Re: Tweaking DSM and DSA limits

On Mon, Oct 21, 2019 at 12:21 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> On Fri, Jun 21, 2019 at 6:52 AM Andres Freund <andres@anarazel.de> wrote:
> > On 2019-06-20 14:20:27 -0400, Robert Haas wrote:
> > > I am not convinced that we really need to GUC-ify this.  How about
> > > just bumping the value up from 2 to say 5?
> >
> > I'm not sure either. Although it's not great if the only way out for a
> > user hitting this is to increase max_connections... But we should really
> > increase the default.
>
> Ok, hard-to-explain GUC abandoned.  Here is a patch that just adjusts
> the two constants.  DSM's array allows for 5 slots per connection (up
> from 2), and DSA doubles its size after every two segments (down from
> 4).

Pushed.  No back-patch for now: the risk/reward ratio doesn't seem
right for that.

> I have a long wish list of improvements I'd like to investigate in
> this area, subject for future emails, but while I'm making small
> tweaks, here's another small thing: there is no "wait event" while
> allocating (in the kernel sense) POSIX shm on Linux, unlike the
> equivalent IO when file-backed segments are filled with write() calls.
> Let's just reuse the same wait event, so that you can see what's going
> on in pg_stat_activity.

Also pushed.