blkio.throttle.read_bps_device |
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81 | |
82 | Above will put a limit of 1MB/second on reads happening for root group | |
83 | on device having major/minor number 8:16. | |
84 | ||
85 | - Run dd to read a file and see if rate is throttled to 1MB/s or not. | |
86 | ||
e11362bb | 87 | # dd iflag=direct if=/mnt/common/zerofile of=/dev/null bs=4K count=1024 |
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88 | 1024+0 records in |
89 | 1024+0 records out | |
90 | 4194304 bytes (4.2 MB) copied, 4.0001 s, 1.0 MB/s | |
91 | ||
9b61fc4c | 92 | Limits for writes can be put using blkio.throttle.write_bps_device file. |
2786c4e5 | 93 | |
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94 | Hierarchical Cgroups |
95 | ==================== | |
bdc85df7 | 96 | |
9138125b TH |
97 | Both CFQ and throttling implement hierarchy support; however, |
98 | throttling's hierarchy support is enabled iff "sane_behavior" is | |
99 | enabled from cgroup side, which currently is a development option and | |
100 | not publicly available. | |
101 | ||
102 | If somebody created a hierarchy like as follows. | |
bdc85df7 VG |
103 | |
104 | root | |
105 | / \ | |
106 | test1 test2 | |
107 | | | |
108 | test3 | |
109 | ||
9138125b TH |
110 | CFQ by default and throttling with "sane_behavior" will handle the |
111 | hierarchy correctly. For details on CFQ hierarchy support, refer to | |
112 | Documentation/block/cfq-iosched.txt. For throttling, all limits apply | |
113 | to the whole subtree while all statistics are local to the IOs | |
114 | directly generated by tasks in that cgroup. | |
115 | ||
116 | Throttling without "sane_behavior" enabled from cgroup side will | |
117 | practically treat all groups at same level as if it looks like the | |
118 | following. | |
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119 | |
120 | pivot | |
67de0162 | 121 | / / \ \ |
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122 | root test1 test2 test3 |
123 | ||
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124 | Various user visible config options |
125 | =================================== | |
72f924f6 | 126 | CONFIG_BLK_CGROUP |
afc24d49 | 127 | - Block IO controller. |
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128 | |
129 | CONFIG_DEBUG_BLK_CGROUP | |
afc24d49 VG |
130 | - Debug help. Right now some additional stats file show up in cgroup |
131 | if this option is enabled. | |
132 | ||
133 | CONFIG_CFQ_GROUP_IOSCHED | |
134 | - Enables group scheduling in CFQ. Currently only 1 level of group | |
135 | creation is allowed. | |
72f924f6 | 136 | |
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137 | CONFIG_BLK_DEV_THROTTLING |
138 | - Enable block device throttling support in block layer. | |
139 | ||
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140 | Details of cgroup files |
141 | ======================= | |
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142 | Proportional weight policy files |
143 | -------------------------------- | |
72f924f6 | 144 | - blkio.weight |
da69da18 GJ |
145 | - Specifies per cgroup weight. This is default weight of the group |
146 | on all the devices until and unless overridden by per device rule. | |
147 | (See blkio.weight_device). | |
df457f84 | 148 | Currently allowed range of weights is from 10 to 1000. |
72f924f6 | 149 | |
da69da18 GJ |
150 | - blkio.weight_device |
151 | - One can specify per cgroup per device rules using this interface. | |
152 | These rules override the default value of group weight as specified | |
153 | by blkio.weight. | |
154 | ||
155 | Following is the format. | |
156 | ||
f6e07d38 | 157 | # echo dev_maj:dev_minor weight > blkio.weight_device |
da69da18 GJ |
158 | Configure weight=300 on /dev/sdb (8:16) in this cgroup |
159 | # echo 8:16 300 > blkio.weight_device | |
160 | # cat blkio.weight_device | |
161 | dev weight | |
162 | 8:16 300 | |
163 | ||
164 | Configure weight=500 on /dev/sda (8:0) in this cgroup | |
