linux-kernel - Re: [GIT PULL, RFC] Full dynticks, CONFIG_NO_HZ

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Message-ID: <518A9614.2010805@tilera.com>
Date:	Wed, 8 May 2013 14:14:44 -0400
From:	Chris Metcalf <cmetcalf@...era.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Fr?d?ric Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL, RFC] Full dynticks, CONFIG_NO_HZ_FULL feature

On 5/6/2013 3:32 PM, Linus Torvalds wrote:
> On Mon, May 6, 2013 at 8:35 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
>>>
>>> I think Linus might have referred to my 'future plans' entry:
> 
> Indeed. I feel that HPC is entirely irrelevant to anybody,
> *especially* HPC benchmarks. In real life, even HPC doesn't tend to
> have the nice behavior their much-touted benchmarks have.
> 
> So as long as the NOHZ is for HPC-style loads, then quite frankly, I
> don't feel it is worth it. The _only_ thing that makes it worth it is
> that "future plans" part where it would actually help real loads.

The work is very relevant to a lot of the actual customer applications
that Tilera chips are sold into: running very low latency userspace
applications that handle packet processing (or, to a lesser extent,
video frame processing).  For packet processing in particular, you really
want to be able to guarantee that you can set up a core handling packets
in userspace and get NO INTERRUPTS at all, ever.  If you do get an
interrupt, you end up dropping a bunch of packets on the floor.

We are still using code I developed internally for this (for the curious,
in my dataplane branch on kernel.org), but I expect in the relatively near
future we will be trying to switch to the NOHZ stuff instead.  I'm
really pleased to see it start getting merged up.
-- 
Chris Metcalf, Tilera Corp.
http://www.tilera.com
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