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

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Message-ID: <20150424145459.GY5561@linux.vnet.ibm.com>
Date:	Fri, 24 Apr 2015 07:54:59 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jerome Glisse <j.glisse@...il.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 Fri, Apr 24, 2015 at 09:30:40AM -0500, Christoph Lameter wrote:
> On Thu, 23 Apr 2015, Paul E. McKenney wrote:
> 
> > If by "entire industry" you mean everyone who might want to use hardware
> > acceleration, for example, including mechanical computer-aided design,
> > I am skeptical.
> 
> The industry designs GPUs with super fast special ram and accellerators
> with special ram designed to do fast searches and you think you can demand page
> that stuff in from the main processor?

The demand paging is indeed a drawback for the option of using autonuma
to handle the migration.  And again, this is not intended to replace the
careful hand-tuning that is required to get the last drop of performance
out of the system.  It is instead intended to handle the cases where
the application needs substantially more performance than the CPUs alone
can deliver, but where the cost of full-fledge hand tuning cannot be
justified.

You seem to believe that this latter category is the empty set, which
I must confess does greatly surprise me.

							Thanx, Paul

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