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

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Date:   Fri, 29 Dec 2017 22:58:45 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Brice Goglin <brice.goglin@...il.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Michal Hocko <mhocko@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Anaczkowski, Lukasz" <lukasz.anaczkowski@...el.com>,
        "Box, David E" <david.e.box@...el.com>,
        "Kogut, Jaroslaw" <Jaroslaw.Kogut@...el.com>,
        "Koss, Marcin" <marcin.koss@...el.com>,
        "Koziej, Artur" <artur.koziej@...el.com>,
        "Lahtinen, Joonas" <joonas.lahtinen@...el.com>,
        "Moore, Robert" <robert.moore@...el.com>,
        "Nachimuthu, Murugasamy" <murugasamy.nachimuthu@...el.com>,
        "Odzioba, Lukasz" <lukasz.odzioba@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "Schmauss, Erik" <erik.schmauss@...el.com>,
        "Verma, Vishal L" <vishal.l.verma@...el.com>,
        "Zheng, Lv" <lv.zheng@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Balbir Singh <bsingharora@...il.com>,
        Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Len Brown <lenb@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>, devel@...ica.org,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Linux API <linux-api@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v3 0/3] create sysfs representation of ACPI HMAT

On Wed, Dec 27, 2017 at 10:10:34AM +0100, Brice Goglin wrote:
> > Perhaps we can enlist /proc/iomem or a similar enumeration interface
> > to tell userspace the NUMA node and whether the kernel thinks it has
> > better or worse performance characteristics relative to base
> > system-RAM, i.e. new IORES_DESC_* values. I'm worried that if we start
> > publishing absolute numbers in sysfs userspace will default to looking
> > for specific magic numbers in sysfs vs asking the kernel for memory
> > that has performance characteristics relative to base "System RAM". In
> > other words the absolute performance information that the HMAT
> > publishes is useful to the kernel, but it's not clear that userspace
> > needs that vs a relative indicator for making NUMA node preference
> > decisions.
> 
> Some HPC users will benchmark the machine to discovery actual
> performance numbers anyway.
> However, most users won't do this. They will want to know relative
> performance of different nodes. If you normalize HMAT values by dividing
> them with system-RAM values, that's likely OK. If you just say "that
> node is faster than system RAM", it's not precise enough.

So "this memory has 800% bandwidth of normal" and "this memory has 70%
bandwidth of normal"?