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    SubjectRe: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node
    On Wed, Apr 17, 2019 at 10:13:44AM -0700, Dave Hansen wrote:
    > On 4/17/19 2:23 AM, Michal Hocko wrote:
    > > yes. This could be achieved by GFP_NOWAIT opportunistic allocation for
    > > the migration target. That should prevent from loops or artificial nodes
    > > exhausting quite naturaly AFAICS. Maybe we will need some tricks to
    > > raise the watermark but I am not convinced something like that is really
    > > necessary.
    >
    > I don't think GFP_NOWAIT alone is good enough.
    >
    > Let's say we have a system full of clean page cache and only two nodes:
    > 0 and 1. GFP_NOWAIT will eventually kick off kswapd on both nodes.
    > Each kswapd will be migrating pages to the *other* node since each is in
    > the other's fallback path.
    >
    > I think what you're saying is that, eventually, the kswapds will see
    > allocation failures and stop migrating, providing hysteresis. This is
    > probably true.
    >
    > But, I'm more concerned about that window where the kswapds are throwing
    > pages at each other because they're effectively just wasting resources
    > in this window. I guess we should figure our how large this window is
    > and how fast (or if) the dampening occurs in practice.

    I'm still refining tests to help answer this and have some preliminary
    data. My test rig has CPU + memory Node 0, memory-only Node 1, and a
    fast swap device. The test has an application strict mbind more than
    the total memory to node 0, and forever writes random cachelines from
    per-cpu threads.

    I'm testing two memory pressure policies:

    Node 0 can migrate to Node 1, no cycles
    Node 0 and Node 1 migrate with each other (0 -> 1 -> 0 cycles)

    After the initial ramp up time, the second policy is ~7-10% slower than
    no cycles. There doesn't appear to be a temporary window dealing with
    bouncing pages: it's just a slower overall steady state. Looks like when
    migration fails and falls back to swap, the newly freed pages occasionaly
    get sniped by the other node, keeping the pressure up.

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