linux-kernel - Re: [PATCH v14 04/10] mm/demotion/dax/kmem: Set node's abstract distance to MEMTIER_DEFAULT_DAX

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Date:   Tue, 16 Aug 2022 20:15:55 +0530
From:   Bharata B Rao <bharata@....com>
To:     huang ying <huang.ying.caritas@...il.com>
Cc:     Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>,
        "Huang, Ying" <ying.huang@...el.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, Wei Xu <weixugc@...gle.com>,
        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>,
        Johannes Weiner <hannes@...xchg.org>, jvgediya.oss@...il.com
Subject: Re: [PATCH v14 04/10] mm/demotion/dax/kmem: Set node's abstract
 distance to MEMTIER_DEFAULT_DAX_ADISTANCE

On 8/16/2022 1:56 PM, huang ying wrote:
<snip>
>>>
>>> If my understanding were correct, you are suggesting to use a kind of
>>> logarithmic mapping from latency to abstract distance?  That is,
>>>
>>>   abstract_distance = log2(latency)
>>>
>>> While I am suggesting to use a kind of linear mapping from latency to
>>> abstract distance.  That is,
>>>
>>>   abstract_distance = C * latency
>>>
>>> I think that linear mapping is easy to understand.
>>>
>>> Are there some good reasons to use logarithmic mapping?
>>
>> Also, what is the recommendation for using bandwidth measure which
>> may be available from HMAT for CXL memory? How is bandwidth going
>> to influence the abstract distance?
> 
> This is a good question.
> 
> Per my understanding, latency stands for idle latency by default.  But
> in practice, the latency under some reasonable memory accessing
> throughput is the "real" latency.  So the memory with lower bandwidth
> should have a larger abstract distance than the memory with higher
> bandwidth even if the idle latency is the same.  But I don't have a
> perfect formula to combine idle latency and bandwidth into abstract
> distance.  One possibility is to increase abstract distance if the
> bandwidth of the memory is much lower than that of DRAM.

So if the firmware/platforms differ in their definition of latency and
bandwidth (like idle vs real value etc) in the firmware tables
(like HMAT), then the low level drivers (like ACPI) would have to be
aware of these and handle the conversion from latency and bw to
abstract distance correctly?

Regards,
Bharata.