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SubjectRe: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node

On 4/18/19 11:16 AM, Keith Busch wrote:
> 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.

Thanks for the test. A follow-up question, how about the size for each
node? Is node 1 bigger than node 0? Since PMEM typically has larger
capacity, so I'm wondering whether the capacity may make things
different or not.

> 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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