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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:00 EST.

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	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.
17	
18	HOWTO
19	=====
20	Proportional Weight division of bandwidth
21	-----------------------------------------
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	
25	- Enable Block IO controller
26		CONFIG_BLK_CGROUP=y
27	
28	- Enable group scheduling in CFQ
29		CONFIG_CFQ_GROUP_IOSCHED=y
30	
31	- Compile and boot into kernel and mount IO controller (blkio); see
32	  cgroups.txt, Why are cgroups needed?.
33	
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
37	
38	- Create two cgroups
39		mkdir -p /sys/fs/cgroup/blkio/test1/ /sys/fs/cgroup/blkio/test2
40	
41	- Set weights of group test1 and test2
42		echo 1000 > /sys/fs/cgroup/blkio/test1/blkio.weight
43		echo 500 > /sys/fs/cgroup/blkio/test2/blkio.weight
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 &
52		echo $! > /sys/fs/cgroup/blkio/test1/tasks
53		cat /sys/fs/cgroup/blkio/test1/tasks
54	
55		dd if=/mnt/sdb/zerofile2 of=/dev/null &
56		echo $! > /sys/fs/cgroup/blkio/test2/tasks
57		cat /sys/fs/cgroup/blkio/test2/tasks
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
62	  much disk time (in milli seconds), each group got and how many secotors each
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	
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	
74	- Mount blkio controller (see cgroups.txt, Why are cgroups needed?)
75	        mount -t cgroup -o blkio none /sys/fs/cgroup/blkio
76	
77	- Specify a bandwidth rate on particular device for root group. The format
78	  for policy is "<major>:<minor>  <bytes_per_second>".
79	
80	        echo "8:16  1048576" > /sys/fs/cgroup/blkio/blkio.throttle.read_bps_device
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	
87			# dd if=/mnt/common/zerofile of=/dev/null bs=4K count=1024
88			# iflag=direct
89	        1024+0 records in
90	        1024+0 records out
91	        4194304 bytes (4.2 MB) copied, 4.0001 s, 1.0 MB/s
92	
93	 Limits for writes can be put using blkio.throttle.write_bps_device file.
94	
95	Hierarchical Cgroups
96	====================
97	
98	Both CFQ and throttling implement hierarchy support; however,
99	throttling's hierarchy support is enabled iff "sane_behavior" is
100	enabled from cgroup side, which currently is a development option and
101	not publicly available.
102	
103	If somebody created a hierarchy like as follows.
104	
105				root
106				/  \
107			     test1 test2
108				|
109			     test3
110	
111	CFQ by default and throttling with "sane_behavior" will handle the
112	hierarchy correctly.  For details on CFQ hierarchy support, refer to
113	Documentation/block/cfq-iosched.txt.  For throttling, all limits apply
114	to the whole subtree while all statistics are local to the IOs
115	directly generated by tasks in that cgroup.
116	
117	Throttling without "sane_behavior" enabled from cgroup side will
118	practically treat all groups at same level as if it looks like the
119	following.
120	
121					pivot
122				     /  /   \  \
123				root  test1 test2  test3
124	
125	Various user visible config options
126	===================================
127	CONFIG_BLK_CGROUP
128		- Block IO controller.
129	
130	CONFIG_DEBUG_BLK_CGROUP
131		- Debug help. Right now some additional stats file show up in cgroup
132		  if this option is enabled.
133	
134	CONFIG_CFQ_GROUP_IOSCHED
135		- Enables group scheduling in CFQ. Currently only 1 level of group
136		  creation is allowed.
137	
138	CONFIG_BLK_DEV_THROTTLING
139		- Enable block device throttling support in block layer.
140	
141	Details of cgroup files
142	=======================
143	Proportional weight policy files
144	--------------------------------
145	- blkio.weight
146		- Specifies per cgroup weight. This is default weight of the group
147		  on all the devices until and unless overridden by per device rule.
148		  (See blkio.weight_device).
149		  Currently allowed range of weights is from 10 to 1000.
150	
151	- blkio.weight_device
152		- One can specify per cgroup per device rules using this interface.
153		  These rules override the default value of group weight as specified
154		  by blkio.weight.
155	
156		  Following is the format.
157	
158		  # echo dev_maj:dev_minor weight > blkio.weight_device
159		  Configure weight=300 on /dev/sdb (8:16) in this cgroup
160		  # echo 8:16 300 > blkio.weight_device
161		  # cat blkio.weight_device
162		  dev     weight
163		  8:16    300
164	
165		  Configure weight=500 on /dev/sda (8:0) in this cgroup
166		  # echo 8:0 500 > blkio.weight_device
167		  # cat blkio.weight_device
168		  dev     weight
169		  8:0     500
170		  8:16    300
171	
172		  Remove specific weight for /dev/sda in this cgroup
173		  # echo 8:0 0 > blkio.weight_device
174		  # cat blkio.weight_device
175		  dev     weight
176		  8:16    300
177	
178	- blkio.leaf_weight[_device]
179		- Equivalents of blkio.weight[_device] for the purpose of
180	          deciding how much weight tasks in the given cgroup has while
181	          competing with the cgroup's child cgroups. For details,
182	          please refer to Documentation/block/cfq-iosched.txt.
