On Tue, Sep 24, 2019 at 09:29:50AM +0800, Yunsheng Lin wrote: > On 2019/9/24 4:34, Peter Zijlstra wrote: > > I'm saying the ACPI standard is wrong. Explain to me how it is > > physically possible to have a device without NUMA affinity in a NUMA > > system? > > > > 1) The fundamental interconnect is not uniform. > > 2) The device needs to actually be somewhere. > > > > From what I can see, NUMA_NO_NODE may make sense in the below case: > > 1) Theoretically, there would be a device that can access all the memory > uniformly and can be accessed by all cpus uniformly even in a NUMA system. > Suppose we have two nodes, and the device just sit in the middle of the > interconnect between the two nodes. > > Even we define a third node solely for the device, we may need to look at > the node distance to decide the device can be accessed uniformly. > > Or we can decide that the device can be accessed uniformly by setting > it's node to NUMA_NO_NODE. This is indeed a theoretical case; it doesn't scale. The moment you're adding multiple sockets or even board interconnects this all goes out the window. And in this case, forcing the device to either node is fine. > 2) For many virtual deivces, such as tun or loopback netdevice, they > are also accessed uniformly by all cpus. Not true; the virtual device will sit in memory local to some node. And as with physical devices, you probably want at least one (virtual) queue per node.