165 | # echo 8:0 500 > blkio.weight_device | |
166 | # cat blkio.weight_device | |
167 | dev weight | |
168 | 8:0 500 | |
169 | 8:16 300 | |
170 | ||
171 | Remove specific weight for /dev/sda in this cgroup | |
172 | # echo 8:0 0 > blkio.weight_device | |
173 | # cat blkio.weight_device | |
174 | dev weight | |
175 | 8:16 300 | |
176 | ||
d02f7aa8 TH |
177 | - blkio.leaf_weight[_device] |
178 | - Equivalents of blkio.weight[_device] for the purpose of | |
179 | deciding how much weight tasks in the given cgroup has while | |
180 | competing with the cgroup's child cgroups. For details, | |
181 | please refer to Documentation/block/cfq-iosched.txt. | |
182 | ||
72f924f6 VG |
183 | - blkio.time |
184 | - disk time allocated to cgroup per device in milliseconds. First | |
185 | two fields specify the major and minor number of the device and | |
186 | third field specifies the disk time allocated to group in | |
187 | milliseconds. | |
188 | ||
189 | - blkio.sectors | |
190 | - number of sectors transferred to/from disk by the group. First | |
191 | two fields specify the major and minor number of the device and | |
192 | third field specifies the number of sectors transferred by the | |
193 | group to/from the device. | |
194 | ||
84c124da DS |
195 | - blkio.io_service_bytes |
196 | - Number of bytes transferred to/from the disk by the group. These | |
197 | are further divided by the type of operation - read or write, sync | |
198 | or async. First two fields specify the major and minor number of the | |
199 | device, third field specifies the operation type and the fourth field | |
200 | specifies the number of bytes. | |
201 | ||
202 | - blkio.io_serviced | |
77ea7338 | 203 | - Number of IOs (bio) issued to the disk by the group. These |
84c124da DS |
204 | are further divided by the type of operation - read or write, sync |
205 | or async. First two fields specify the major and minor number of the | |
206 | device, third field specifies the operation type and the fourth field | |
207 | specifies the number of IOs. | |
208 | ||
209 | - blkio.io_service_time | |
210 | - Total amount of time between request dispatch and request completion | |
211 | for the IOs done by this cgroup. This is in nanoseconds to make it | |
212 | meaningful for flash devices too. For devices with queue depth of 1, | |
213 | this time represents the actual service time. When queue_depth > 1, | |
214 | that is no longer true as requests may be served out of order. This | |
215 | may cause the service time for a given IO to include the service time | |
216 | of multiple IOs when served out of order which may result in total | |
217 | io_service_time > actual time elapsed. This time is further divided by | |
218 | the type of operation - read or write, sync or async. First two fields | |
219 | specify the major and minor number of the device, third field | |
220 | specifies the operation type and the fourth field specifies the | |
221 | io_service_time in ns. | |
222 | ||
223 | - blkio.io_wait_time | |
224 | - Total amount of time the IOs for this cgroup spent waiting in the | |
225 | scheduler queues for service. This can be greater than the total time | |
226 | elapsed since it is cumulative io_wait_time for all IOs. It is not a | |
227 | measure of total time the cgroup spent waiting but rather a measure of | |
228 | the wait_time for its individual IOs. For devices with queue_depth > 1 | |
229 | this metric does not include the time spent waiting for service once | |
230 | the IO is dispatched to the device but till it actually gets serviced | |
231 | (there might be a time lag here due to re-ordering of requests by the | |
232 | device). This is in nanoseconds to make it meaningful for flash | |
233 | devices too. This time is further divided by the type of operation - | |
234 | read or write, sync or async. First two fields specify the major and | |
235 | minor number of the device, third field specifies the operation type | |
