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

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Message-ID: <1429828197.4915.45.camel@kernel.crashing.org>
Date:	Fri, 24 Apr 2015 08:29:57 +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 Thu, 2015-04-23 at 09:10 -0500, Christoph Lameter wrote:
> 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.

How would it be impacted ? You can still do dedicated allocations etc...
if you want to do so. I think Jerome gave a pretty good explanation of
the need for the usage model we are proposing, it's also coming from the
industry ...

Ben.


> 
> 
> 
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