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

From: Arjan van de Ven
Date: Fri Feb 20 2015 - 13:29:48 EST

On 2/20/2015 10:27 AM, Paul E. McKenney wrote:
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?

I'll make you pretty graphs when I get home from collab summit, which
should be later today

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