Re: Autovacuum improvements

Alvaro Herrera <alvherre@commandprompt.com> writes:
> I'm cooking a patch for this which seems pretty reasonable, but I'm
> having a problem: what mechanism do we have for waiting until a process
> exits?

None, and I think you probably don't want to sit on the database lock
while waiting, either.  I was envisioning a simple sleep loop, viz

	for(;;)
	{
		acquire database lock;
		foreach(PGPROC entry in that database)
		{
			if (it's autovac)
				send sigint;
			else
				fail;
		}
		if (found any autovacs)
		{
			release database lock;
			sleep(100ms or so);
			/* loop back and try again */
		}
		else
			break;
	}

Also see Peter's nearby suggestion that we ought to wait instead of fail
for *all* cases of somebody attached to the database.  This would adapt
readily enough to that.

I was complaining elsewhere that I didn't want to use a sleep loop
for fixing the fsync-synchronization issue, but CREATE/DROP DATABASE
seems a much heavier-weight operation, so I don't feel that a sleep
is inappropriate here.

> Maybe make autovacuum acquire an LWLock at start, which it then
> keeps until it's gone, but it seems wasteful to have a lwlock just for
> that purpose.

And it doesn't scale to multiple autovacs anyway, much less the wait-for-
everybody variant.

			regards, tom lane