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diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt new file mode 100644 index 000000000000..630879cd9a42 --- /dev/null +++ b/Documentation/cgroups/blkio-controller.txt @@ -0,0 +1,135 @@ + Block IO Controller + =================== +Overview +======== +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. + +HOWTO +===== +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 + CONFIG_CFQ_GROUP_IOSCHED=y + +- 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 +=================================== +CONFIG_CFQ_GROUP_IOSCHED + - Enables group scheduling in CFQ. Currently only 1 level of group + creation is allowed. + +CONFIG_DEBUG_CFQ_IOSCHED + - 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. + +CONFIG_BLK_CGROUP + - Block IO controller. Selected by CONFIG_CFQ_GROUP_IOSCHED. + +CONFIG_DEBUG_BLK_CGROUP + - 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 +================= +/sys/block/<disk>/queue/iosched/group_isolation + +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 +system. + +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. |