linux-kernel - Re: [PATCH 3/7] doc/vm: New documentation for memory performance

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Date:   Tue, 20 Nov 2018 14:51:50 +0100
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Keith Busch <keith.busch@...el.com>
Cc:     linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-mm@...ck.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Wysocki <rafael@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 3/7] doc/vm: New documentation for memory performance

Hi,

Sorry if I'm jumping too late.

On Wed, Nov 14, 2018 at 03:49:16PM -0700, Keith Busch wrote:
> Platforms may provide system memory where some physical address ranges
> perform differently than others. These heterogeneous memory attributes are
> common to the node that provides the memory and exported by the kernel.
> 
> Add new documentation providing a brief overview of such systems and
> the attributes the kernel makes available to aid applications wishing
> to query this information.
> 
> Signed-off-by: Keith Busch <keith.busch@...el.com>
> ---
>  Documentation/vm/numaperf.rst | 71 +++++++++++++++++++++++++++++++++++++++++++

As this document describes user-space interfaces it belongs to
Documentation/admin-guide/mm.

>  1 file changed, 71 insertions(+)
>  create mode 100644 Documentation/vm/numaperf.rst
> 
> diff --git a/Documentation/vm/numaperf.rst b/Documentation/vm/numaperf.rst
> new file mode 100644
> index 000000000000..5a3ecaff5474
> --- /dev/null
> +++ b/Documentation/vm/numaperf.rst
> @@ -0,0 +1,71 @@
> +.. _numaperf:
> +
> +================
> +NUMA Performance
> +================
> +
> +Some platforms may have multiple types of memory attached to a single
> +CPU. These disparate memory ranges share some characteristics, such as
> +CPU cache coherence, but may have different performance. For example,
> +different media types and buses affect bandwidth and latency.
> +
> +A system supporting such heterogeneous memory groups each memory type
> +under different "nodes" based on similar CPU locality and performance
> +characteristics.  Some memory may share the same node as a CPU, and
> +others are provided as memory-only nodes. While memory only nodes do not
> +provide CPUs, they may still be local to one or more compute nodes. The
> +following diagram shows one such example of two compute noes with local
> +memory and a memory only node for each of compute node:
> +
> + +------------------+     +------------------+
> + | Compute Node 0   +-----+ Compute Node 1   |
> + | Local Node0 Mem  |     | Local Node1 Mem  |
> + +--------+---------+     +--------+---------+
> +          |                        |
> + +--------+---------+     +--------+---------+
> + | Slower Node2 Mem |     | Slower Node3 Mem |
> + +------------------+     +--------+---------+
> +
> +A "memory initiator" is a node containing one or more devices such as
> +CPUs or separate memory I/O devices that can initiate memory requests. A
> +"memory target" is a node containing one or more CPU-accessible physical
> +address ranges.
> +
> +When multiple memory initiators exist, accessing the same memory
> +target may not perform the same as each other. The highest performing
> +initiator to a given target is considered to be one of that target's
> +local initiators.
> +
> +To aid applications matching memory targets with their initiators,
> +the kernel provide symlinks to each other like the following example::
> +
> +	# ls -l /sys/devices/system/node/nodeX/initiator*
> +	/sys/devices/system/node/nodeX/targetY -> ../nodeY
> +
> +	# ls -l /sys/devices/system/node/nodeY/target*
> +	/sys/devices/system/node/nodeY/initiatorX -> ../nodeX
> +
> +Applications may wish to consider which node they want their memory to
> +be allocated from based on the nodes performance characteristics. If
> +the system provides these attributes, the kernel exports them under the
> +node sysfs hierarchy by appending the initiator_access directory under
> +the node as follows::
> +
> +	/sys/devices/system/node/nodeY/initiator_access/
> +
> +The kernel does not provide performance attributes for non-local memory
> +initiators. The performance characteristics the kernel provides for
> +the local initiators are exported are as follows::
> +
> +	# tree /sys/devices/system/node/nodeY/initiator_access
> +	/sys/devices/system/node/nodeY/initiator_access
> +	|-- read_bandwidth
> +	|-- read_latency
> +	|-- write_bandwidth
> +	`-- write_latency
> +
> +The bandwidth attributes are provided in MiB/second.
> +
> +The latency attributes are provided in nanoseconds.
> +
> +See also: https://www.uefi.org/sites/default/files/resources/ACPI_6_2.pdf
> -- 
> 2.14.4
> 

-- 
Sincerely yours,
Mike.