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

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Message-ID: <935fc14d-91f2-bc2a-f8b5-665e4145e148@deltatee.com>
Date:   Thu, 6 Dec 2018 16:28:44 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Dave Hansen <dave.hansen@...el.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 2018-12-06 4:09 p.m., Dave Hansen wrote:
> This looks great.  But, we don't _have_ this kind of information for any
> system that I know about or any system available in the near future.
> 
> We basically have two different world views:
> 1. The system is described point-to-point.  A connects to B @
>    100GB/s.  B connects to C at 50GB/s.  Thus, C->A should be
>    50GB/s.
>    * Less information to convey
>    * Potentially less precise if the properties are not perfectly
>      additive.  If A->B=10ns and B->C=20ns, A->C might be >30ns.
>    * Costs must be calculated instead of being explicitly specified
> 2. The system is described endpoint-to-endpoint.  A->B @ 100GB/s
>    B->C @ 50GB/s, A->C @ 50GB/s.
>    * A *lot* more information to convey O(N^2)?
>    * Potentially more precise.
>    * Costs are explicitly specified, not calculated
> 
> These patches are really tied to world view #1.  But, the HMAT is really
> tied to world view #1.

I didn't think this was meant to describe actual real world performance
between all of the links. If that's the case all of this seems like a
pipe dream to me.

Attributes like cache coherency, atomics, etc should fit well in world
view #1... and, at best, some kind of flag saying whether or not to use
a particular link if you care about transfer speed. -- But we don't need
special "link" directories to describe the properties of existing buses.

You're not *really* going to know bandwidth or latency for any of this
unless you actually measure it on the system in question.

Logan