linux-kernel - Re: [RFC PATCH 00/14] Heterogeneous Memory System (HMS) and hbind()

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Message-ID: <f7eb9939-d550-706a-946d-acbb7383172e@intel.com>
Date:   Thu, 6 Dec 2018 14:04:46 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        Jerome Glisse <jglisse@...hat.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Keith Busch <keith.busch@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Haggai Eran <haggaie@...lanox.com>,
        Balbir Singh <bsingharora@...il.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Felix Kuehling <felix.kuehling@....com>,
        Philip Yang <Philip.Yang@....com>,
        Christian König <christian.koenig@....com>,
        Paul Blinzer <Paul.Blinzer@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Michal Hocko <mhocko@...nel.org>,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        Mark Hairgrove <mhairgrove@...dia.com>,
        Vivek Kini <vkini@...dia.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Dave Airlie <airlied@...hat.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Ben Woodard <woodard@...hat.com>, linux-acpi@...r.kernel.org
Subject: Re: [RFC PATCH 00/14] Heterogeneous Memory System (HMS) and hbind()

On 12/6/18 12:11 PM, Logan Gunthorpe wrote:
>> My concern with having folks do per-program parsing, *and* having a huge
>> amount of data to parse makes it unusable.  The largest systems will
>> literally have hundreds of thousands of objects in /sysfs, even in a
>> single directory.  That makes readdir() basically impossible, and makes
>> even open() (if you already know the path you want somehow) hard to do fast.
> Is this actually realistic? I find it hard to imagine an actual hardware
> bus that can have even thousands of devices under a single node, let
> alone hundreds of thousands.

Jerome's proposal, as I understand it, would have generic "links".
They're not an instance of bus, but characterize a class of "link".  For
instance, a "link" might characterize the characteristics of the QPI bus
between two CPU sockets. The link directory would enumerate the list of
all *instances* of that link

So, a "link" directory for QPI would say Socket0<->Socket1,
Socket1<->Socket2, Socket1<->Socket2, Socket2<->PCIe-1.2.3.4 etc...  It
would have to enumerate the connections between every entity that shared
those link properties.

While there might not be millions of buses, there could be millions of
*paths* across all those buses, and that's what the HMAT describes, at
least: the net result of all those paths.