236 | and the fourth field specifies the io_wait_time in ns. | |
237 | ||
812d4026 DS |
238 | - blkio.io_merged |
239 | - Total number of bios/requests merged into requests belonging to this | |
240 | cgroup. This is further divided by the type of operation - read or | |
241 | write, sync or async. | |
242 | ||
cdc1184c DS |
243 | - blkio.io_queued |
244 | - Total number of requests queued up at any given instant for this | |
245 | cgroup. This is further divided by the type of operation - read or | |
246 | write, sync or async. | |
247 | ||
248 | - blkio.avg_queue_size | |
afc24d49 | 249 | - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. |
cdc1184c DS |
250 | The average queue size for this cgroup over the entire time of this |
251 | cgroup's existence. Queue size samples are taken each time one of the | |
252 | queues of this cgroup gets a timeslice. | |
253 | ||
812df48d | 254 | - blkio.group_wait_time |
afc24d49 | 255 | - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. |
812df48d DS |
256 | This is the amount of time the cgroup had to wait since it became busy |
257 | (i.e., went from 0 to 1 request queued) to get a timeslice for one of | |
258 | its queues. This is different from the io_wait_time which is the | |
259 | cumulative total of the amount of time spent by each IO in that cgroup | |
260 | waiting in the scheduler queue. This is in nanoseconds. If this is | |
261 | read when the cgroup is in a waiting (for timeslice) state, the stat | |
262 | will only report the group_wait_time accumulated till the last time it | |
263 | got a timeslice and will not include the current delta. | |
264 | ||
265 | - blkio.empty_time | |
afc24d49 | 266 | - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. |
812df48d DS |
267 | This is the amount of time a cgroup spends without any pending |
268 | requests when not being served, i.e., it does not include any time | |
269 | spent idling for one of the queues of the cgroup. This is in | |
270 | nanoseconds. If this is read when the cgroup is in an empty state, | |
271 | the stat will only report the empty_time accumulated till the last | |
272 | time it had a pending request and will not include the current delta. | |
273 | ||
274 | - blkio.idle_time | |
afc24d49 | 275 | - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. |
812df48d | 276 | This is the amount of time spent by the IO scheduler idling for a |
40e47125 | 277 | given cgroup in anticipation of a better request than the existing ones |
812df48d DS |
278 | from other queues/cgroups. This is in nanoseconds. If this is read |
279 | when the cgroup is in an idling state, the stat will only report the | |
280 | idle_time accumulated till the last idle period and will not include | |
281 | the current delta. | |
282 | ||
72f924f6 | 283 | - blkio.dequeue |
afc24d49 | 284 | - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. This |
72f924f6 VG |
285 | gives the statistics about how many a times a group was dequeued |
286 | from service tree of the device. First two fields specify the major | |
287 | and minor number of the device and third field specifies the number | |
288 | of times a group was dequeued from a particular device. | |
289 | ||
d02f7aa8 TH |
290 | - blkio.*_recursive |
291 | - Recursive version of various stats. These files show the | |
292 | same information as their non-recursive counterparts but | |
293 | include stats from all the descendant cgroups. | |
294 | ||
2786c4e5 VG |
295 | Throttling/Upper limit policy files |
296 | ----------------------------------- | |
297 | - blkio.throttle.read_bps_device | |
298 | - Specifies upper limit on READ rate from the device. IO rate is | |
40e47125 | 299 | specified in bytes per second. Rules are per device. Following is |
2786c4e5 VG |
300 | the format. |
301 | ||
9b61fc4c | 302 | echo "<major>:<minor> <rate_bytes_per_second>" > /cgrp/blkio.throttle.read_bps_device |
2786c4e5 VG |
303 | |
304 | - blkio.throttle.write_bps_device | |
