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

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Date:   Thu, 10 Jan 2019 18:07:02 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Keith Busch <keith.busch@...el.com>, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org, linux-mm@...ck.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Wysocki <rafael@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Keith Busch <keith.busch@...el.com>
Subject: Re: [PATCHv3 07/13] node: Add heterogenous memory access attributes

Keith Busch <keith.busch@...el.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