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Date: Mon, 23 Mar 2015 21:57:20 -0400
From: Sowmini Varadhan <sowmini.varadhan@oracle.com>
To: David Miller <davem@davemloft.net>
Subject: Re: Generic IOMMU pooled allocator
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benh> It might be sufficient to add a flush counter and compare it between runs
benh> if actual wall-clock benchmarks are too hard to do (especially if you
benh> don't have things like very fast network cards at hand).
benh>
benh> Number of flush / number of packets might be a sufficient metric, it..

I was just going to say: I can add those counters tomorrow,
and get some stats, but seems like it doesn't really matter what 
the outcome is, because: 

On (03/23/15 21:44), David Miller wrote:
> 
> From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Date: Tue, 24 Mar 2015 09:21:05 +1100
> 
> > Dave, what's your feeling there ? Does anybody around still have
> > some HW that we can test with ?
> 
> I don't see what the actual problem is.
> 
> Even if you use multiple pools, which we should for scalability on
> sun4u too, just do the flush when allocation in _any_ pool wraps
> around.
> 
> That's still better than not doing the optimization at all.
> 
> That is always going to be correct, and you can use a separate
> spinlock to make sure only one thread of control does the full
> IOMMU flush at a time.

--Sowmini