305 | - Specifies upper limit on WRITE rate to the device. IO rate is | |
40e47125 | 306 | specified in bytes per second. Rules are per device. Following is |
2786c4e5 VG |
307 | the format. |
308 | ||
9b61fc4c | 309 | echo "<major>:<minor> <rate_bytes_per_second>" > /cgrp/blkio.throttle.write_bps_device |
2786c4e5 VG |
310 | |
311 | - blkio.throttle.read_iops_device | |
312 | - Specifies upper limit on READ rate from the device. IO rate is | |
40e47125 | 313 | specified in IO per second. Rules are per device. Following is |
2786c4e5 VG |
314 | the format. |
315 | ||
9b61fc4c | 316 | echo "<major>:<minor> <rate_io_per_second>" > /cgrp/blkio.throttle.read_iops_device |
2786c4e5 VG |
317 | |
318 | - blkio.throttle.write_iops_device | |
319 | - Specifies upper limit on WRITE rate to the device. IO rate is | |
40e47125 | 320 | specified in io per second. Rules are per device. Following is |
2786c4e5 VG |
321 | the format. |
322 | ||
9b61fc4c | 323 | echo "<major>:<minor> <rate_io_per_second>" > /cgrp/blkio.throttle.write_iops_device |
2786c4e5 VG |
324 | |
325 | Note: If both BW and IOPS rules are specified for a device, then IO is | |
40e47125 | 326 | subjected to both the constraints. |
2786c4e5 VG |
327 | |
328 | - blkio.throttle.io_serviced | |
77ea7338 TH |
329 | - Number of IOs (bio) issued to the disk by the group. These |
330 | are further divided by the type of operation - read or write, sync | |
331 | or async. First two fields specify the major and minor number of the | |
332 | device, third field specifies the operation type and the fourth field | |
333 | specifies the number of IOs. | |
2786c4e5 VG |
334 | |
335 | - blkio.throttle.io_service_bytes | |
336 | - Number of bytes transferred to/from the disk by the group. These | |
337 | are further divided by the type of operation - read or write, sync | |
338 | or async. First two fields specify the major and minor number of the | |
339 | device, third field specifies the operation type and the fourth field | |
340 | specifies the number of bytes. | |
341 | ||
2786c4e5 VG |
342 | Common files among various policies |
343 | ----------------------------------- | |
84c124da DS |
344 | - blkio.reset_stats |
345 | - Writing an int to this file will result in resetting all the stats | |
346 | for that cgroup. | |
347 | ||
72f924f6 VG |
348 | CFQ sysfs tunable |
349 | ================= | |
6d6ac1c1 VG |
350 | /sys/block/<disk>/queue/iosched/slice_idle |
351 | ------------------------------------------ | |
352 | On a faster hardware CFQ can be slow, especially with sequential workload. | |
353 | This happens because CFQ idles on a single queue and single queue might not | |
354 | drive deeper request queue depths to keep the storage busy. In such scenarios | |
355 | one can try setting slice_idle=0 and that would switch CFQ to IOPS | |
356 | (IO operations per second) mode on NCQ supporting hardware. | |
357 | ||
358 | That means CFQ will not idle between cfq queues of a cfq group and hence be | |
359 | able to driver higher queue depth and achieve better throughput. That also | |
360 | means that cfq provides fairness among groups in terms of IOPS and not in | |
361 | terms of disk time. | |
362 | ||
363 | /sys/block/<disk>/queue/iosched/group_idle | |
364 | ------------------------------------------ | |
365 | If one disables idling on individual cfq queues and cfq service trees by | |
366 | setting slice_idle=0, group_idle kicks in. That means CFQ will still idle | |
367 | on the group in an attempt to provide fairness among groups. | |
368 | ||
369 | By default group_idle is same as slice_idle and does not do anything if | |
370 | slice_idle is enabled. | |
371 | ||
372 | One can experience an overall throughput drop if you have created multiple | |
373 | groups and put applications in that group which are not driving enough | |
374 | IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle | |
375 | on individual groups and throughput should improve. |