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    SubjectRe: [RFC V2 03/12] mm: Change generic FALLBACK zonelist creation process
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    On 01/31/2017 12:04 PM, Dave Hansen wrote:
    > 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.
    >
    I think John's point is that latency might not be the predominant factor
    anymore
    for certain sections of the CPU and GPU world. What if a Phi has MCDRAM
    physically attached, but DDR4 connected via QPI that still has lower
    total latency (might
    be a stretch for Phi but not a stretch for GPUs with deep sorting memory
    controllers)? Lowest latency is probably the wrong choice. Latency has
    really been a
    numeric proxy for physical proximity, under assumption most closely
    coupled memory is
    the right placement, but HBM/MCDRAM is causing that relationship to
    break down in all
    sorts of interesting ways.

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