Re: [RFC V2 03/12] mm: Change generic FALLBACK zonelist creation process

[Dave Hansen]

On 01/30/2017 11:25 PM, John Hubbard wrote:
> I also don't like having these policies hard-coded, and your 100x
> example above helps clarify what can go wrong about it. It would be
> nicer if, instead, we could better express the "distance" between nodes
> (bandwidth, latency, relative to sysmem, perhaps), and let the NUMA
> system figure out the Right Thing To Do.
> 
> I realize that this is not quite possible with NUMA just yet, but I
> wonder if that's a reasonable direction to go with this?

In the end, I don't think the kernel can make the "right" decision very
widely here.

Intel's Xeon Phis have some high-bandwidth memory (MCDRAM) that
evidently has a higher latency than DRAM.  Given a plain malloc(), how
is the kernel to know that the memory will be used for AVX-512
instructions that need lots of bandwidth vs. some random data structure
that's latency-sensitive?

In the end, I think all we can do is keep the kernel's existing default
of "low latency to the CPU that allocated it", and let apps override
when that policy doesn't fit them.