linux-kernel - Re: [PATCH RFC tip/core/rcu] Parallelize and economize NOCB kthread wakeups

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Date:	Fri, 04 Jul 2014 08:01:34 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, riel@...hat.com, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	sbw@....edu
Subject: Re: [PATCH RFC tip/core/rcu] Parallelize and economize NOCB kthread
 wakeups

On Thu, 2014-07-03 at 22:05 -0700, Paul E. McKenney wrote: 
> On Fri, Jul 04, 2014 at 05:23:56AM +0200, Mike Galbraith wrote:

> > Turn it on and don't worry about it is exactly what distros want the
> > obscure feature with very few users to be.  Last time I did a drive-by,
> > my boxen said I should continue to worry about it ;-)
> 
> Yep, which is the reason for the patch on the last email.
> 
> Then again, exactly which feature and which reason for worry?

NO_HZ_FULL.  I tried ALL a while back, box instantly called me an idiot.
Maybe that has improved since, dunno.

Last drive-by I didn't do much overhead measurement, stuck mostly
functionality, and it still had rough edges that enterprise users may
not fully appreciate.  Trying to let 60 of 64 cores do 100% compute
showed some cores having a hard time entering tickless at all, and
~200us spikes that I think are due to tick losing skew.. told Frederic
I'd take a peek at that, but haven't had time yet.  There were other
known things as well, like timers and workqueues for which there are
patches floating around.  All in all, it was waving the men at work
sign, pointing at the "Say N" by the config option, and suggesting that
ignoring that would not be the cleverest of moves.

-Mike

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