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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
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