Re: Eagerly scan all-visible pages to amortize aggressive vacuum

On Tue, Feb 4, 2025 at 2:57 PM Robert Treat <rob(at)xzilla(dot)net> wrote:
> > Yea, I thought that counting them as failures made sense because we
> > did fail to freeze them. However, now that you mention it, we didn't
> > fail to freeze them because of age, so maybe we don't want to count
> > them as failures. I don't expect us to have a bunch of contended
> > all-visible pages, so I think the question is about what makes it more
> > clear in the code. What do you think? Should I reset was_eager_scanned
> > to false if we don't get the cleanup lock?
>
> I feel like if we are making the trade-off in resources to attempt
> eager scanning, and we weren't making progress for whatever reason
> (and in the lock failure cases, wouldn't some of those be things that
> would prevent us from freezing?) then it would generally be ok to bias
> towards bailing sooner rather than later.

Failures to acquire cleanup locks are, hopefully, rare, so it may not
matter that much. Having said that, if we skip a page because we can't
acquire a cleanup lock on it, I think that means that it was already
present in shared_buffers, which means that we didn't have to do an
I/O to get it. Since I think the point of the failure cap is mostly to
limit wasted I/O, I would lean toward NOT counting such cases as
failures.

--
Robert Haas
EDB: http://www.enterprisedb.com