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

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Message-ID: <553917F4.4080300@redhat.com>
Date:	Thu, 23 Apr 2015 12:04:04 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Christoph Lameter <cl@...ux.com>,
	Jerome Glisse <j.glisse@...il.com>
CC:	"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,
	airlied@...hat.com, benh@...nel.crashing.org,
	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 04/21/2015 08:50 PM, Christoph Lameter wrote:
> On Tue, 21 Apr 2015, Jerome Glisse wrote:

>> So big use case here, let say you have an application that rely on a
>> scientific library that do matrix computation. Your application simply
>> use malloc and give pointer to this scientific library. Now let say
>> the good folks working on this scientific library wants to leverage
>> the GPU, they could do it by allocating GPU memory through GPU specific
>> API and copy data in and out. For matrix that can be easy enough, but
>> still inefficient. What you really want is the GPU directly accessing
>> this malloced chunk of memory, eventualy migrating it to device memory
>> while performing the computation and migrating it back to system memory
>> once done. Which means that you do not want some kind of filesystem or
>> anything like that.
> 
> With a filesystem the migration can be controlled by the application.

Which is absolutely the wrong thing to do when using the "GPU"
(or whatever co-processor it is) transparently from libraries,
without the applications having to know about it.

Your use case is legitimate, but so is this other case.
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