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

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Message-ID: <20120906175324.GN2448@linux.vnet.ibm.com>
Date:	Thu, 6 Sep 2012 10:53:24 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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, Sep 06, 2012 at 04:15:30PM +0200, Peter Zijlstra wrote:
> On Thu, 2012-08-30 at 11:18 -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paul.mckenney@...aro.org>
> > 
> > Some uses of RCU benefit from shorter grace periods, while others benefit
> > more from the greater efficiency provided by longer grace periods.
> > Therefore, this commit allows the durations to be controlled from sysfs.
> > There are two sysfs parameters, one named "jiffies_till_first_fqs" that
> > specifies the delay in jiffies from the end of grace-period initialization
> > until the first attempt to force quiescent states, and the other named
> > "jiffies_till_next_fqs" that specifies the delay (again in jiffies)
> > between subsequent attempts to force quiescent states.  They both default
> > to three jiffies, which is compatible with the old hard-coded behavior.
> 
> A number of questions:
> 
>  - 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.  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.

>  - the above implies a measure of good/bad-ness associated with fqs,
>    can one formulate this?

Maybe.  I do not yet have enough data on really big systems to have a
good formula just yet.

>  - if we can, should we not do this 'automagically' and avoid burdening
>    our already hard pressed sysads of the world with trying to figure
>    this out?

I do expect to get there at some point.

> Also, whatever made you want to provide this 'feature' in the first
> place?

Complaints from the two groups called out above.

							Thanx, Paul

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