linux-kernel - Re: [PATCH tip/core/rcu 13/23] rcu: Control grace-period duration from sysfs

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Message-ID: <1346956101.18408.61.camel@twins>
Date:	Thu, 06 Sep 2012 20:28:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 13/23] rcu: Control grace-period duration
 from sysfs

On Thu, 2012-09-06 at 10:53 -0700, Paul E. McKenney wrote:

> >  - how do I know if my workload wants a longer or shorter forced qs
> >    period?
> 
> Almost everyone can do just fine with the defaults.  If you have more
> than about 1,000 CPUs, you might need a longer period. 

Because the cost of starting a grace period is on the same order (or
larger) in cost as this period?

>  Some embedded
> systems might need a shorter period -- the only specific example I know
> of is network diagnostic equipment running wireshark, which starts up
> slowly due to grace-period length.

But but but 3 jiffies.. however is that too long?

> > Also, whatever made you want to provide this 'feature' in the first
> > place?
> 
> Complaints from the two groups called out above.

Does this really warrant a boot time knob for which even you cannot
quite explain what values to use when?


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