On Thu, Feb 25, 2016 at 11:19:20AM -0800, Jeff Janes wrote:
> On Wed, Feb 24, 2016 at 8:51 AM, Teodor Sigaev <teodor@sigaev.ru> wrote:
> > Thank you for remembering this problem, at least for me.
> >
> >>> Well, turns out there's a quite significant difference, actually. The
> >>> index sizes I get (quite stable after multiple runs):
> >>>
> >>> 9.5 : 2428 MB
> >>> 9.6 + alone cleanup : 730 MB
> >>> 9.6 + pending lock : 488 MB
> >
> > Interesting, I don't see why alone_cleanup and pending_lock are so differ.
> > I'd like to understand that, but does somebody have an good theory?
>
> Under my patch, anyone who wanted to do a clean up and detected
> someone else was doing one would wait for the concurrent one to end.
> (This is more consistent with the existing behavior, I just made it so
> they don't do any damage while they wait.)
>
> Under your patch, if a backend wants to do a clean up and detects
> someone else is already doing one, it would just skip the clean up and
> proceed on with whatever it was doing. This allows one process
> (hopefully a vacuum, but maybe a user backend) to get pinned down
> indefinitely, as other processes keep putting stuff onto the end of
> the pending_list with no throttle.
>
> Since the freespace recycling only takes place once the list is
> completely cleaned, allowing some processes to add to the end while
> one poor process is trying to clean can lead to less effective
> recycling.
>
> That is my theory, anyway.
[This is a generic notification.]
The above-described topic is currently a PostgreSQL 9.6 open item. Teodor,
since you committed the patch believed to have created it, you own this open
item. If that responsibility lies elsewhere, please let us know whose
responsibility it is to fix this. Since new open items may be discovered at
any time and I want to plan to have them all fixed well in advance of the ship
date, I will appreciate your efforts toward speedy resolution. Please
present, within 72 hours, a plan to fix the defect within seven days of this
message. Thanks.