183	
184	- blkio.time
185		- disk time allocated to cgroup per device in milliseconds. First
186		  two fields specify the major and minor number of the device and
187		  third field specifies the disk time allocated to group in
188		  milliseconds.
189	
190	- blkio.sectors
191		- number of sectors transferred to/from disk by the group. First
192		  two fields specify the major and minor number of the device and
193		  third field specifies the number of sectors transferred by the
194		  group to/from the device.
195	
196	- blkio.io_service_bytes
197		- Number of bytes transferred to/from the disk by the group. These
198		  are further divided by the type of operation - read or write, sync
199		  or async. First two fields specify the major and minor number of the
200		  device, third field specifies the operation type and the fourth field
201		  specifies the number of bytes.
202	
203	- blkio.io_serviced
204		- Number of IOs completed to/from the disk by the group. These
205		  are further divided by the type of operation - read or write, sync
206		  or async. First two fields specify the major and minor number of the
207		  device, third field specifies the operation type and the fourth field
208		  specifies the number of IOs.
209	
210	- blkio.io_service_time
211		- Total amount of time between request dispatch and request completion
212		  for the IOs done by this cgroup. This is in nanoseconds to make it
213		  meaningful for flash devices too. For devices with queue depth of 1,
214		  this time represents the actual service time. When queue_depth > 1,
215		  that is no longer true as requests may be served out of order. This
216		  may cause the service time for a given IO to include the service time
217		  of multiple IOs when served out of order which may result in total
218		  io_service_time > actual time elapsed. This time is further divided by
219		  the type of operation - read or write, sync or async. First two fields
220		  specify the major and minor number of the device, third field
221		  specifies the operation type and the fourth field specifies the
222		  io_service_time in ns.
223	
224	- blkio.io_wait_time
225		- Total amount of time the IOs for this cgroup spent waiting in the
226		  scheduler queues for service. This can be greater than the total time
227		  elapsed since it is cumulative io_wait_time for all IOs. It is not a
228		  measure of total time the cgroup spent waiting but rather a measure of
229		  the wait_time for its individual IOs. For devices with queue_depth > 1
230		  this metric does not include the time spent waiting for service once
231		  the IO is dispatched to the device but till it actually gets serviced
232		  (there might be a time lag here due to re-ordering of requests by the
233		  device). This is in nanoseconds to make it meaningful for flash
234		  devices too. This time is further divided by the type of operation -
235		  read or write, sync or async. First two fields specify the major and
236		  minor number of the device, third field specifies the operation type
237		  and the fourth field specifies the io_wait_time in ns.
238	
239	- blkio.io_merged
240		- Total number of bios/requests merged into requests belonging to this
241		  cgroup. This is further divided by the type of operation - read or
242		  write, sync or async.
243	
244	- blkio.io_queued
245		- Total number of requests queued up at any given instant for this
246		  cgroup. This is further divided by the type of operation - read or
247		  write, sync or async.
248	
249	- blkio.avg_queue_size
250		- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
251		  The average queue size for this cgroup over the entire time of this
252		  cgroup's existence. Queue size samples are taken each time one of the
253		  queues of this cgroup gets a timeslice.
254	
255	- blkio.group_wait_time
256		- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
257		  This is the amount of time the cgroup had to wait since it became busy
258		  (i.e., went from 0 to 1 request queued) to get a timeslice for one of
259		  its queues. This is different from the io_wait_time which is the
260		  cumulative total of the amount of time spent by each IO in that cgroup
261		  waiting in the scheduler queue. This is in nanoseconds. If this is
262		  read when the cgroup is in a waiting (for timeslice) state, the stat
263		  will only report the group_wait_time accumulated till the last time it
264		  got a timeslice and will not include the current delta.
265	
266	- blkio.empty_time
267		- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
268		  This is the amount of time a cgroup spends without any pending
269		  requests when not being served, i.e., it does not include any time
270		  spent idling for one of the queues of the cgroup. This is in
271		  nanoseconds. If this is read when the cgroup is in an empty state,
272		  the stat will only report the empty_time accumulated till the last
273		  time it had a pending request and will not include the current delta.
274	
275	- blkio.idle_time
276		- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
277		  This is the amount of time spent by the IO scheduler idling for a
278		  given cgroup in anticipation of a better request than the existing ones
279		  from other queues/cgroups. This is in nanoseconds. If this is read
280		  when the cgroup is in an idling state, the stat will only report the
281		  idle_time accumulated till the last idle period and will not include
282		  the current delta.
