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

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Date:   Thu, 6 Dec 2018 11:31:21 -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 12/6/18 11:20 AM, Jerome Glisse wrote:
>>> For case 1 you can pre-parse stuff but this can be done by helper library
>> How would that work?  Would each user/container/whatever do this once?
>> Where would they keep the pre-parsed stuff?  How do they manage their
>> cache if the topology changes?
> Short answer i don't expect a cache, i expect that each program will have
> a init function that query the topology and update the application codes
> accordingly.

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.

I just don't think sysfs (or any filesystem, really) can scale to
express large, complicated topologies in a way that any normal program
can practically parse it.

My suspicion is that we're going to need to have the kernel parse and
cache these things.  We *might* have the data available in sysfs, but we
can't reasonably expect anyone to go parsing it.