Re: show size of DSAs and dshash tables in pg_dsm_registry_allocations

Hi,

The patch LGTM overall. I had tested the v1 and it worked fine.

> That function was added by commit ee1b30f, which AFAICT used an exclusive
> lock just to stay consistent with the rest of dsa.c [0]. I don't see any
> discussion about this in the original DSA thread [1]. Perhaps we could go
> through dsa.c and switch to LW_SHARED where appropriate, although I doubt
> it makes much difference.
>

Thank you for highlighting the discussions. I'm unsure about the best
approach here, but I think it would be safe to stay consistent with the
rest of the code in dsa.c, especially since it's unclear that the use of
LW_EXCLUSIVE for reading values in dsa is a mistake.

>
> > +size_t
> > +dsa_get_total_size_from_handle(dsa_handle handle)
> >
> > I believe this function will report the size as long as the dsa control
> > structure is created within a dsm segment, since all dsm segments are
> > tracked by the global list - dsm_segment_list, regardless of whether the
> > dsa is created with dsa_create or dsa_create_in_place. In that case,
> > perhaps we should update the comment above to reflect this.
>
> Sorry, I'm not following what you think we should update the comment to
> say.
>

Sorry for the confusion, I am trying to say that we can change the
following comment

+ *The area must have
+ * been created with dsa_create (not dsa_create_in_place).

to say this:

"The area must have been created using dsm_segments"

Since, this function can report the size of an area created with
dsa_create_in_place
too, as long as the area is created using dsm_segments.

> > 4. Since, with this change, the size column will show memory allocation
> > regardless of whether it is currently mapped in the local process, I
> > think it would be helpful to add a boolean column to display the mapped
> > status as a future enhancement.
>
> Maybe, although I'm struggling to think of a scenario where that
> information would be useful.
>
>
Fair enough. I was thinking of a scenario where a user might want
to see how much dsa memory is allocated in the client backend process.
However, I understand now that this view is designed for the entire cluster,
and adding a column which is process-specific could lead to confusion.

Thank you,
Rahila Syed