Re: Add last commit LSN to pg_last_committed_xact()

On Wed, Jan 19, 2022 at 10:12 PM Andres Freund <andres@anarazel.de> wrote:
>
> Hi,
>
> On 2022-01-19 21:23:12 -0500, James Coleman wrote:
> >  { oid => '3537', descr => 'get identification of SQL object',
> > diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
> > index a58888f9e9..2a026b0844 100644
> > --- a/src/include/storage/proc.h
> > +++ b/src/include/storage/proc.h
> > @@ -258,6 +258,11 @@ struct PGPROC
> >       PGPROC     *lockGroupLeader;    /* lock group leader, if I'm a member */
> >       dlist_head      lockGroupMembers;       /* list of members, if I'm a leader */
> >       dlist_node      lockGroupLink;  /* my member link, if I'm a member */
> > +
> > +     /*
> > +      * Last transaction metadata.
> > +      */
> > +     XLogRecPtr      lastCommitLSN;          /* cache of last committed LSN */
> >  };
>
> Might be worth forcing this to be on a separate cacheline than stuff more
> hotly accessed by other backends, like the lock group stuff.

What's the best way to do that? I'm poking around and don't see any
obvious cases of doing that in a struct definition. I could add a
char* of size PG_CACHE_LINE_SIZE, but that seems unnecessarily
wasteful, and the other ALIGN macros seem mostly used in situations
where we're allocating memory. Is it possible in C to get the size of
the struct so far to be able to subtract from PG_CACHE_LINE_SIZE?
Maybe there's some other approach I'm missing...

Thanks,
James Coleman