On Wed, 12 Aug 2020 at 02:42, Alvaro Herrera <alvhe...@2ndquadrant.com> wrote: > > On 2020-Aug-11, Masahiko Sawada wrote: > > > On Tue, 11 Aug 2020 at 07:56, Alvaro Herrera <alvhe...@2ndquadrant.com> > > wrote: > > > > So if you have some table where tuples gain hint bits in bulk, and > > > rarely modify the pages afterwards, and promote before those pages are > > > frozen, then you may end up with a massive amount of pages that will > > > need hinting after the promote, which can become troublesome. > > > > Did the case you observed not use hot standby? I thought the impact of > > this issue could be somewhat alleviated in hot standby cases since > > read queries on the hot standby can set hint bits. > > Oh, interesting, I didn't know that. However, it's not 100% true: the > standby can set the bit in shared buffers, but it does not mark the page > dirty. So when the page is evicted, those bits that were set are lost. > That's not great. See MarkBufferDirtyHint:
Yeah, you're right. > > > > One simple idea to try to forestall this problem would be to modify the > > > algorithm so that all tuples are scanned and hinted if the page is going > > > to be dirtied -- then send a single FPI setting bits for all tuples, > > > instead of just on the first tuple. > > > > This idea seems good to me but I'm concerned a bit that the > > probability of concurrent processes writing FPI for the same page > > might get higher since concurrent processes could set hint bits at the > > same time. If it's true, I wonder if we can advertise hint bits are > > being updated to prevent concurrent FPI writes for the same page. > > Hmm, a very good point. Sounds like we would need to obtain an > exclusive lock on the page .. but that would be very problematic. > I think that when the page is going to be dirty only updating hint bits on the page and writing FPI need to be performed exclusively. So perhaps we can add a flag, say BM_UPDATE_HINTBITS, to buffer descriptor indicating the hint bits are being updated. Regards, -- Masahiko Sawada http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
