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Subject: Re: Interacting with coherent memory on external devices
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Christoph Lameter <cl@linux.com>
Cc: Jerome Glisse <j.glisse@gmail.com>,
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Date: Thu, 23 Apr 2015 12:36:32 +1000
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On Wed, 2015-04-22 at 13:17 -0500, Christoph Lameter wrote:
> 
> > But again let me stress that application that want to be in control will
> > stay in control. If you want to make the decission yourself about where
> > things should end up then nothing in all we are proposing will preclude
> > you from doing that. Please just think about others people application,
> > not just yours, they are a lot of others thing in the world and they do
> > not want to be as close to the metal as you want to be. We just want to
> > accomodate the largest number of use case.
> 
> What I think you want to do is to automatize something that should not be
> automatized and cannot be automatized for performance reasons.

You don't know that.

>  Anyone
> wanting performance (and that is the prime reason to use a GPU) would
> switch this off because the latencies are otherwise not controllable and
> those may impact performance severely. There are typically multiple
> parallel strands of executing that must execute with similar performance
> in order to allow a data exchange at defined intervals. That is no longer
> possible if you add variances that come with the "transparency" here.

Stop trying to apply your unique usage model to the entire world :-)

Ben.