Re: Reducing power consumption on idle servers

On Sat, 26 Feb 2022 at 17:44, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Magnus Hagander <magnus(at)hagander(dot)net> writes:
> >> Deprecating explicit file-based promotion is possible and simple, so
> >> that is the approach in the latest version of the patch.
>
> > Is there any actual use-case for this other than backwards
> > compatibility?
>
> The fundamental problem with signal-based promotion is that it's
> an edge-triggered rather than level-triggered condition, ie if you
> miss the single notification event you're screwed. I'm not sure
> why we'd want to go there, considering all the problems we're having
> with edge-triggered APIs in nearby threads.
>
> We could combine the two approaches, perhaps: have "pg_ctl promote"
> create a trigger file and then send a signal. The trigger file
> would record the existence of the promotion request, so that if the
> postmaster isn't running right now or misses the signal, it'd still
> promote when restarted or otherwise at the next opportunity.
> But with an approach like this, we could expect quick response to
> the signal in normal conditions, without need for constant wakeups
> to check for the file. Also, this route would allow overloading
> of the signal, since it would just mean "wake up, I asked you to
> do something" rather than needing to carry the full semantics of
> what is to be done.

The above is how this works now, luckily, but few comments explain why.

This current state evolved because I first added file-based promotion,
for the reasons you give, but it was slow, so the signal approach was
added later, with new API.

What we are discussing deprecating is the ability to create the
trigger file directly (the original API). Once that is gone the
mechanism would be to request promotion via pg_ctl promote (the new
API), which then creates the trigger file and sends a signal. So in
both APIs the trigger file is still used.

In this patch, if you create the trigger file
directly/explicitly/yourself (the old API) then it will still trigger
when hibernating, but it will be slower. The new API is unaffected by
this change.

--
Simon Riggs http://www.EnterpriseDB.com/