On Wed, Dec 22, 2010 at 4:54 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Josh Berkus <j...@agliodbs.com> writes: >> Regarding the contention which Tom expects: the extra load on the CLOG >> would be 100% reads, no? If it's *all* reads, why would we have any >> more contention than we have now? > > Read involves sharelock which still causes contention. Those bufmgr > contention storms we saw before were completely independent of whether > the pages were accessed for read or for write. > > Another thing to keep in mind is that the current clog access code is > designed on the assumption that there's considerable locality of access > to pg_clog, ie, you usually only need to consult it for recent XIDs > because older ones have been hinted. Turn off hint bits, that behavior > goes out the window.
That's not always going to be the case though. In olap-ish environments you will see cases of scans over many records that come from a single transaction. This is also the case where hint bits can really drill you -- you insert a bunch of records, log the bits, delete, log the bits, and vacuum eventually. I started investigating this on behalf of a friend who is experiencing basically the worst case with regularity. merlin -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
