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

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Date:   Fri, 28 Oct 2022 16:33:48 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Bharata B Rao <bharata@....com>
Cc:     Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>,
        <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

Bharata B Rao <bharata@....com> writes:

> 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?

Only if we set abstract distance chunk size large.  So, I think that
it's better to set chunk size as small as possible to avoid potential
issue.  What is the downside to set the chunk size small?

Best Regards,
Huang, Ying