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

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Message-ID: <20120906203742.GV2448@linux.vnet.ibm.com>
Date:	Thu, 6 Sep 2012 13:37:42 -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 08:28:21PM +0200, Peter Zijlstra wrote:
> 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?

Because the overhead of rcu_gp_fqs() can then be multiple jiffies, so
it doesn't make sense to run it so often.  If nothing else, the
rcu_gp_kthread() will start chewing up appreciable CPU time.

> >  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?

Because wireshark startup runs through a great many grace periods when
starting up, and those 3-jiffy time periods add up.

> > > 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?

If people look at me funny when I explain, I just tell them to leave
it alone.

One alternative at the low end would be to have a sysfs variable that
converted normal grace periods to expedited grace periods.  Would that
be preferable?

							Thanx, Paul

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