Re: [PATCH v2 1/3] sched/numa: make free-space tiering promotion bypass configurable

[Bharata B Rao]

On 28-Oct-26 9:42 AM, Li Zhe wrote:
> Memory tiering normally promotes any recently accessed slow-tier page if
> the target fast-tier node has enough free space. This preserves the
> existing policy of filling available fast memory before relying on
> hotness and rate-limit control.
> 
> That is not always the desired policy. Some deployments may intentionally
> keep part of a workload's memory on CXL-attached memory, for example to
> preserve a desired DRAM/CXL capacity split or bandwidth distribution. With
> the free-space bypass, short cold accesses and bursts of hot accesses can
> refill the fast tier and pull the workload away from that distribution
> whenever DRAM has free space.
> 
> Redis testing exercises this policy issue. On a CXL system with node0 as
> DRAM and node3 as slow-tier memory, a Redis test was run with
> memory-tiering-only mode enabled. The test used a Redis dataset with 12
> million 4KB values. It reclaimed memory until about 85% of Redis
> anonymous memory was on node3, used a Redis benchmark configuration with
> a 10% hot-key working set, and then issued a short 256MB access burst to
> cold keys outside that working set. The cold-key burst mainly touches
> memory outside the hot working set, which is expected to remain mostly on
> the slow tier.

Can you please clarify if 85% of redis memory ended up on lower tier naturally
due to overflow from toptier? I ask this question because, if it is a natural
overflow/demotion, then it will be hard to find head room in the toptier to
allow promotions that bypass hotness or rate limit checks."

Regards,
Bharata.