On Wed, Nov 27, 2024 at 9:01 AM Robert Haas <robertmh...@gmail.com> wrote: > > On Tue, Nov 26, 2024 at 8:38 PM Masahiko Sawada <sawada.m...@gmail.com> wrote: > > > visibility map: 5 pages set all-visible, 4 pages set all-frozen. > > > > Which means that 5 pages were marked as only all-visible and 4 pages > > were marked as all-frozen (and possibly all-visible too). The total > > number of pages' whose VM status changed is the sum of these two > > numbers, 9 pages. We would have no idea how many total VM bits were > > set, though. > > To me, the above output means 9 bits changed, 5 of which were > all-visible bits and 4 of which were all-frozen bits. It doesn't say > whether they were the same pages or not (although we might be able to > infer that based on only 3 of the 4 states being valid). > > If you want to count the number of pages that changed state, then I > think the message wording needs to be different,
Agreed. > but personally I > think counting the number of flipped bits of each type seems easier to > understand. I agree that counting the number of flipped bits is easier to understand. But I think there is still ambiguity when these two numbers are mostly the same. For example, suppose that we report the number of flipped bits and we have: visibility map: 10 pages set all-visible, 10000 pages set all-frozen. It's likely that many all-visible pages became all-frozen and 10 non-all-visible pages became all-visible. Overall, we can interpret it that the number of all-frozen pages in the table increased much and the number of all-visible pages (but not all-frozen) increased a bit by this vacuum. Then, suppose we have: visibility map: 10000 pages set all-visible, 10 pages set all-frozen. It's likely that many non-all-visible pages became all-visible but most of which didn't become all-frozen. Overall, we can interpret it that the number of all-visible pages (but not all-frozen) in the table increased much and the number of all-frozen pages increased a bit by this vacuum. Finally, in case where we have: visibility map: 10000 pages set all-visible, 10000 pages set all-frozen. We can understand that 10000 pages newly became all-frozen, but have no idea how many pages became all-visible but not all-frozen. It could be even 0. Users might want to know it to understand how (auto)vacuum and freezing are working well. Regards, -- Masahiko Sawada Amazon Web Services: https://aws.amazon.com
