linux-kernel - Re: [PATCH v11 1/8] mm/demotion: Add support for explicit memory tiers

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Date:   Fri, 29 Jul 2022 12:54:36 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     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,
        Jagdish Gediya <jvgediya@...ux.ibm.com>
Subject: Re: [PATCH v11 1/8] mm/demotion: Add support for explicit memory tiers

"Huang, Ying" <ying.huang@...el.com> writes:

> "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:
>
>> In the current kernel, memory tiers are defined implicitly via a demotion path
>> relationship between NUMA nodes, which is created during the kernel
>> initialization and updated when a NUMA node is hot-added or hot-removed. The
>> current implementation puts all nodes with CPU into the highest tier, and builds
>> the tier hierarchy tier-by-tier by establishing the per-node demotion targets
>> based on the distances between nodes.
>>
>> This current memory tier kernel implementation needs to be improved for several
>> important use cases,
>>
>> The current tier initialization code always initializes each memory-only NUMA
>> node into a lower tier. But a memory-only NUMA node may have a high performance
>> memory device (e.g. a DRAM-backed memory-only node on a virtual machine) that
>> should be put into a higher tier.
>>
>> The current tier hierarchy always puts CPU nodes into the top tier. But on a
>> system with HBM or GPU devices, the memory-only NUMA nodes mapping these devices
>> should be in the top tier, and DRAM nodes with CPUs are better to be placed into
>> the next lower tier.
>>
>> With current kernel higher tier node can only be demoted to nodes with shortest
>> distance on the next lower tier as defined by the demotion path, not any other
>> node from any lower tier. This strict, demotion order does not work in all use
>> cases (e.g. some use cases may want to allow cross-socket demotion to another
>> node in the same demotion tier as a fallback when the preferred demotion node is
>> out of space), This demotion order is also inconsistent with the page allocation
>> fallback order when all the nodes in a higher tier are out of space: The page
>> allocation can fall back to any node from any lower tier, whereas the demotion
>> order doesn't allow that.
>>
>> This patch series address the above by defining memory tiers explicitly.
>>
>> Linux kernel presents memory devices as NUMA nodes and each memory device is of
>> a specific type. The memory type of a device is represented by its abstract
>> distance. A memory tier corresponds to a range of abstract distance. This allows
>> for classifying memory devices with a specific performance range into a memory
>> tier.
>>
>> This patch configures the range/chunk size to be 128. The default DRAM
>> abstract distance is 512. We can have 4 memory tiers below the default DRAM
>> abstract distance which cover the range 0 - 127, 127 - 255, 256- 383, 384 - 511.
>> Slower memory devices like persistent memory will have abstract distance below
>> the default DRAM level and hence will be placed in these 4 lower tiers.
>
> For abstract distance, the lower value means higher performance, higher
> value means lower performance.  So the abstract distance of PMEM should
> be smaller than that of DRAM.

I noticed that after sending v11 and did send v12 fixing that already
which can be found 

https://lore.kernel.org/linux-mm/20220729061349.968148-1-aneesh.kumar@linux.ibm.com

>
>> A kernel parameter is provided to override the default memory tier.
>
> Forget to delete?

yes. Also fixed in v12.

-aneesh