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

Keith Busch
2019-04-18T18:23:09Z

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.