Re: [RFC] Heterogeneous memory management (mirror process address space on a device mmu).

["H. Peter Anvin" <hpa@zytor.com>]

<ogerlitz@mellanox.com>,Sagi Grimberg <sagig@mellanox.com>,Shachar Raindel <raindel@mellanox.com>,Liran Liss <liranl@mellanox.com>,Roland Dreier <roland@purestorage.com>,"Sander, Ben" <ben.sander@amd.com>,"Stoner, Greg" <Greg.Stoner@amd.com>,"Bridgman, John" <John.Bridgman@amd.com>,"Mantor, Michael" <Michael.Mantor@amd.com>,"Blinzer, Paul" <Paul.Blinzer@amd.com>,"Morichetti, Laurent" <Laurent.Morichetti@amd.com>,"Deucher, Alexander" <Alexander.Deucher@amd.com>,"Gabbay, Oded" <Oded.Gabbay@amd.com>,Davidlohr Bueso <davidlohr@hp.com>
Message-ID: <0bf54468-3ed1-4cd4-b771-4836c78dde14@email.android.com>

Nothing wrong with device-side memory, but not having it accessible by the CPU seems fundamentally brown from the point of view of unified memory addressing.

On May 6, 2014 9:54:08 AM PDT, Jerome Glisse <j.glisse@gmail.com> wrote:
>On Tue, May 06, 2014 at 12:47:16PM -0400, Rik van Riel wrote:
>> On 05/06/2014 12:34 PM, Linus Torvalds wrote:
>> > On Tue, May 6, 2014 at 9:30 AM, Rik van Riel <riel@redhat.com>
>wrote:
>> >>
>> >> The GPU runs a lot faster when using video memory, instead
>> >> of system memory, on the other side of the PCIe bus.
>> > 
>> > The nineties called, and they want their old broken model back.
>> > 
>> > Get with the times. No high-performance future GPU will ever run
>> > behind the PCIe bus. We still have a few straggling historical
>> > artifacts, but everybody knows where the future is headed.
>> > 
>> > They are already cache-coherent because flushing caches etc was too
>> > damn expensive. They're getting more so.
>> 
>> I suppose that VRAM could simply be turned into a very high
>> capacity CPU cache for the GPU, for the case where people
>> want/need an add-on card.
>> 
>> With a few hundred MB of "CPU cache" on the video card, we
>> could offload processing to the GPU very easily, without
>> having to worry about multiple address or page table formats
>> on the CPU side.
>> 
>> A new generation of GPU hardware seems to come out every
>> six months or so, so I guess we could live with TLB
>> invalidations to the first generations of hardware being
>> comically slow :)
>> 
>
>I do not want to speak for any GPU manufacturer but i think it is safe
>to say that there will be dedicated memory for GPU for a long time. It
>is not going anywhere soon and it is a lot more than few hundred MB,
>think several GB. If you think about 4k, 8k screen you really gonna
>want
>8GB at least on desktop computer and for compute you will likely see
>16GB or 32GB as common size.
>
>Again i stress that there is nothing on the horizon that let me believe
>that regular memory associated to CPU will ever come close to the
>bandwith
>that exist with memory associated to GPU. It is already more than 10
>times
>faster on GPU and as far as i know the gap will grow even more in the
>next
>generation.
>
>So dedicated memory to gpu should not be discarded as something that is
>vanishing quite the contrary it should be acknowledge as something that
>is
>here to stay a lot longer afaict.
>
>Cheers,
>Jérôme

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