Hello Mike and others,
The autogroup feature that you added in 2.6.38 remains poorly
documented, so I took a stab at adding some text to the sched(7)
manual page. There are still a few pieces to be fixed, and you
may also see some other pieces that should be added. Could I
ask you to take a look at the text below?
Cheers,
Michael
The autogroup feature
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 workloads such as building
the Linux kernel with large numbers of parallel build processes
(i.e., the make(1) -j flag).
This feature operates in conjunction with the CFS scheduler and
requires a kernel that is configured with CONFIG_SCHED_AUTO‐
GROUP. On a running system, this feature is enabled or dis‐
abled via the file /proc/sys/kernel/sched_autogroup_enabled; a
value of 0 disables the feature, while a value of 1 enables it.
The default value in this file is 1, unless the kernel was
booted with the 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 setsid(2), as happens, for example,
when a new terminal window is created. A task group is auto‐
matically destroyed when the last process in the group termi‐
nates.
┌─────────────────────────────────────────────────────┐
│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 pro‐
cesses in task groups that contain multiple CPU-intensive pro‐
cesses are in effect disfavored by the scheduler.
A process's autogroup (task group) membership can be viewed via
the file /proc/[pid]/autogroup:
$ cat /proc/1/autogroup
/autogroup-1 nice 0
This file can also be used to modify the CPU bandwidth allo‐
cated to a task group. This is done by writing a number in the
"nice" range to the file to set the task group's nice value.
The allowed range is from +19 (low priority) to -20 (high pri‐
ority). Note that all values in this range cause a task group
to be further disfavored by the scheduler, with -20 resulting
in the scheduler mildy disfavoring the task group and +19
greatly disfavoring it.
┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Regarding the previous paragraph... My tests indi‐ │
│cate that writing *any* value to the autogroup file │
│causes the task group to get a lower priority. This │
│somewhat surprised me, since I assumed (based on the │
│parallel with the process nice(2) value) that nega‐ │
│tive values might boost the task group's priority │
│above a task group whose autogroup file had not been │
│touched. │
│ │
│Is this the expected behavior? I presume it is... │
│ │
│But then there's a small surprise in the interface. │
│Suppose that the value 0 is written to the autogroup │
│file, then this results in the task group being sig‐ │
│nificantly disfavored. But, the nice value *shown* │
│in the autogroup file will be the same as if the │
│file had not been modified. So, the user has no way │
│of discovering the difference. That seems odd. Am I │
│missing something? │
└─────────────────────────────────────────────────────┘
┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Is the following correct? Does the statement need to │
│be more precise? (E.g., in precisely which circum‐ │
│stances does the use of cgroups override autogroup?) │
└─────────────────────────────────────────────────────┘
The use of the cgroups(7) CPU controller overrides the effect
of autogrouping.
┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│What needs to be said about autogroup and real-time │
│tasks? │
└─────────────────────────────────────────────────────┘
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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