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

[Li Zhe]

On 10/29/26 5:34 PM, Bharata B Rao wrote:
> 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."


It was not a natural overflow from a full top tier.

The test intentionally reclaimed/demoted Redis memory to the lower tier
to model a deployment that keeps only a limited fraction of the workload
in DRAM and leaves the rest on CXL memory.  In that setup the top tier can
still have enough free space, so the current free-space bypass can promote
short cold accesses back to DRAM.

Those pages are not hot enough to be promoted by the normal
hotness/rate-limit path, but the free-space bypass can still promote them
when the top tier has enough free space.

Thanks for pointing this out.  I will clarify this setup in the commit
message.

Thanks,
Zhe

>
> Regards,
> Bharata.