linux-kernel - Re: TLB evaluation for Linux

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Date:	Tue, 2 Sep 2008 06:43:53 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	"CHADHA,VINEET" <vineet@....edu>
Cc:	nickpiggin@...oo.com.au, linux-kernel@...r.kernel.org,
	vineet@....edu
Subject: Re: TLB evaluation for Linux

On Tue, 2 Sep 2008 00:12:03 -0400 (EDT)
"CHADHA,VINEET" <vineet@....edu> wrote:

> Hi,
> 
> I have been working to evaluate TLB performance for Linux O/S and 
> virtualized workloads(such as Xen) in a Full system simulator(e.g. 
> simics). While my evaluation is in nascent stage, I do notice that 
> most of the IPIs in multi-core environments cause complete TLB 
> Flush.

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)
> 
> I want to evaluate cost of TLB shootdown including re-population 
> vs. each entry shootdown (invlpg). While a similar study has been 
> done in other kernels (e.g. L4 kernel), I am not aware if it has 
> been done for Linux O/S.
> 

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. 

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