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-rw-r--r--Documentation/sched-design-CFS.txt67
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diff --git a/Documentation/sched-design-CFS.txt b/Documentation/sched-design-CFS.txt
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@@ -117,3 +117,70 @@ Some implementation details:
117 iterators of the scheduling modules are used. The balancing code got 117 iterators of the scheduling modules are used. The balancing code got
118 quite a bit simpler as a result. 118 quite a bit simpler as a result.
119 119
120
121Group scheduler extension to CFS
122================================
123
124Normally the scheduler operates on individual tasks and strives to provide
125fair CPU time to each task. Sometimes, it may be desirable to group tasks
126and provide fair CPU time to each such task group. For example, it may
127be desirable to first provide fair CPU time to each user on the system
128and then to each task belonging to a user.
129
130CONFIG_FAIR_GROUP_SCHED strives to achieve exactly that. It lets
131SCHED_NORMAL/BATCH tasks be be grouped and divides CPU time fairly among such
132groups. At present, there are two (mutually exclusive) mechanisms to group
133tasks for CPU bandwidth control purpose:
134
135 - Based on user id (CONFIG_FAIR_USER_SCHED)
136 In this option, tasks are grouped according to their user id.
137 - Based on "cgroup" pseudo filesystem (CONFIG_FAIR_CGROUP_SCHED)
138 This options lets the administrator create arbitrary groups
139 of tasks, using the "cgroup" pseudo filesystem. See
140 Documentation/cgroups.txt for more information about this
141 filesystem.
142
143Only one of these options to group tasks can be chosen and not both.
144
145Group scheduler tunables:
146
147When CONFIG_FAIR_USER_SCHED is defined, a directory is created in sysfs for
148each new user and a "cpu_share" file is added in that directory.
149
150 # cd /sys/kernel/uids
151 # cat 512/cpu_share # Display user 512's CPU share
152 1024
153 # echo 2048 > 512/cpu_share # Modify user 512's CPU share
154 # cat 512/cpu_share # Display user 512's CPU share
155 2048
156 #
157
158CPU bandwidth between two users are divided in the ratio of their CPU shares.
159For ex: if you would like user "root" to get twice the bandwidth of user
160"guest", then set the cpu_share for both the users such that "root"'s
161cpu_share is twice "guest"'s cpu_share
162
163
164When CONFIG_FAIR_CGROUP_SCHED is defined, a "cpu.shares" file is created
165for each group created using the pseudo filesystem. See example steps
166below to create task groups and modify their CPU share using the "cgroups"
167pseudo filesystem
168
169 # mkdir /dev/cpuctl
170 # mount -t cgroup -ocpu none /dev/cpuctl
171 # cd /dev/cpuctl
172
173 # mkdir multimedia # create "multimedia" group of tasks
174 # mkdir browser # create "browser" group of tasks
175
176 # #Configure the multimedia group to receive twice the CPU bandwidth
177 # #that of browser group
178
179 # echo 2048 > multimedia/cpu.shares
180 # echo 1024 > browser/cpu.shares
181
182 # firefox & # Launch firefox and move it to "browser" group
183 # echo <firefox_pid> > browser/tasks
184
185 # #Launch gmplayer (or your favourite movie player)
186 # echo <movie_player_pid> > multimedia/tasks