Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

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

On 6/8/22 9:25 PM, Johannes Weiner wrote:
> Hello,
> 
> On Wed, Jun 08, 2022 at 10:11:31AM -0400, Johannes Weiner wrote:
>> On Fri, Jun 03, 2022 at 07:12:29PM +0530, Aneesh Kumar K.V wrote:
>>> @@ -0,0 +1,20 @@
>>> +/* SPDX-License-Identifier: GPL-2.0 */
>>> +#ifndef _LINUX_MEMORY_TIERS_H
>>> +#define _LINUX_MEMORY_TIERS_H
>>> +
>>> +#ifdef CONFIG_TIERED_MEMORY
>>> +
>>> +#define MEMORY_TIER_HBM_GPU	0
>>> +#define MEMORY_TIER_DRAM	1
>>> +#define MEMORY_TIER_PMEM	2
>>> +
>>> +#define MEMORY_RANK_HBM_GPU	300
>>> +#define MEMORY_RANK_DRAM	200
>>> +#define MEMORY_RANK_PMEM	100
>>> +
>>> +#define DEFAULT_MEMORY_TIER	MEMORY_TIER_DRAM
>>> +#define MAX_MEMORY_TIERS  3
>>
>> I understand the names are somewhat arbitrary, and the tier ID space
>> can be expanded down the line by bumping MAX_MEMORY_TIERS.
>>
>> But starting out with a packed ID space can get quite awkward for
>> users when new tiers - especially intermediate tiers - show up in
>> existing configurations. I mentioned in the other email that DRAM !=
>> DRAM, so new tiers seem inevitable already.
>>
>> It could make sense to start with a bigger address space and spread
>> out the list of kernel default tiers a bit within it:
>>
>> MEMORY_TIER_GPU		0
>> MEMORY_TIER_DRAM	10
>> MEMORY_TIER_PMEM	20
> 
> Forgive me if I'm asking a question that has been answered. I went
> back to earlier threads and couldn't work it out - maybe there were
> some off-list discussions? Anyway...
> 
> Why is there a distinction between tier ID and rank? I undestand that
> rank was added because tier IDs were too few. But if rank determines
> ordering, what is the use of a separate tier ID? IOW, why not make the
> tier ID space wider and have the kernel pick a few spread out defaults
> based on known hardware, with plenty of headroom to be future proof.
> 
>    $ ls tiers
>    100				# DEFAULT_TIER
>    $ cat tiers/100/nodelist
>    0-1				# conventional numa nodes
> 
>    <pmem is onlined>
> 
>    $ grep . tiers/*/nodelist
>    tiers/100/nodelist:0-1	# conventional numa
>    tiers/200/nodelist:2		# pmem
> 
>    $ grep . nodes/*/tier
>    nodes/0/tier:100
>    nodes/1/tier:100
>    nodes/2/tier:200
> 
>    <unknown device is online as node 3, defaults to 100>
> 
>    $ grep . tiers/*/nodelist
>    tiers/100/nodelist:0-1,3
>    tiers/200/nodelist:2
> 
>    $ echo 300 >nodes/3/tier
>    $ grep . tiers/*/nodelist
>    tiers/100/nodelist:0-1
>    tiers/200/nodelist:2
>    tiers/300/nodelist:3
> 
>    $ echo 200 >nodes/3/tier
>    $ grep . tiers/*/nodelist
>    tiers/100/nodelist:0-1	
>    tiers/200/nodelist:2-3
> 
> etc.

tier ID is also used as device id memtier.dev.id. It was discussed that 
we would need the ability to change the rank value of a memory tier. If 
we make rank value same as tier ID or tier device id, we will not be 
able to support that.

-aneesh