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

[Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>]

On 8/2/22 9:10 AM, Dan Williams wrote:
> Huang, Ying wrote:
>> Dan Williams <dan.j.williams@intel.com> writes:
>>
>>> Aneesh Kumar K.V wrote:
>>>> 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.
>>>>
>>>> A kernel parameter is provided to override the default memory tier.
>>>>
>>>> Link: https://lore.kernel.org/linux-mm/CAAPL-u9Wv+nH1VOZTj=9p9S70Y3Qz3+63EkqncRDdHfubsrjfw@mail.gmail.com
>>>> Link: https://lore.kernel.org/linux-mm/7b72ccf4-f4ae-cb4e-f411-74d055482026@linux.ibm.com
>>>>
>>>> Signed-off-by: Jagdish Gediya <jvgediya@linux.ibm.com>
>>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>>>> ---
>>>>  include/linux/memory-tiers.h |  17 ++++++
>>>>  mm/Makefile                  |   1 +
>>>>  mm/memory-tiers.c            | 102 +++++++++++++++++++++++++++++++++++
>>>>  3 files changed, 120 insertions(+)
>>>>  create mode 100644 include/linux/memory-tiers.h
>>>>  create mode 100644 mm/memory-tiers.c
>>>>
>>>> diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
>>>> new file mode 100644
>>>> index 000000000000..8d7884b7a3f0
>>>> --- /dev/null
>>>> +++ b/include/linux/memory-tiers.h
>>>> @@ -0,0 +1,17 @@
>>>> +/* SPDX-License-Identifier: GPL-2.0 */
>>>> +#ifndef _LINUX_MEMORY_TIERS_H
>>>> +#define _LINUX_MEMORY_TIERS_H
>>>> +
>>>> +/*
>>>> + * Each tier cover a abstrace distance chunk size of 128
>>>> + */
>>>> +#define MEMTIER_CHUNK_BITS	7
>>>> +#define MEMTIER_CHUNK_SIZE	(1 << MEMTIER_CHUNK_BITS)
>>>> +/*
>>>> + * For now let's have 4 memory tier below default DRAM tier.
>>>> + */
>>>> +#define MEMTIER_ADISTANCE_DRAM	(1 << (MEMTIER_CHUNK_BITS + 2))
>>>> +/* leave one tier below this slow pmem */
>>>> +#define MEMTIER_ADISTANCE_PMEM	(1 << MEMTIER_CHUNK_BITS)
>>>
>>> Why is memory type encoded in these values? There is no reason to
>>> believe that PMEM is of a lower performance tier than DRAM. Consider
>>> high performance energy backed DRAM that makes it "PMEM", consider CXL
>>> attached DRAM over a switch topology and constrained links that makes it
>>> a lower performance tier than locally attached DRAM. The names should be
>>> associated with tiers that indicate their usage. Something like HOT,
>>> GENERAL, and COLD. Where, for example, HOT is low capacity high
>>> performance compared to the general purpose pool, and COLD is high
>>> capacity low performance intended to offload the general purpose tier.
>>>
>>> It does not need to be exactly that ontology, but please try to not
>>> encode policy meaning behind memory types. There has been explicit
>>> effort to avoid that to date because types are fraught for declaring
>>> relative performance characteristics, and the relative performance
>>> changes based on what memory types are assembled in a given system.
>>
>> Yes.  MEMTIER_ADISTANCE_PMEM is something over simplified.  That is only
>> used in this very first version to make it as simple as possible.  
> 
> I am failing to see the simplicity of using names that convey a
> performance contract that are invalid depending on the system.
> 
>> I think we can come up with something better in the later version.
>> For example, identify the abstract distance of a PMEM device based on
>> HMAT, etc. 
> 
> Memory tiering has nothing to do with persistence why is PMEM in the
> name at all?
> 

How about

MEMTIER_DEFAULT_DAX_ADISTANCE with a comment there explaining if low level drivers don't
initialize a memory_dev_type for a device/NUMA node, dax/kmem will consider the node
slower than DRAM?


>>  And even in this first version, we should put MEMTIER_ADISTANCE_PMEM
>>  in dax/kmem.c.  Because it's just for that specific type of memory
>>  used now, not for all PMEM.
> 
> dax/kmem.c also handles HBM and "soft reserved" memory in general. There
> is also nothing PMEM specific about the device-dax subsystem.
> 
>> In the current design, memory type is used to report the performance of
>> the hardware, in terms of abstract distance, per Johannes' suggestion.
> 
> That sounds fine, just pick an abstract name, not an explicit memory
> type.
> 
>> Which is an abstraction of memory latency and bandwidth.  Policy is
>> described via memory tiers.  Several memory types may be put in one
>> memory tier.  The abstract distance chunk size of the memory tier may
>> be adjusted according to policy.
> 
> That part all sounds good. That said, I do not see the benefit of
> waiting to run away from these inadequate names.

-aneesh