On 18 Apr 2019, at 15:23, Yang Shi wrote: > 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. In addition to these two policies, I am curious about how MPOL_PREFERRED to Node 0 performs. I just wonder how bad static page allocation does. -- Best Regards, Yan Zi
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