Re: [PATCH v3 0/3] create sysfs representation of ACPI HMAT

From: Matthew Wilcox
Date: Wed Dec 20 2017 - 13:19:46 EST

On Mon, Dec 18, 2017 at 01:35:47PM -0700, Ross Zwisler wrote:
> What I'm hoping to do with this series is to just provide a sysfs
> representation of the HMAT so that applications can know which NUMA nodes to
> select with existing utilities like numactl. This series does not currently
> alter any kernel behavior, it only provides a sysfs interface.
> Say for example you had a system with some high bandwidth memory (HBM), and
> you wanted to use it for a specific application. You could use the sysfs
> representation of the HMAT to figure out which memory target held your HBM.
> You could do this by looking at the local bandwidth values for the various
> memory targets, so:
> # grep . /sys/devices/system/hmat/mem_tgt*/local_init/write_bw_MBps
> /sys/devices/system/hmat/mem_tgt2/local_init/write_bw_MBps:81920
> /sys/devices/system/hmat/mem_tgt3/local_init/write_bw_MBps:40960
> /sys/devices/system/hmat/mem_tgt4/local_init/write_bw_MBps:40960
> /sys/devices/system/hmat/mem_tgt5/local_init/write_bw_MBps:40960
> and look for the one that corresponds to your HBM speed. (These numbers are
> made up, but you get the idea.)

Presumably ACPI-based platforms will not be the only ones who have the
ability to expose different bandwidth memories in the future. I think
we need a platform-agnostic way ... right, PowerPC people?

I don't know what the right interface is, but my laptop has a set of
/sys/devices/system/memory/memoryN/ directories. Perhaps this is the
right place to expose write_bw (etc).

> Once you know the NUMA node of your HBM, you can figure out the NUMA node of
> it's local initiator:
> # ls -d /sys/devices/system/hmat/mem_tgt2/local_init/mem_init*
> /sys/devices/system/hmat/mem_tgt2/local_init/mem_init0
> So, in our made-up example our HBM is located in numa node 2, and the local
> CPU for that HBM is at numa node 0.

initiator is a CPU? I'd have expected you to expose a memory controller
abstraction rather than re-use storage terminology.