linux-kernel - Re: Does anyone use CONFIG_TINY_PREEMPT

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Message-ID: <20121113224040.GA14594@elliptictech.com>
Date:	Tue, 13 Nov 2012 17:40:40 -0500
From:	Nick Bowler <nbowler@...iptictech.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, edumazet@...gle.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org
Subject: Re: Does anyone use CONFIG_TINY_PREEMPT_RCU?

On 2012-11-13 14:25 -0800, Paul E. McKenney wrote:
> On Tue, Nov 13, 2012 at 04:47:20PM -0500, Nick Bowler wrote:
> > On 2012-11-13 13:19 -0800, Paul E. McKenney wrote:
> > > On Tue, Nov 13, 2012 at 12:56:54PM -0500, Nick Bowler wrote:
> > > > On 2012-11-13 09:08 -0800, Paul E. McKenney wrote:
> > > > > Suppose that TREE_PREEMPT_RCU was available for !SMP && PREEMPT builds.
> > > > > Would that work for you?
> > > > 
> > > > To be honest I don't really know what the difference is, other than what
> > > > the help text says, which is:
> > > > 
> > > >   [TINY_PREEMPT_RCU] greatly reduces the memory footprint of RCU.
> > > >   
> > > > "Greatly reduced memory footprint" sounds pretty useful...
> > > 
> > > OK, so from your viewpoint, the only possible benefit is smaller
> > > memory?
> > 
> > Well, I have no idea.  If I was given the choice between TREE_PREEMPT_RCU
> > and TINY_PREEMPT_RCU, absent any information not in the description of
> > these options, I would choose TINY.  The description suggests that the
> > memory savings come at the expense of SMP support, which sounds like a
> > great tradeoff to make for a UP system.
> > 
> > > How much memory does your device have, if I may ask?
> > 
> > It's a (pretty old!) desktop.  I recently had to upgrade it to two
> > gigabytes due to unbearable thrashing with only one...
> 
> If you have two gigabytes (or even one gigabyte), you won't notice the
> few kilobytes of difference between TINY_PREEMPT_RCU and TREE_PREEMPT_RCU.

Well then TINY_PREEMPT_RCU doesn't sound all that useful for me!
Perhaps the help text could be improved... such as changing the words
"greatly reduced" to "marginally reduced" as a first step?

Is there no significant cache impact due to the larger implementation?
I don't really have the time or expertise to do measurements in this
regard, but if TREE_PREEMPT_RCU was actually a selectable option I could
at least choose it to see if anything explodes horribly...

Cheers,
-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)
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