Re: R: Proposal: Shared Work Mem Area

Greetings,

Please don't top-post on the PG mailing lists, it makes it harder to
follow the discussion.

* Marco Fortina (marco_fortina(at)hotmail(dot)it) wrote:
> https://commitfest.postgresql.org/42/3867/ is not exacly what I proposed as new feature to developers.

I understood what you were proposing.

> If I'm not wrong, almost all main memory areas have a fixed size:
>
> shared_buffers
> effective_cache_size
> wal_buffers
>
> Instead, work_mem is per-process dynamically allocated up to defined size limit.

That's not how work_mem works actually. It's a per-node amount and it's
not a per-process overall limit, nor is it really a hard limit though
some nodes will do their best to respect the amount configured.

> What I suggested is to replace work_mem from per-process allocation to global and fixed size allocation (see pga_aggregate_target on Oracle) and shared to worker processes.

I understood the suggestion and it's a lot of work for an unclear gain.
You noted that having it be pulled from a single area would allow
administrators to configure an overall memory usage limit- but that's
not the only way to do that and there's an existing effort to do exactly
that already underway that's a lot simpler than what you're proposing.
While there might be other advantages to having a shared memory segment
be used for work_mem, you've not outlined any.

> Let's assume the new parameter name is worker_mem_area and this was set to 8GB: with my proposal method each worker process do not use it's own dedicated work_mem but the shared one.

I understand your suggestion, but making such a large change just to
make it isn't sensible, there should be reasoning behind why that's
better than what we're doing already or proposing to do.

> In this way each worker is also able to peek free pages from the worker_mem_area if needed.

This can be done with the existing approach and doesn't require a shared
memory segment for work_mem. We are pretty far from having an actual
acceptance system for queries though but I do agree that would be a
useful thing to work towards. I don't know that it requires work_mem
being in shared memory though.

Thanks,

Stephen