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

[Johannes Weiner <hannes@cmpxchg.org>]

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.