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sched.7: Rework discussion of autogroups 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2016-11-24 21:20:03 +01:00
parent 576b74eec2
commit ee1f3c18a2
1 changed files with 44 additions and 25 deletions
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@ -683,7 +683,7 @@ or
.\" commit 5091faa449ee0b7d73bc296a93bca9540fc51d0a
Since Linux 2.6.38,
the kernel provides a feature known as autogrouping to improve interactive
desktop performance in the face of multiprocess CPU-intensive
desktop performance in the face of multiprocess, CPU-intensive
workloads such as building the Linux kernel with large numbers of
parallel build processes (i.e., the
.BR make (1)
@ -700,25 +700,39 @@ The default value in this file is 1, unless the kernel was booted with the
.IR noautogroup
parameter.
When autogrouping is enabled, processes are automatically placed
into "task groups" for the purposes of scheduling.
In the current implementation,
a new task group is created when a new session is created via
.BR setsid (2),
as happens, for example, when a new terminal window is created.
A task group is automatically destroyed when the last process
A new autogroup is created created when a new session is created via
.BR setsid (2);
this happens, for example, when a new terminal window is started.
A new process created by
.BR fork (2)
inherits its parent's autogroup membership.
Thus, all of the processes in a session are members of the same autogroup.
An autogroup is automatically destroyed when the last process
in the group terminates.
.\" FIXME The following is a little vague. Does it need to be
.\" made more precise?
The CFS scheduler employs an algorithm that distributes the CPU
across task groups.
As a result of this algorithm,
the processes in task groups that contain multiple CPU-intensive
processes are in effect disfavored by the scheduler.
When autogrouping is enabled, all of the members of an autogroup
are placed in the same kernel scheduler "task group".
The CFS scheduler employs an algorithm that equalizes the
distribution of CPU cycles across task groups.
The benefits of this for interactive desktop performance
can be described via the following example.
A process's autogroup (task group) membership can be viewed via
the file
Suppose that there are two autogroups competing for the same CPU.
The first group contains ten CPU-bound processes from
a kernel build started with
.IR "make\ \-j10" .
The other contains a single CPU-bound process: a video player.
The effect of autogrouping is that the two groups will
each receive half of the CPU cycles.
That is, the video player will receive 50% of the CPU cycles,
rather just 9% of the cycles,
which would likely lead to degraded video playback.
Or to put things another way:
an autogroup that contains a large number of CPU-bound processes
does not end up overwhelming the CPU at the expense of the other
jobs on the system.
A process's autogroup (task group) membership can be viewed via the file
.IR /proc/[pid]/autogroup :
.nf
@ -729,13 +743,15 @@ $ \fBcat /proc/1/autogroup\fP
.fi
This file can also be used to modify the CPU bandwidth allocated
to a task group.
to an autogroup.
This is done by writing a number in the "nice" range to the file
to set the task group's nice value.
to set the autogroup's nice value.
The allowed range is from +19 (low priority) to \-20 (high priority),
and the setting has the same effect as modifying the nice level via
.BR getpriority (2).
(For a discussion of the nice value, see
.BR getpriority (2).)
The allowed range is from +19 (low priority) to \-20 (high priority).
.\" FIXME
.\" FIXME .
.\" Because of a bug introduced in Linux 4.7
.\" (commit 2159197d66770ec01f75c93fb11dc66df81fd45b made changes
.\" that exposed the fact that autogroup didn't call scale_load()),
@ -747,14 +763,17 @@ The allowed range is from +19 (low priority) to \-20 (high priority).
.\" A patch was posted on 23 Nov 2016
.\" ("sched/autogroup: Fix 64bit kernel nice adjustment";
.\" check later to see in which kernel version it lands.
.\"
.\" FIXME How do the nice value of a process and the nice value of
.\" an autogroup interact? Which has priority?
.\" FIXME Is the following correct? Does the statement need to
.\" be more precise? (E.g., in precisely which circumstances does
.\" the use of cgroups override autogroup?)
The use of the
.BR cgroups (7)
CPU controller overrides the effect of autogrouping.
.\" FIXME What needs to be said about autogroup and real-time tasks?
The autogroup feature does not group processes
that are scheduled under a real-time and deadline policies.
Those processes are scheduled according to the rules described earlier.
.SH NOTES
The
.BR cgroups (7)