Re: [PATCHv3 07/13] node: Add heterogenous memory access attributes

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

Keith Busch <keith.busch@intel.com> 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?

-aneesh