linux-kernel - Re: [PATCH v10 1/8] mm/demotion: Add support for explicit memory tiers

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Message-ID: <YuLF/GG8x5lQvg/f@cmpxchg.org>
Date:   Thu, 28 Jul 2022 13:23:08 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>,
        Wei Xu <weixugc@...gle.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        jvgediya.oss@...il.com, Jagdish Gediya <jvgediya@...ux.ibm.com>
Subject: Re: [PATCH v10 1/8] mm/demotion: Add support for explicit memory
 tiers

On Wed, Jul 27, 2022 at 09:16:08AM +0800, Huang, Ying wrote:
> Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> writes:
> > It is an abstract concept that indicates the performance of the
> > device. As we learn more about which device attribute makes more impact in
> > defining hierarchy, performance level will give more weightage to that specific
> > attribute. It could be write latency or bandwidth. For me, distance has a direct
> > linkage to latency because that is how we define numa distance now. Adding
> > abstract to the name is not making it more abstract than perf_level. 
> >
> > I am open to suggestions from others.  Wei Xu has also suggested perf_level name.
> > I can rename this to abstract_distance if that indicates the goal better.
> 
> I'm open to naming.  But I think that it's good to define it at some
> degree instead of completely opaque stuff.  If it's latency based, then
> low value corresponds to high performance.  If it's bandwidth based,
> then low value corresponds to low performance.
> 
> Hi, Wei and Johannes,
> 
> What do you think about this?

I'm also partial to distance. It's a familiar metric in non-uniform
memory for guiding placement decisions, and that is how we continue to
use it here too.

It's historically meant bus latency, but given how the kernel
perceives and acts on the metric IMO the term works just fine to
express differences in bandwidth and chip resonpse times as well.