Re: [RFC PATCH 0/3] Cgroup accounting of memory tier usage

[Tim Chen <tim.c.chen@linux.intel.com>]

On Wed, 2022-06-15 at 12:58 +0800, Ying Huang wrote:
> On Tue, 2022-06-14 at 15:25 -0700, Tim Chen wrote:
> > For controlling usage of a top tiered memory by a cgroup, accounting
> > of top tier memory usage is needed.  This patch set implements the
> > following:
> > 
> > Patch 1 introduces interface and simple implementation to retrieve
> > 	cgroup tiered memory usage
> > Patch 2 introduces more efficient accounting with top tier memory page counter 
> > Patch 3 provides a sysfs interface to repot the the top tiered memory
> > 	usage.
> > 
> > The patchset works with Aneesh's v6 memory-tiering implementation [1].
> > It is a preparatory patch set before introducing features to
> > control top tiered memory in cgroups.
> > 
> > I'll like to first get feedback to see if 
> > (1) Controllng the topmost tiered memory is enough 
> > or
> > (2) Multiple tiers at the top levels need to be grouped into "toptier"
> > or
> 
> If we combine top-N tiers, I think the better name could be "fast-tier",
> in contrast to "slow-tier".
> 

I can see use cases for grouping tiers. For example, it makese sense for HBM and 
DRAM tiers be grouped together into a "fast-tier-group".

To make things simple, we can define any tiers above or equal
to the rank of DRAM will belong to this fast-tier-group.

An implication for page promotion/demotion is it needs
to take tier grouping into consideration.  You want to demote
pages away from current tier-group.  For example,
you want to demote HBM (fast-tier-group) into PMEM (slow-tier-group)
instead of into DRAM (fast-tier-group).  

The question is whether fast/slow tier groups are sufficient.
Or you need fast/slow/slower groups?

> > (3) There are use cases not covered by (1) and (2). 
> 
> Is it necessary to control memory usage of each tier (except the
> lowest/slowest)?  I am not the right person to answer the question, but
> I want to ask it.
> 

I have the same question.

Tim