linux-kernel - Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace periods

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Message-ID: <20150220182745.GN5745@linux.vnet.ibm.com>
Date:	Fri, 20 Feb 2015 10:27:45 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace
 periods

On Fri, Feb 20, 2015 at 09:32:39AM -0800, Arjan van de Ven wrote:
> >>>>Does it really make a machine boot much faster? Why are people using
> >>>>synchronous gp primitives if they care about speed? Should we not fix
> >>>>that instead?
> >>>
> >>>The report I heard was that it provided 10-15% faster boot times.
> >>
> >>That's not insignificant; got more details? I think we should really
> >>look at why people are using the sync primitives.
> >
> >I must defer to the people who took the exact measurements.
> >
> >But yes, once I have that info, I should add it to the commit log.
> 
> so the two most obvious cases are
> 
> Registering sysrq keys ... even when the old key code had no handler
> (have a patch pending for this)
> 
> registering idle handlers
> (this is more tricky, it's very obvious abuse but the fix is less clear)
> 
> there's a few others as well that I'm chasing down...
> .. but the flip side, prior to running ring 3 code, why NOT do fast expedites?

It would be good to have before-and-after measurements of actual
boot time.  Are these numbers available?

							Thanx, Paul

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