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

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Message-ID: <alpine.DEB.2.11.1504230907330.32297@gentwo.org>
Date:	Thu, 23 Apr 2015 09:10:13 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
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 Thu, 23 Apr 2015, Benjamin Herrenschmidt wrote:

> >  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 :-)

Much of the HPC apps that the world is using is severely impacted by what
you are proposing. Its the industries usage model not mine. That is why I
was asking about the use case. Does not seem to fit the industry you are
targeting. This is also the basic design principle that got GPUs to work
as fast as they do today. Introducing random memory latencies there will
kill much of the benefit of GPUs there too.




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