[Devel] Re: A question about group CFS scheduling

[Peter Zijlstra]

On Thu, 2008-06-26 at 15:19 +0800, Zhao Forrest wrote:
> Hi experts,
> 
> In Documentation/sched-design-CFS.txt it reads:
> Group scheduler tunables:
> 
> When CONFIG_FAIR_USER_SCHED is defined, a directory is created in sysfs for
> each new user and a "cpu_share" file is added in that directory.
>         # cd /sys/kernel/uids
>         # cat 512/cpu_share             # Display user 512's CPU share
>         1024
>         # echo 2048 > 512/cpu_share     # Modify user 512's CPU share
>         # cat 512/cpu_share             # Display user 512's CPU share
>         2048
>         #
> CPU bandwidth between two users are divided in the ratio of their CPU shares.
> For ex: if you would like user "root" to get twice the bandwidth of user
> "guest", then set the cpu_share for both the users such that "root"'s
> cpu_share is twice "guest"'s cpu_share.
> 
> My question is: how is CPU bandwidth divided between cgroup and
> regular processes?
> For example,
> 1 the cpu_share of user "root" is set to 2048
> 2 the cpu_share of user "guest" is set to 1024
> 3 there're many processes owned by other users, which don't belong to any cgroup

A process always belongs to a (c)group.

> if the relative CPU bandwidth allocated to cgroup of "root" is 2,
> allocated to cgroup of "guest" is 1, then what's the relative CPU
> bandwidth allocated to other regular processes? 2 or 1?

Are you interested in UID based group scheduling or cgroup scheduling?

Let me explain the cgroup case (the sanest option IMHO):

initially all your tasks will belong to the root cgroup, eg:

assuming:
mkdir -p /cgroup/cpu
mount none /cgroup/cpu -t cgroup -o cpu

Then the root cgroup (cgroup:/) is /cgroup/cpu/ and all tasks will be
found in /cgroup/cpu/tasks.

You can then create new groups as sibling from this root group, eg:

cgroup:/foo
cgroup:/bar

They will get a weigth of 1024 by default, exactly as heavy as a nice 0
task.

That means that no matter how many tasks you stuff into foo, their
combined cpu time will be as much as a single tasks in cgroup:/ would
get.

This is fully recursive, so you can also create:

cgroup:/foo/bar and its tasks in turn will get as much combined cpu time
as a single task in cgroup:/foo would get.

In theory this should go on indefinitely, in practise we'll run into
serious numerical issues quite quickly.


The USER grouping basically creates a fake root and all uids (including
0) are its siblings. The only special case is that uid-0 (aka root) will
get twice the weight of the others.




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