path: root/Documentation/sched-design-CFS.txt
diff options
Diffstat (limited to 'Documentation/sched-design-CFS.txt')
1 files changed, 67 insertions, 0 deletions
diff --git a/Documentation/sched-design-CFS.txt b/Documentation/sched-design-CFS.txt
index 84901e7c0508..88bcb8767335 100644
--- a/Documentation/sched-design-CFS.txt
+++ b/Documentation/sched-design-CFS.txt
@@ -117,3 +117,70 @@ Some implementation details:
iterators of the scheduling modules are used. The balancing code got
quite a bit simpler as a result.
+Group scheduler extension to CFS
+Normally the scheduler operates on individual tasks and strives to provide
+fair CPU time to each task. Sometimes, it may be desirable to group tasks
+and provide fair CPU time to each such task group. For example, it may
+be desirable to first provide fair CPU time to each user on the system
+and then to each task belonging to a user.
+CONFIG_FAIR_GROUP_SCHED strives to achieve exactly that. It lets
+SCHED_NORMAL/BATCH tasks be be grouped and divides CPU time fairly among such
+groups. At present, there are two (mutually exclusive) mechanisms to group
+tasks for CPU bandwidth control purpose:
+ - Based on user id (CONFIG_FAIR_USER_SCHED)
+ In this option, tasks are grouped according to their user id.
+ - Based on "cgroup" pseudo filesystem (CONFIG_FAIR_CGROUP_SCHED)
+ This options lets the administrator create arbitrary groups
+ of tasks, using the "cgroup" pseudo filesystem. See
+ Documentation/cgroups.txt for more information about this
+ filesystem.
+Only one of these options to group tasks can be chosen and not both.
+Group scheduler tunables:
+When CONFIG_FAIR_USER_SCHED is defined, a directory is created in sysfs for
+each new user and a "cpu_share" file is added in that directory.
+ # cd /sys/kernel/uids
+ # cat 512/cpu_share # Display user 512's CPU share
+ 1024
+ # echo 2048 > 512/cpu_share # Modify user 512's CPU share
+ # cat 512/cpu_share # Display user 512's CPU share
+ 2048
+ #
+CPU bandwidth between two users are divided in the ratio of their CPU shares.
+For ex: if you would like user "root" to get twice the bandwidth of user
+"guest", then set the cpu_share for both the users such that "root"'s
+cpu_share is twice "guest"'s cpu_share
+When CONFIG_FAIR_CGROUP_SCHED is defined, a "cpu.shares" file is created
+for each group created using the pseudo filesystem. See example steps
+below to create task groups and modify their CPU share using the "cgroups"
+pseudo filesystem
+ # mkdir /dev/cpuctl
+ # mount -t cgroup -ocpu none /dev/cpuctl
+ # cd /dev/cpuctl
+ # mkdir multimedia # create "multimedia" group of tasks
+ # mkdir browser # create "browser" group of tasks
+ # #Configure the multimedia group to receive twice the CPU bandwidth
+ # #that of browser group
+ # echo 2048 > multimedia/cpu.shares
+ # echo 1024 > browser/cpu.shares
+ # firefox & # Launch firefox and move it to "browser" group
+ # echo <firefox_pid> > browser/tasks
+ # #Launch gmplayer (or your favourite movie player)
+ # echo <movie_player_pid> > multimedia/tasks