Re: Creating a function for exposing memory usage of backend process

Hi,

On 2020-08-19 21:29:06 -0400, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > On 2020-08-19 11:01:37 -0400, Tom Lane wrote:
> >> I agree with that, but I think this patch has a bigger problem:
> >> why bother at all? It seems like a waste of code space and future
> >> maintenance effort, because there is no use-case.
>
> > I don't agree with this at all. I think there's plenty use cases. It's
> > e.g. very common to try to figure out why the memory usage of a process
> > is high. Is it memory not returned to the OS? Is it caches that have
> > grown too much etc.
>
> Oh, I agree completely that there are lots of use-cases for finding
> out what a process' memory map looks like. But this patch fails to
> address any of them in a usable way.

Even just being able to see the memory usage in a queryable way is a
huge benefit. The difference over having to parse the log, then parse
the memory usage dump, and then aggregate the data in there in a
meaningful way is *huge*. We've been slacking around lowering our
memory usage, and I think the fact that it's annoying to analyze is a
partial reason for that.

I totally agree that it's not *enough*, but in contrast to you I think
it's a good step. Subsequently we should add a way to get any backends
memory usage.
It's not too hard to imagine how to serialize it in a way that can be
easily deserialized by another backend. I am imagining something like
sending a procsignal that triggers (probably at CFR() time) a backend to
write its own memory usage into pg_memusage/<pid> or something roughly
like that.

Greetings,

Andres Freund