Re: Inefficiency in parallel pg_restore with many tables

On Sun, Jul 16, 2023 at 09:45:54AM -0400, Tom Lane wrote:
> Actually, as long as we're talking about approximately-correct behavior:
> let's make the ready_list be a priority heap, and then just make
> pop_next_work_item scan forward from the array start until it finds an
> item that's runnable per the lock heuristic. If the heap root is
> blocked, the next things we'll examine will be its two children.
> We might pick the lower-priority of those two, but it's still known to
> be higher priority than at least 50% of the remaining heap entries, so
> it shouldn't be too awful as a choice. The argument gets weaker the
> further you go into the heap, but we're not expecting that having most
> of the top entries blocked will be a common case. (Besides which, the
> priorities are pretty crude to begin with.) Once selected, pulling out
> an entry that is not the heap root is no problem: you just start the
> sift-down process from there.
>
> The main advantage of this over the only-sort-sometimes idea is that
> we can guarantee that the largest ready item will always be dispatched
> as soon as it can be (because it will be the heap root). So cases
> involving one big table (with big indexes) and a lot of little ones
> should get scheduled sanely, which is the main thing we want this
> algorithm to ensure. With the other approach we can't really promise
> much at all.

This seems worth a try. IIUC you are suggesting making binaryheap.c
frontend-friendly and expanding its API a bit. If no one has volunteered,
I could probably hack something together.

--
Nathan Bossart
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