Re: TLB evaluation for Linux

[Arjan van de Ven <arjan@infradead.org>]

On Tue, 2 Sep 2008 10:59:25 -0400 (EDT)
"CHADHA,VINEET" <vineet@ufl.edu> wrote:

> 
> On Tue Sep 02 09:43:53 EDT 2008, Arjan van de Ven 
> <arjan@infradead.org> wrote:
> 
> > On Tue, 2 Sep 2008 00:12:03 -0400 (EDT)
> > "CHADHA,VINEET" <vineet@ufl.edu> wrote:
> > 
> 
> > note that linux only does an ipi to processors that actually are
> > currently running a thread of the same program (or a kernel 
> > thread).
> > Old versions didn't do this (they also IPI'd idle processors), 
> > but
> > on modern cpus and modern kernels that's not supposed to happen 
> > anymore
> > (the C-states that flush the tlb anyway now do the kernel side
> > bookkeeping as well to avoid the wakeup+useless flush)
> 
> Interesting to know about it.
> 
> > one of the problems is that invlpg is rather expensive; in 
> > long-ago
> > experiments the threshold was like around a handful of pages 
> > already.
> > At that point.. all the bookkeeping isn't likely to be a win.
> > Esp since a tlb refill on x86 is quite cheap.
> 
> Yeah that is possible. Do you have link to any published work ? It 
> would be still interesting to characterize and  compare behavior 
> for new workloads scenarios such as virtual machines.
> 

I don't have a reference, but I'd not be surprised to see on virtual
machines for this to be even stronger; each invlpg would be a
hypercall, compared to only one for the total flush.