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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.
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