On Thu, Jan 10, 2019 at 06:07:02PM +0530, Aneesh Kumar K.V wrote: > Keith Busch <keith.busch@xxxxxxxxx> writes: > > > Heterogeneous memory systems provide memory nodes with different latency > > and bandwidth performance attributes. Provide a new kernel interface for > > subsystems to register the attributes under the memory target node's > > initiator access class. If the system provides this information, applications > > may query these attributes when deciding which node to request memory. > > > > The following example shows the new sysfs hierarchy for a node exporting > > performance attributes: > > > > # tree -P "read*|write*" /sys/devices/system/node/nodeY/classZ/ > > /sys/devices/system/node/nodeY/classZ/ > > |-- read_bandwidth > > |-- read_latency > > |-- write_bandwidth > > `-- write_latency > > > > The bandwidth is exported as MB/s and latency is reported in nanoseconds. > > Memory accesses from an initiator node that is not one of the memory's > > class "Z" initiator nodes may encounter different performance than > > reported here. When a subsystem makes use of this interface, initiators > > of a lower class number, "Z", have better performance relative to higher > > class numbers. When provided, class 0 is the highest performing access > > class. > > How does the definition of performance relate to bandwidth and latency here?. The > initiator in this class has the least latency and high bandwidth? Can there > be a scenario where both are not best for the same node? ie, for a > target Node Y, initiator Node A gives the highest bandwidth but initiator > Node B gets the least latency. How such a config can be represented? Or is > that not possible? I am not aware of a real platform that has an initiator-target pair with better latency but worse bandwidth than any different initiator paired to the same target. If such a thing exists and a subsystem wants to report that, you can register any arbitrary number of groups or classes and rank them according to how you want them presented.
