Messages in this thread | | | Subject | Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node | From | Yang Shi <> | Date | Thu, 18 Apr 2019 12:23:41 -0700 |
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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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