Re: [PATCH 2/2] smp_call_function: use rwlocks on queues ratherthan rcu

[Paul E. McKenney]

On Fri, Aug 22, 2008 at 08:33:46PM +0200, Andi Kleen wrote:
> > Would it make sense to push the freed-by-RCU memory further up the
> > hierarchy, so that such memory is not mistaken for recently freed
> > hot-in-cache memory?
> 
> Right now my impression is that it is not well understood why
> the kmalloc makes the IPI that much slower. In theory a kmalloc
> shouldn't be all that slow, it's essentially just a 
> "disable interrupts; unlink object from cpu cache; enable interrupts"
> with some window dressing. kfree() is similar.
> 
> Does it bounce a cache line on freeing perhaps?

It is indeed easy to focus on the wrong area when attempting to
improve performance.  Done it many times myself...  :-/

							Thanx, Paul
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