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

Paul E. McKenney
2015-05-21T12:58:09Z

On Thu, May 21, 2015 at 05:42:46PM +0530, Afzal Mohammed wrote:
> Hi,
>
> On Wed, May 20, 2015 at 02:00:26PM -0700, Paul E. McKenney wrote:
>
> > > > 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?
>
> I was just playing NO_HZ_FULL with tip-[sched,timers]-* changes.
>
> Thought that shutting down ticks as much as possible would be
> beneficial to normal loads too, though it has been mentioned to be used
> for specialized loads. Seems like drawbacks due to it weigh against
> normal loads, but haven't so far observed any (on a laptop with normal
> activities) before this change.

Indeed, NO_HZ_FULL is special purpose. You normally would select
NO_HZ_FULL_ALL only on a system intended for heavy compute without
normal-workload distractions or for some real-time systems. For mixed
workloads, you would build with NO_HZ_FULL (but not NO_HZ_FULL_ALL) and
use the boot parameters to select which CPUs are to be running the
specialized portion of the workload.

And you would of course need to lead enough CPUs running normally to
handle the non-specialized portion of the workload.

This sort of thing has traditionally required specialized kernels,
so the cool thing here is that we can make Linux do it. Though, as
you noticed, careful configuration is still required.

Seem reasonable?

Thanx, Paul