linux-kernel - Re: [RFC PATCH 7/9] housekeeping: Use own boot option, independant from nohz

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Date:   Tue, 15 Aug 2017 11:26:11 -0500 (CDT)
From:   Christopher Lameter <cl@...ux.com>
To:     Mike Galbraith <efault@....de>
cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>
Subject: Re: [RFC PATCH 7/9] housekeeping: Use own boot option, independant
 from nohz

On Tue, 15 Aug 2017, Mike Galbraith wrote:

> On Tue, 2017-08-15 at 10:52 -0500, Christopher Lameter wrote:
> > On Tue, 15 Aug 2017, Paul E. McKenney wrote:
> >
> > > Don't the HPC guys just disable idle_balance(), or am I out of date again?
> >
> > Ummm.. Why does idle management matter when your goal is to keep all
> > processor busy working at maximum throughput?
>
> If you _never_ idle, you never have to worry about it.  Is 100% CPU
> until the end of time all there is to HPC?

Most of the time that is true for HPC loads. They may also go through a
I/O throughput constrained processing phase or synchronization phase where
idle activity occurs.

However, there are also low latency loads that are often confused with
HPC. Those are usually waiting idle until an event happens and then have
to react to it in the fastest way possible. After that they go back to
idle. You could call this a RT load (since the term seems to be so
flexible...) but its more event based than the typical idea of realtime
(do something at this and thata time).