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

From: Paul E. McKenney
Date: Fri Feb 20 2015 - 13:27:59 EST

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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