[PATCH] blkio-controller.txt: Remove references to CFQ
From: Andreas Herrmann
Date: Wed Jun 12 2019 - 05:24:49 EST
CFQ is gone. No need anymore to document its "proportional weight time
based division of disk policy".
Signed-off-by: Andreas Herrmann <aherrmann@xxxxxxxx>
Documentation/cgroup-v1/blkio-controller.txt | 96 ++--------------------------
1 file changed, 7 insertions(+), 89 deletions(-)
diff --git a/Documentation/cgroup-v1/blkio-controller.txt b/Documentation/cgroup-v1/blkio-controller.txt
index 673dc34d3f78..d1a1b7bdd03a 100644
@@ -8,61 +8,13 @@ both at leaf nodes as well as at intermediate nodes in a storage hierarchy.
Plan is to use the same cgroup based management interface for blkio controller
and based on user options switch IO policies in the background.
-Currently two IO control policies are implemented. First one is proportional
-weight time based division of disk policy. It is implemented in CFQ. Hence
-this policy takes effect only on leaf nodes when CFQ is being used. The second
-one is throttling policy which can be used to specify upper IO rate limits
-on devices. This policy is implemented in generic block layer and can be
-used on leaf nodes as well as higher level logical devices like device mapper.
+One IO control policy is throttling policy which can be used to
+specify upper IO rate limits on devices. This policy is implemented in
+generic block layer and can be used on leaf nodes as well as higher
+level logical devices like device mapper.
-Proportional Weight division of bandwidth
-You can do a very simple testing of running two dd threads in two different
-cgroups. Here is what you can do.
-- Enable Block IO controller
-- Enable group scheduling in CFQ
-- Compile and boot into kernel and mount IO controller (blkio); see
- cgroups.txt, Why are cgroups needed?.
- mount -t tmpfs cgroup_root /sys/fs/cgroup
- mkdir /sys/fs/cgroup/blkio
- mount -t cgroup -o blkio none /sys/fs/cgroup/blkio
-- Create two cgroups
- mkdir -p /sys/fs/cgroup/blkio/test1/ /sys/fs/cgroup/blkio/test2
-- Set weights of group test1 and test2
- echo 1000 > /sys/fs/cgroup/blkio/test1/blkio.weight
- echo 500 > /sys/fs/cgroup/blkio/test2/blkio.weight
-- Create two same size files (say 512MB each) on same disk (file1, file2) and
- launch two dd threads in different cgroup to read those files.
- echo 3 > /proc/sys/vm/drop_caches
- dd if=/mnt/sdb/zerofile1 of=/dev/null &
- echo $! > /sys/fs/cgroup/blkio/test1/tasks
- cat /sys/fs/cgroup/blkio/test1/tasks
- dd if=/mnt/sdb/zerofile2 of=/dev/null &
- echo $! > /sys/fs/cgroup/blkio/test2/tasks
- cat /sys/fs/cgroup/blkio/test2/tasks
-- At macro level, first dd should finish first. To get more precise data, keep
- on looking at (with the help of script), at blkio.disk_time and
- blkio.disk_sectors files of both test1 and test2 groups. This will tell how
- much disk time (in milliseconds), each group got and how many sectors each
- group dispatched to the disk. We provide fairness in terms of disk time, so
- ideally io.disk_time of cgroups should be in proportion to the weight.
Throttling/Upper Limit policy
- Enable Block IO controller
@@ -94,7 +46,7 @@ Throttling/Upper Limit policy
-Both CFQ and throttling implement hierarchy support; however,
+Throttling implements hierarchy support; however,
throttling's hierarchy support is enabled iff "sane_behavior" is
enabled from cgroup side, which currently is a development option and
not publicly available.
@@ -107,9 +59,8 @@ If somebody created a hierarchy like as follows.
-CFQ by default and throttling with "sane_behavior" will handle the
-hierarchy correctly. For details on CFQ hierarchy support, refer to
-Documentation/block/cfq-iosched.txt. For throttling, all limits apply
+Throttling with "sane_behavior" will handle the
+hierarchy correctly. For throttling, all limits apply
to the whole subtree while all statistics are local to the IOs
directly generated by tasks in that cgroup.
@@ -130,10 +81,6 @@ CONFIG_DEBUG_BLK_CGROUP
- Debug help. Right now some additional stats file show up in cgroup
if this option is enabled.
- - Enables group scheduling in CFQ. Currently only 1 level of group
- creation is allowed.
- Enable block device throttling support in block layer.
@@ -344,32 +291,3 @@ Common files among various policies
- Writing an int to this file will result in resetting all the stats
for that cgroup.
-CFQ sysfs tunable
-On a faster hardware CFQ can be slow, especially with sequential workload.
-This happens because CFQ idles on a single queue and single queue might not
-drive deeper request queue depths to keep the storage busy. In such scenarios
-one can try setting slice_idle=0 and that would switch CFQ to IOPS
-(IO operations per second) mode on NCQ supporting hardware.
-That means CFQ will not idle between cfq queues of a cfq group and hence be
-able to driver higher queue depth and achieve better throughput. That also
-means that cfq provides fairness among groups in terms of IOPS and not in
-terms of disk time.
-If one disables idling on individual cfq queues and cfq service trees by
-setting slice_idle=0, group_idle kicks in. That means CFQ will still idle
-on the group in an attempt to provide fairness among groups.
-By default group_idle is same as slice_idle and does not do anything if
-slice_idle is enabled.
-One can experience an overall throughput drop if you have created multiple
-groups and put applications in that group which are not driving enough
-IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle
-on individual groups and throughput should improve.