Re: shared-memory based stats collector

On Mon, Mar 15, 2021 at 10:56 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> I did roughly the first steps of the split as I had outlined. I moved:
>
> 1) wait event related functions into utils/activity/wait_event.c /
> wait_event.h
>
> 2) "backend status" functionality (PgBackendStatus stuff) into
> utils/activity/backend_status.c
>
> 3) "progress" related functionality into
> utils/activity/backend_progress.c

In general, I like this. I'm not too sure about the names. I realize
you don't want to have functions called status.c and progress.c,
because that's awful generic, but now you have backend_progress.c,
backend_status.c, and wait_event.c, which makes the last one look a
little strange. Maybe command_progress.c instead of
backend_progress.c?

> I think 1 and 2 are good (albeit in need of further polish). I'm a bit
> less sure about 3:
> - There's dependency from backend_status.h to backend_progress.h,
> because it PGSTAT_NUM_PROGRESS_PARAM etc.

That doesn't seem like a catastrophe.

> - it's a fairly small amount of code

But it's not bad to have it separate.

> - there's potential for confusion, because there's also
> include/commands/progress.h

That could be merged, perhaps. I think I only created that because I
didn't want to jam too much stuff into pgstat.h. But if it has its own
header then jamming some more stuff in there seems more OK.

> - I'm inclined to leave pgstat_report_{activity, tmpfile, appname,
> timestamp, ..} alone naming-wise, but to rename pgstat_bestart() to
> something like pgbestat_start()?

I'd probably rename them e.g. command_progress_start(),
comand_progress_update_param(), etc.

> - I've not gone through all the files that could now remove pgstat.h,
> replacing it with wait_event.h - I'm thinking it might be worth
> waiting till just after code freeze with that (there'll new additions,
> and it's likely to cause conflicts)?

Don't care.

> - backend_status.h needs miscadmin.h, due to BackendType. Imo that's a
> header we should try to avoid exposing in headers if possible. But
> right now there's no good place to move BackendType to. So I'd let
> this slide for now.

Why the concern? miscadmin.h is extremely widely-included already.
Maybe it should be broken up into pieces so that we're not including
so MUCH stuff in a zillion places, but the header that contains the
definition of CHECK_FOR_INTERRUPTS() is always going to be needed in a
ton of spots. Honestly, I wonder why we don't just put that part in
postgres.h. If you're writing any significant amount of code and you
don't have at least one CHECK_FOR_INTERRUPTS() in there, you're
probably doing it wrong.

--
Robert Haas
EDB: http://www.enterprisedb.com