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.
