Re: [PATCH 4/4] nohz: Set isolcpus when nohz_full is set

Paul E. McKenney
2015-05-20T21:00:41Z

On Thu, May 21, 2015 at 02:08:09AM +0530, Afzal Mohammed wrote:
> Hi,
>
> On Sun, May 17, 2015 at 07:30:50AM +0200, Mike Galbraith wrote:
>
> > Yeah, tying nohz_full set to isolcpus set up an initial condition that
> > you have to tear down with cpusets if you want those cpus returned to
> > the general purpose pool. I had considered the kernel setting initial
> > state to be an optimization, but have reconsidered.
>
> > Given that kernel initiated association to isolcpus, a user turning
> > NO_HZ_FULL_ALL on had better not have much generic load to manage. If
>
> On a quad-core desktop system with NO_HZ_FULL_ALL, hackbench took 3x
> time as compared to w/o this patch, except boot cpu every one else
> jobless. Though NO_HZ_FULL_ALL (afaik) is not meant for generic load,
> it was working fine, but not after this - it is now like a single core
> system.

I have to ask... What is your use case? What are you wanting NO_HZ_FULL
to do for you?

Thanx, Paul

> Regards
> Afzal
>
> > he/she does not have CPUSETS enabled, or should Rik's patch rendering
> > isolcpus immutable be merged, he/she would quickly discover that the
> > generic box has been transformed into an utterly inflexible specialist
> > incapable of performing mundane tasks ever again.
>