linux-kernel - Re: [RFC] memory tiering: use small chunk size and more tiers

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Date:   Fri, 28 Oct 2022 13:34:46 +0530
From:   Bharata B Rao <bharata@....com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Jagdish Gediya <jvgediya.oss@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Michal Hocko <mhocko@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>, Wei Xu <weixugc@...gle.com>,
        Yang Shi <shy828301@...il.com>
Subject: Re: [RFC] memory tiering: use small chunk size and more tiers

On 10/28/2022 11:16 AM, Huang, Ying wrote:
> If my understanding were correct, you think the latency / bandwidth of
> these NUMA nodes will near each other, but may be different.
> 
> Even if the latency / bandwidth of these NUMA nodes isn't exactly same,
> we should deal with that in memory types instead of memory tiers.
> There's only one abstract distance for each memory type.
> 
> So, I still believe we will not have many memory tiers with my proposal.
> 
> I don't care too much about the exact number, but want to discuss some
> general design choice,
> 
> a) Avoid to group multiple memory types into one memory tier by default
>    at most times.

Do you expect the abstract distances of two different types to be
close enough in real life (like you showed in your example with
CXL - 5000 and PMEM - 5100) that they will get assigned into same tier
most times?

Are you foreseeing that abstract distance that get mapped by sources
like HMAT would run into this issue?

Regards,
Bharata.