Re: Missing wait events (gap analysis)

Hi,

On 2025-11-21 18:43:31 -0600, Nikolay Samokhvalov wrote:
> Many tools that implement wait event analysis, when visualizing samples
> with "wait_event is null" use green color and "CPU" (perhaps, it started
> with RDS Performance Insights and PASH Viewer and, I suppose, originally
> came from the Oracle world, and now I see it in many more places).
>
> I don't have any concerns with green color, but always had a feeling that
> "coalesce(wait_event, 'CPU')" is an assumption that can make analysis
> inaccurate, because there may be a lot of places in the code that are not
> covered by wait events, but technically should -- and such places cannot be
> named "CPU".
>
> I asked Claude Code to analyze Postgres source code and find such places,
> that we could potentially cover with more wait events. Here is the first
> result:
> https://github.com/NikolayS/postgres/blob/claude/cpu-asterisk-wait-events-01CyiYYMMcFMovuqPqLNcp8T/WAIT_EVENTS_ANALYSIS.md
>
> Before moving forward with proposals of specific patches, I wanted to hear
> opinions -- does it make sense to work in this direction?

Some of this seems sensible. However, I vehemently oppose turning wait events
into a poor emulation of a CPU profiler. I think it would lead us down a bad
path to add wait events for CPU activity. It'd just lead us to adding them
everywhere, ending up with wait events (CPU activity is not a wait!) having
significant costs.

Greetings,

Andres Freund