linux-kernel - Re: Interacting with coherent memory on external devices

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Message-ID: <1429756592.4915.23.camel@kernel.crashing.org>
Date:	Thu, 23 Apr 2015 12:36:32 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Jerome Glisse <j.glisse@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	jglisse@...hat.com, mgorman@...e.de, aarcange@...hat.com,
	riel@...hat.com, airlied@...hat.com,
	aneesh.kumar@...ux.vnet.ibm.com,
	Cameron Buschardt <cabuschardt@...dia.com>,
	Mark Hairgrove <mhairgrove@...dia.com>,
	Geoffrey Gerfin <ggerfin@...dia.com>,
	John McKenna <jmckenna@...dia.com>, akpm@...ux-foundation.org
Subject: Re: Interacting with coherent memory on external devices

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.


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