Date: Fri, 29 Dec 2017 22:58:45 -0800
From: Matthew Wilcox <willy@infradead.org>
To: Brice Goglin <brice.goglin@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>, Michal Hocko <mhocko@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Anaczkowski, Lukasz" <lukasz.anaczkowski@intel.com>,
        "Box, David E" <david.e.box@intel.com>,
        "Kogut, Jaroslaw" <Jaroslaw.Kogut@intel.com>,
        "Koss, Marcin" <marcin.koss@intel.com>,
        "Koziej, Artur" <artur.koziej@intel.com>,
        "Lahtinen, Joonas" <joonas.lahtinen@intel.com>,
        "Moore, Robert" <robert.moore@intel.com>,
        "Nachimuthu, Murugasamy" <murugasamy.nachimuthu@intel.com>,
        "Odzioba, Lukasz" <lukasz.odzioba@intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        "Schmauss, Erik" <erik.schmauss@intel.com>,
        "Verma, Vishal L" <vishal.l.verma@intel.com>,
        "Zheng, Lv" <lv.zheng@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Jerome Glisse <jglisse@redhat.com>, John Hubbard <jhubbard@nvidia.com>,
        Len Brown <lenb@kernel.org>, Tim Chen <tim.c.chen@linux.intel.com>,
        devel@acpica.org, Linux ACPI <linux-acpi@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
        Linux API <linux-api@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v3 0/3] create sysfs representation of ACPI HMAT
Message-ID: <20171230065845.GD27959@bombadil.infradead.org>
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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"?