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sched.7: Add a subsection on group scheduling 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Michael Kerrisk 2016-11-27 20:18:08 +01:00
parent e9c1649aa7
commit e92070f8cc
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@ -439,6 +439,52 @@ an unprivileged process's nice value can be raised; see
.BR setrlimit (2)
for details.
.\"
.SS The nice value and group scheduling
When scheduling non-real-time processes (i.e., those scheduled under the
.BR SCHED_OTHER ,
.BR SCHED_BATCH ,
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
option (which is typical).
Scheduling groups (also known as 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.
.IP *
All of the processes 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.
.IP *
If autogrouping is disabled, then all processes in the root CPU cgroup
form a scheduling group (sometimes called the "root 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" .
This has some surprising consequences in terms of the traditional semantics
of the nice value on UNIX systems.
In particular, if autogrouping
is enabled (which is the default), then employing
.BR setpriority (2)
or
.BR nice (1)
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),
.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.
.\"
.SS SCHED_BATCH: Scheduling batch processes
(Since Linux 2.6.16.)
\fBSCHED_BATCH\fP can be used only at static priority 0.