diff options
| author | Michael Kerrisk <mtk.manpages@gmail.com> | 2016-11-24 21:20:03 +0100 |
|---|---|---|
| committer | Michael Kerrisk <mtk.manpages@gmail.com> | 2016-11-29 21:50:15 +0100 |
| commit | ee1f3c18a2ce4cee4abfbff7c20e6a102ade81c1 (patch) | |
| tree | 204fc8d83c03dfe3b70454a84fe7240c78f3c587 /man7/sched.7 | |
| parent | 576b74eec2e9d0243e3169b95efef8ce6425083d (diff) | |
| download | man-pages-ee1f3c18a2ce4cee4abfbff7c20e6a102ade81c1.tar.gz | |
sched.7: Rework discussion of autogroups
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Diffstat (limited to 'man7/sched.7')
| -rw-r--r-- | man7/sched.7 | 71 |
1 files changed, 45 insertions, 26 deletions
diff --git a/man7/sched.7 b/man7/sched.7 index f372638baf..4787d752f7 100644 --- a/man7/sched.7 +++ b/man7/sched.7 @@ -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. - -A process's autogroup (task group) membership can be viewed via -the file +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. + +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) |