On 16 Apr 2019, at 10:30, Dave Hansen wrote: > On 4/16/19 12:47 AM, Michal Hocko wrote: >> You definitely have to follow policy. You cannot demote to a node which >> is outside of the cpuset/mempolicy because you are breaking contract >> expected by the userspace. That implies doing a rmap walk. > > What *is* the contract with userspace, anyway? :) > > Obviously, the preferred policy doesn't have any strict contract. > > The strict binding has a bit more of a contract, but it doesn't prevent > swapping. Strict binding also doesn't keep another app from moving the > memory. > > We have a reasonable argument that demotion is better than swapping. > So, we could say that even if a VMA has a strict NUMA policy, demoting > pages mapped there pages still beats swapping them or tossing the page > cache. It's doing them a favor to demote them. I just wonder whether page migration is always better than swapping, since SSD write throughput keeps improving but page migration throughput is still low. For example, my machine has a SSD with 2GB/s writing throughput but the throughput of 4KB page migration is less than 1GB/s, why do we want to use page migration for demotion instead of swapping? -- Best Regards, Yan Zi
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