Re: How much do the hint bits help?
От | Simon Riggs |
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Тема | Re: How much do the hint bits help? |
Дата | |
Msg-id | 1293035030.1193.28678.camel@ebony обсуждение исходный текст |
Ответ на | Re: How much do the hint bits help? (Tom Lane <tgl@sss.pgh.pa.us>) |
Список | pgsql-hackers |
On Wed, 2010-12-22 at 10:59 -0500, Tom Lane wrote: > Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes: > > My gut feeling is that a reasonable compromise is to set hint bits like > > we do today, but don't mark the page as dirty when only hint bits are > > set. That way you get the benefit of hint bits for tuples that are > > frequently accessed and stay in buffer cache. But you don't spend any > > extra I/O to set them. > > I think it's far more likely that that could be acceptable than the > radical method of removing hint bits altogether. I haven't argued to remove them, just have an option to not set them. > I have not looked into what's wrong with Merlin's test case, but my > thinking about it goes like this: we know that contention for buffer > lookup is significant at high loads, despite the facts that the accesses > are distributed across a lot of independently-usable buffers and we've > done much work to partition the lookup locks. If we remove hint bits > and thereby force an access to clog for every tuple touch, we can expect > that the contention for clog access will be comparable to the worst case > for buffer access contention ... except that in many cases, it will be > distributed across far fewer pages and so the actual interference rate > will be far higher. This will make our past experiences with "context > swap storms" look like a day at the beach. I think you're right, but I also think there are other ways we could optimise that other than hint bits. For example, the single item cache might be changed, or we might buffer/batch clog updates, or we might use a hash table of known aborted transactions etc. As Merlin points out, we don't have much evidence for their value or lack of value, so we need a parameter to allow wide scale testing. -- Simon Riggs http://www.2ndQuadrant.com/books/PostgreSQL Development, 24x7 Support, Training and Services
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