Re: [RFC PATCH 0/4] Kernel daemon for detecting and promoting hot pages

Balbir Singh
2025-03-18T05:28:32Z

On 3/6/25 16:45, Bharata B Rao wrote:
> Hi,
>
> This is an attempt towards having a single subsystem that accumulates
> hot page information from lower memory tiers and does hot page
> promotion.
>
> At the heart of this subsystem is a kernel daemon named kpromoted that
> does the following:
>
> 1. Exposes an API that other subsystems which detect/generate memory
> access information can use to inform the daemon about memory
> accesses from lower memory tiers.
> 2. Maintains the list of hot pages and attempts to promote them to
> toptiers.
>
> Currently I have added AMD IBS driver as one source that provides
> page access information as an example. This driver feeds info to
> kpromoted in this RFC patchset. More sources were discussed in a
> similar context here at [1].
>

Is hot page promotion mandated or good to have? Memory tiers today
are a function of latency and bandwidth, specifically in
mt_aperf_to_distance()

adist ~ k * R(B)/R(L) where R(x) is relatively performance of the
memory w.r.t DRAM. Do we want hot pages in the top tier all the time?
Are we optimizing for bandwidth or latency?

> This is just an early attempt to check what it takes to maintain
> a single source of page hotness info and also separate hot page
> detection mechanisms from the promotion mechanism. There are too
> many open ends right now and I have listed a few of them below.
>

<snip>

> This is just an early RFC posted now to ignite some discussion
> in the context of LSFMM [2].
>

I look forward to any summary of the discussions

Balbir Singh