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Date:   Fri, 26 Apr 2019 11:51:23 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Aubrey Li <aubrey.intel@gmail.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Vineeth Remanan Pillai <vpillai@digitalocean.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Turner <pjt@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Subhra Mazumdar <subhra.mazumdar@oracle.com>,
        Fr?d?ric Weisbecker <fweisbec@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>, Phil Auld <pauld@redhat.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jiri Kosina <jkosina@suse.cz>
Subject: Re: [RFC PATCH v2 00/17] Core scheduling v2
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* Mel Gorman <mgorman@techsingularity.net> wrote:

> > Interesting.
> > 
> > Here too I'm wondering whether the scheduler could do something to 
> > improve the saturated case: which *is* an important workload, as kernel 
> > hackers tend to over-load their systems a bit when building kernel, to 
> > make sure the system is at least 100% utilized. ;-)
> > 
> 
> Every so often I try but I never managed to settle on a heuristic that 
> helped this case without breaking others. The biggest hurdle is that 
> typically things are better if migrations are low but it's hard to do 
> that in a way that does not also stack tasks on the same CPUs 
> prematurely.

So instead of using a heuristic (which are fragile and most of them are 
also annoyingly non-deterministic and increase overall noise and make 
measurements harder) I'd suggest using SCHED_BATCH just as a hardcore 
toggle to maximize for CPU-bound throughput.

It's not used very much, but the kernel build could use it by default 
(i.e. we could use a "chrt -b" call within the main Makefile), so it 
would be the perfect guinea pig and wouldn't affect anything else.

I.e. we could use SCHED_BATCH to maximize kernel build speed, with no no 
regard to latency (within SCHED_BATCH workloads). I suspect this will 
also maximize bandwidth of a lot of other real-world, highly parallel but 
interacting processing workloads.

[ I'd even be willing to rename it to SCHED_KBUILD, internally. ;-) ]

Thanks,

	Ingo