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

From: Peter Zijlstra
Date: Fri Feb 20 2015 - 11:54:29 EST

On Fri, Feb 20, 2015 at 08:37:37AM -0800, Paul E. McKenney wrote:
> On Fri, Feb 20, 2015 at 10:11:07AM +0100, Peter Zijlstra wrote:
> > So I though we wanted to get rid / limit the expedited stuff because its
> > IPI happy, and here its spreading.
> Well, at least it no longer IPIs idle CPUs. ;-)
> And this is during boot, when a few extra IPIs should not be a big deal.

Well the one application now is during boot; but you expose the
interface for all to use, and therefore someone will.

> > 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.
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