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On 2016-11-22 04:03 PM, Daniel Vetter
wrote:
On Tue, Nov 22, 2016 at 9:35 PM, Serguei Sagalovitch
<serguei.sagalovitch@xxxxxxx> wrote:
On 2016-11-22 03:10 PM, Daniel Vetter wrote:
On Tue, Nov 22, 2016 at 9:01 PM, Dan Williams <dan.j.williams@xxxxxxxxx>
wrote:
On Tue, Nov 22, 2016 at 10:59 AM, Serguei Sagalovitch
<serguei.sagalovitch@xxxxxxx> wrote:
I personally like "device-DAX" idea but my concerns are:
- How well it will co-exists with the DRM infrastructure /
implementations
in part dealing with CPU pointers?
Inside the kernel a device-DAX range is "just memory" in the sense
that you can perform pfn_to_page() on it and issue I/O, but the vma is
not migratable. To be honest I do not know how well that co-exists
with drm infrastructure.
- How well we will be able to handle case when we need to
"move"/"evict"
memory/data to the new location so CPU pointer should point to the
new
physical location/address
(and may be not in PCI device memory at all)?
So, device-DAX deliberately avoids support for in-kernel migration or
overcommit. Those cases are left to the core mm or drm. The device-dax
interface is for cases where all that is needed is a direct-mapping to
a statically-allocated physical-address range be it persistent memory
or some other special reserved memory range.
For some of the fancy use-cases (e.g. to be comparable to what HMM can
pull off) I think we want all the magic in core mm, i.e. migration and
overcommit. At least that seems to be the very strong drive in all
general-purpose gpu abstractions and implementations, where memory is
allocated with malloc, and then mapped/moved into vram/gpu address
space through some magic,
It is possible that there is other way around: memory is requested to be
allocated and should be kept in vram for performance reason but due
to possible overcommit case we need at least temporally to "move" such
allocation to system memory.
With migration I meant migrating both ways of course. And with stuff
like numactl we can also influence where exactly the malloc'ed memory
is allocated originally, at least if we'd expose the vram range as a
very special numa node that happens to be far away and not hold any
cpu cores.
-Daniel
One additional item to consider: it is not only "plain" numa case
where
we could have different performance for access but also possibility
that
we will not have access at all (or write only access) particular if
PCIe
devices belong to different root complex. I must admit that I do not
know
how to detect reliably such cases in the kernel.
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