283	
284	- blkio.dequeue
285		- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. This
286		  gives the statistics about how many a times a group was dequeued
287		  from service tree of the device. First two fields specify the major
288		  and minor number of the device and third field specifies the number
289		  of times a group was dequeued from a particular device.
290	
291	- blkio.*_recursive
292		- Recursive version of various stats. These files show the
293	          same information as their non-recursive counterparts but
294	          include stats from all the descendant cgroups.
295	
296	Throttling/Upper limit policy files
297	-----------------------------------
298	- blkio.throttle.read_bps_device
299		- Specifies upper limit on READ rate from the device. IO rate is
300		  specified in bytes per second. Rules are per device. Following is
301		  the format.
302	
303	  echo "<major>:<minor>  <rate_bytes_per_second>" > /cgrp/blkio.throttle.read_bps_device
304	
305	- blkio.throttle.write_bps_device
306		- Specifies upper limit on WRITE rate to the device. IO rate is
307		  specified in bytes per second. Rules are per device. Following is
308		  the format.
309	
310	  echo "<major>:<minor>  <rate_bytes_per_second>" > /cgrp/blkio.throttle.write_bps_device
311	
312	- blkio.throttle.read_iops_device
313		- Specifies upper limit on READ rate from the device. IO rate is
314		  specified in IO per second. Rules are per device. Following is
315		  the format.
316	
317	  echo "<major>:<minor>  <rate_io_per_second>" > /cgrp/blkio.throttle.read_iops_device
318	
319	- blkio.throttle.write_iops_device
320		- Specifies upper limit on WRITE rate to the device. IO rate is
321		  specified in io per second. Rules are per device. Following is
322		  the format.
323	
324	  echo "<major>:<minor>  <rate_io_per_second>" > /cgrp/blkio.throttle.write_iops_device
325	
326	Note: If both BW and IOPS rules are specified for a device, then IO is
327	      subjected to both the constraints.
328	
329	- blkio.throttle.io_serviced
330		- Number of IOs (bio) completed to/from the disk by the group (as
331		  seen by throttling policy). These are further divided by the type
332		  of operation - read or write, sync or async. First two fields specify
333		  the major and minor number of the device, third field specifies the
334		  operation type and the fourth field specifies the number of IOs.
335	
336		  blkio.io_serviced does accounting as seen by CFQ and counts are in
337		  number of requests (struct request). On the other hand,
338		  blkio.throttle.io_serviced counts number of IO in terms of number
339		  of bios as seen by throttling policy.  These bios can later be
340		  merged by elevator and total number of requests completed can be
341		  lesser.
342	
343	- blkio.throttle.io_service_bytes
344		- Number of bytes transferred to/from the disk by the group. These
345		  are further divided by the type of operation - read or write, sync
346		  or async. First two fields specify the major and minor number of the
347		  device, third field specifies the operation type and the fourth field
348		  specifies the number of bytes.
349	
350		  These numbers should roughly be same as blkio.io_service_bytes as
351		  updated by CFQ. The difference between two is that
352		  blkio.io_service_bytes will not be updated if CFQ is not operating
353		  on request queue.
354	
355	Common files among various policies
356	-----------------------------------
357	- blkio.reset_stats
358		- Writing an int to this file will result in resetting all the stats
359		  for that cgroup.
360	
361	CFQ sysfs tunable
362	=================
363	/sys/block/<disk>/queue/iosched/slice_idle
364	------------------------------------------
365	On a faster hardware CFQ can be slow, especially with sequential workload.
366	This happens because CFQ idles on a single queue and single queue might not
367	drive deeper request queue depths to keep the storage busy. In such scenarios
368	one can try setting slice_idle=0 and that would switch CFQ to IOPS
369	(IO operations per second) mode on NCQ supporting hardware.
370	
371	That means CFQ will not idle between cfq queues of a cfq group and hence be
372	able to driver higher queue depth and achieve better throughput. That also
373	means that cfq provides fairness among groups in terms of IOPS and not in
374	terms of disk time.
375	
376	/sys/block/<disk>/queue/iosched/group_idle
377	------------------------------------------
378	If one disables idling on individual cfq queues and cfq service trees by
379	setting slice_idle=0, group_idle kicks in. That means CFQ will still idle
380	on the group in an attempt to provide fairness among groups.
381	
382	By default group_idle is same as slice_idle and does not do anything if
383	slice_idle is enabled.
384	
385	One can experience an overall throughput drop if you have created multiple
386	groups and put applications in that group which are not driving enough
387	IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle
388	on individual groups and throughput should improve.
389	
390	What works
391	==========
392	- Currently only sync IO queues are support. All the buffered writes are
393	  still system wide and not per group. Hence we will not see service
394	  differentiation between buffered writes between groups.
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