Re: [RFC] Make use of non-dynamic dmabuf in RDMA

[John Hubbard]

On 8/24/21 10:32 AM, Jason Gunthorpe wrote:
...
> > > And yes at least for the amdgpu driver we migrate the memory to host
> > > memory as soon as it is pinned and I would expect that other GPU drivers
> > > do something similar.
> >
> > Well...for many topologies, migrating to host memory will result in a
> > dramatically slower p2p setup. For that reason, some GPU drivers may
> > want to allow pinning of video memory in some situations.
> >
> > Ideally, you've got modern ODP devices and you don't even need to pin.
> > But if not, and you still hope to do high performance p2p between a GPU
> > and a non-ODP Infiniband device, then you would need to leave the pinned
> > memory in vidmem.
> >
> > So I think we don't want to rule out that behavior, right? Or is the
> > thinking more like, "you're lucky that this old non-ODP setup works at
> > all, and we'll make it work by routing through host/cpu memory, but it
> > will be slow"?
>
> I think it depends on the user, if the user creates memory which is
> permanently located on the GPU then it should be pinnable in this way
> without force migration. But if the memory is inherently migratable
> then it just cannot be pinned in the GPU at all as we can't
> indefinately block migration from happening eg if the CPU touches it
> later or something.

OK. I just want to avoid creating any API-level assumptions that dma_buf_pin()
necessarily implies or requires migrating to host memory.

thanks,
--
John Hubbard
NVIDIA