diff options
authorVivek Goyal <vgoyal@redhat.com>2009-12-03 12:59:57 -0500
committerJens Axboe <jens.axboe@oracle.com>2009-12-03 19:28:53 +0100
commit72f924f62a6eb375c7c237ecc911f95be0531d1a (patch)
parentc04645e592d4dd60c58def40c913699d4c806727 (diff)
blkio: Documentation
Signed-off-by: Vivek Goyal <vgoyal@redhat.com> Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
1 files changed, 135 insertions, 0 deletions
diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt
new file mode 100644
index 00000000000..630879cd9a4
--- /dev/null
+++ b/Documentation/cgroups/blkio-controller.txt
@@ -0,0 +1,135 @@
+ Block IO Controller
+ ===================
+cgroup subsys "blkio" implements the block io controller. There seems to be
+a need of various kinds of IO control policies (like proportional BW, max BW)
+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.
+In the first phase, this patchset implements 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.
+You can do a very simple testing of running two dd threads in two different
+cgroups. Here is what you can do.
+- Enable group scheduling in CFQ
+- Compile and boot into kernel and mount IO controller (blkio).
+ mount -t cgroup -o blkio none /cgroup
+- Create two cgroups
+ mkdir -p /cgroup/test1/ /cgroup/test2
+- Set weights of group test1 and test2
+ echo 1000 > /cgroup/test1/blkio.weight
+ echo 500 > /cgroup/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.
+ sync
+ echo 3 > /proc/sys/vm/drop_caches
+ dd if=/mnt/sdb/zerofile1 of=/dev/null &
+ echo $! > /cgroup/test1/tasks
+ cat /cgroup/test1/tasks
+ dd if=/mnt/sdb/zerofile2 of=/dev/null &
+ echo $! > /cgroup/test2/tasks
+ cat /cgroup/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 milli seconds), each group got and how many secotors 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.
+Various user visible config options
+ - Enables group scheduling in CFQ. Currently only 1 level of group
+ creation is allowed.
+ - Enables some debugging messages in blktrace. Also creates extra
+ cgroup file blkio.dequeue.
+Config options selected automatically
+These config options are not user visible and are selected/deselected
+automatically based on IO scheduler configuration.
+ - Block IO controller. Selected by CONFIG_CFQ_GROUP_IOSCHED.
+ - Debug help. Selected by CONFIG_DEBUG_CFQ_IOSCHED.
+Details of cgroup files
+- blkio.weight
+ - Specifies per cgroup weight.
+ Currently allowed range of weights is from 100 to 1000.
+- blkio.time
+ - disk time allocated to cgroup per device in milliseconds. First
+ two fields specify the major and minor number of the device and
+ third field specifies the disk time allocated to group in
+ milliseconds.
+- blkio.sectors
+ - number of sectors transferred to/from disk by the group. First
+ two fields specify the major and minor number of the device and
+ third field specifies the number of sectors transferred by the
+ group to/from the device.
+- blkio.dequeue
+ - Debugging aid only enabled if CONFIG_DEBUG_CFQ_IOSCHED=y. This
+ gives the statistics about how many a times a group was dequeued
+ from service tree of the device. First two fields specify the major
+ and minor number of the device and third field specifies the number
+ of times a group was dequeued from a particular device.
+CFQ sysfs tunable
+If group_isolation=1, it provides stronger isolation between groups at the
+expense of throughput. By default group_isolation is 0. In general that
+means that if group_isolation=0, expect fairness for sequential workload
+only. Set group_isolation=1 to see fairness for random IO workload also.
+Generally CFQ will put random seeky workload in sync-noidle category. CFQ
+will disable idling on these queues and it does a collective idling on group
+of such queues. Generally these are slow moving queues and if there is a
+sync-noidle service tree in each group, that group gets exclusive access to
+disk for certain period. That means it will bring the throughput down if
+group does not have enough IO to drive deeper queue depths and utilize disk
+capacity to the fullest in the slice allocated to it. But the flip side is
+that even a random reader should get better latencies and overall throughput
+if there are lots of sequential readers/sync-idle workload running in the
+If group_isolation=0, then CFQ automatically moves all the random seeky queues
+in the root group. That means there will be no service differentiation for
+that kind of workload. This leads to better throughput as we do collective
+idling on root sync-noidle tree.
+By default one should run with group_isolation=0. If that is not sufficient
+and one wants stronger isolation between groups, then set group_isolation=1
+but this will come at cost of reduced throughput.
+What works
+- Currently only sync IO queues are support. All the buffered writes are
+ still system wide and not per group. Hence we will not see service
+ differentiation between buffered writes between groups.