Difference between chrt and nice

Difference between chrt and nice

[Jaakko Sipari]

Hi!

I've already asked this in a couple of forums but got no answers. I
would appreciate it if someone in this list could take some time to
answer to my possibly stupid question(s):

What's the difference of setting process priority with chrt and nice?
And how do using both of these commands compare with a normal vs.
realtime-kernel  (PREEMPT_RT)?

BR, Jaakko

Re: Difference between chrt and nice

[Tobias Knutsson]

Hi Jaakko,

I'm relatively new in this field so take this information will a grain of salt.

From what i understand, the nice command will only change the priority
within the SCHED_OTHER scheduling policy. It is not possible to assign
the process to another policy such as SCHED_RR or SCHED_FIFO, which
would have precedence over all SCHED_OTHER processes. This can be done
with chrt.

Both of these commands should work both with a vanilla and an rt
kernel. The result will be different however. Since the locking
mechanisms in the rt kernel are much more sophisticated, it will be
able to provide much more deterministic latencies to the processes
assigned to the processes assigned to the realtime scheduling policies
(round robin and fifo).

//Tobbe

On Tue, Mar 31, 2009 at 07:43, Jaakko Sipari <jaakko.sipari@gmail.com> wrote:
> Hi!
>
> I've already asked this in a couple of forums but got no answers. I
> would appreciate it if someone in this list could take some time to
> answer to my possibly stupid question(s):
>
> What's the difference of setting process priority with chrt and nice?
> And how do using both of these commands compare with a normal vs.
> realtime-kernel  (PREEMPT_RT)?
>
> BR, Jaakko
> --
> To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in
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-- 

Hälsningar/Regards
Tobias Knutsson
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Re: Difference between chrt and nice

[Thomas Gleixner]

On Tue, 31 Mar 2009, Jaakko Sipari wrote:

> Hi!
> 
> I've already asked this in a couple of forums but got no answers. I
> would appreciate it if someone in this list could take some time to
> answer to my possibly stupid question(s):
> 
> What's the difference of setting process priority with chrt and nice?

"nice" is an historic utility which was used in the early days of batch
computing to be "nice" to other users and give up some CPU time. It's
still in use and useful and applies only to processes which run with
the SCHED_OTHER policy on Linux. 

"chrt" is a tool to change scheduling policy(SCHED_OTHER, _FIFO, RR)
and the priority of a process/task. With chrt you can either start a
process with such a policy or modify an already running process/tasks
policy. You need to have the permissions to do that.

So the main difference is that "nice" can only operate within the nice
levels of the SCHED_OTHER policy while "chrt" can change the policy
and the priority of a process/task.

> And how do using both of these commands compare with a normal vs.
> realtime-kernel  (PREEMPT_RT)?

Both commands are the same on vanilla and -rt kernels. The difference
is just the runtime behaviour of the -rt enabled kernel. processes /
tasks which run with real time scheduling policy have a deterministic
behaviour which is not and can not (yet) be guaranteed by the vanilla
kernel.

Hope that helps.

Thanks,

	tglx

P.S.: further info at http://rt.wiki.kernel.org