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sched.7: Improve section on nice value and group scheduling 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2016-11-29 07:34:53 +01:00
parent 4fbe161bf2
commit a695d35c98
1 changed files with 35 additions and 15 deletions
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@ -815,26 +815,42 @@ and
.BR SCHED_IDLE
policies), the CFS scheduler employs a technique known as "group scheduling",
if the kernel was configured with the
.BR CONFIG_GROUP_SCHED
.BR CONFIG_FAIR_GROUP_SCHED
option (which is typical).
Scheduling groups (also known as task groups)
are formed in the following circumstances:
Under group scheduling, threads are scheduled in "task groups".
Task groups have a hierarchical relationship,
rooted under the initial task group on the system,
known as the "root task group".
Task groups are formed in the following circumstances:
.IP * 3
All of the processes in a non-root CPU cgroup form a scheduling group.
Thus, each non-root CPU cgroup is a scheduling group.
All of the threads in a CPU cgroup form a task group.
The parent of this task group is the task group of the
corresponding parent cgroup.
.IP *
All of the processes that are (implicitly) placed in an autogroup
If autogrouping is enabled,
then all of the threads that are (implicitly) placed in an autogroup
(i.e., the same session, as created by
.BR setsid (2))
form a scheduling group.
Each new autogroup is thus a separate scheduling group.
form a task group.
Each new autogroup is thus a separate task group.
The root task group is the parent of all such autogroups.
.IP *
If autogrouping is disabled, then all processes in the root CPU cgroup
form a scheduling group (sometimes called the "root task group").
If autogrouping is enabled, then the root task group consists of
all processes in the root CPU cgroup that were not
otherwise implicitly placed into a new autogroup.
.IP *
If autogrouping is disabled, then the root task group consists of
all processes in the root CPU cgroup.
.IP *
If group scheduling was disabled (i.e., the kernel was configured without
.BR CONFIG_FAIR_GROUP_SCHED ),
then all of the processes on the system are notionally placed
in a single task group.
.PP
Under group scheduling,
the nice value has an effect for scheduling decisions
.IR "only relative to other threads in the same group" .
a thread's nice value has an effect for scheduling decisions
.IR "only relative to other threads in the same task group" .
This has some surprising consequences in terms of the traditional semantics
of the nice value on UNIX systems.
In particular, if autogrouping
@ -845,13 +861,17 @@ or
on a process has an effect only for scheduling relative
to other processes executed in the same session
(typically: the same terminal window).
Conversely, for two process that are (for example) the sole CPU-bound processes
in different sessions (different terminal windows),
Conversely, for two processes that are (for example)
the sole CPU-bound processes in different sessions
(e.g., different terminal windows,
each of whose jobs are tied to different autogroups),
.IR "modifying the nice value of the process in one of the sessions"
.IR "has no effect"
in terms of the scheduler's decisions relative to the
process in the other session.
.\" More succinctly: the nice(1) command is nearly a no-op since Linux 2.6.38.
.\" More succinctly: the nice(1) command is in many cases a no-op since
.\" Linux 2.6.38.
.\"
.SS Real-time features in the mainline Linux kernel
.\" FIXME . Probably this text will need some minor tweaking