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

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Date:   Wed, 5 Dec 2018 09:27:09 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     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>,
        Logan Gunthorpe <logang@...tatee.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 12/4/18 6:13 PM, Jerome Glisse wrote:
> On Tue, Dec 04, 2018 at 05:06:49PM -0800, Dave Hansen wrote:
>> OK, but there are 1024*1024 matrix cells on a systems with 1024
>> proximity domains (ACPI term for NUMA node).  So it sounds like you are
>> proposing a million-directory approach.
> 
> No, pseudo code:
>     struct list links;
> 
>     for (unsigned r = 0; r < nrows; r++) {
>         for (unsigned c = 0; c < ncolumns; c++) {
>             if (!link_find(links, hmat[r][c].bandwidth,
>                            hmat[r][c].latency)) {
>                 link = link_new(hmat[r][c].bandwidth,
>                                 hmat[r][c].latency);
>                 // add initiator and target correspond to that row
>                 // and columns to this new link
>                 list_add(&link, links);
>             }
>         }
>     }
> 
> So all cells that have same property are under the same link. 

OK, so the "link" here is like a cable.  It's like saying, "we have a
network and everything is connected with an ethernet cable that can do
1gbit/sec".

But, what actually connects an initiator to a target?  I assume we still
need to know which link is used for each target/initiator pair.  Where
is that enumerated?

I think this just means we need a million symlinks to a "link" instead
of a million link directories.  Still not great.

> Note that userspace can parse all this once during its initialization
> and create pools of target to use.

It sounds like you're agreeing that there is too much data in this
interface for applications to _regularly_ parse it.  We need some
central thing that parses it all and caches the results.