Message-ID: <1498792305.28164.68.camel@gmail.com>
Subject: Re: wake_wide mechanism clarification
From: Mike Galbraith <umgwanakikbuti@gmail.com>
To: Josef Bacik <josef@toxicpanda.com>, Joel Fernandes <joelaf@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, Juri Lelli <Juri.Lelli@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Brendan Jackman <brendan.jackman@arm.com>,
        Chris Redpath <Chris.Redpath@arm.com>
Date: Fri, 30 Jun 2017 05:11:45 +0200
In-Reply-To: <20170630004912.GA2457@destiny>
References: <CAJWu+oo0_zAWLjObRZp-r3VH5kKKb4aFVnRevtbYh+uH_tCNag@mail.gmail.com>
         <20170630004912.GA2457@destiny>
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On Thu, 2017-06-29 at 20:49 -0400, Josef Bacik wrote:
> On Thu, Jun 29, 2017 at 05:19:14PM -0700, Joel Fernandes wrote:
> 
> > Why are wanting the master's flip frequency to be higher than the
> > slaves by the factor?
> 
> (Responding from my personal email as my work email is outlook shit and
> impossible to use)
> 
> Because we are trying to detect the case that the master is waking many
> different processes, and the 'slave' processes are only waking up the
> master/some other specific processes to determine if we don't care about cache
> locality.

Yes, the heuristic (large delta implies waker/wakee are NOT 1:1, ie
filter out high frequency communication where eating misses doesn't
merely sting, it hurts like hell) just became bidirectional.

> Actually I think both are wrong, but I need Mike to weigh in.

My weigh in is this: if you have ideas to improve or replace that
heuristic, by all means go for it, just make damn sure it's dirt cheap.
 Heuristics all suck one way or another, problem is that nasty old
"perfect is the enemy of good" adage.  Make it perfect, it'll hurt.

	-Mike