On Sat, 2009-03-14 at 12:09 -0400, Tom Lane wrote: > Heikki Linnakangas <heikki.linnakangas@xxxxxxxxxxxxxxxx> writes: > > WALInsertLock is also quite high on Jignesh's list. That I've seen > > become the bottleneck on other tests too. > > Yeah, that's been seen to be an issue before. I had the germ of an idea > about how to fix that: > > ... with no lock, determine size of WAL record ... > obtain WALInsertLock > identify WAL start address of my record, advance insert pointer > past record end > *release* WALInsertLock > without lock, copy record into the space just reserved > > The idea here is to allow parallelization of the copying of data into > the buffers. The hold time on WALInsertLock would be very short. Maybe > it could even become a spinlock, though I'm not sure, because the > "advance insert pointer" bit is more complicated than it looks (you have > to allow for the extra overhead when crossing a WAL page boundary). > > Now the fly in the ointment is that there would need to be some way to > ensure that we didn't write data out to disk until it was valid; in > particular how do we implement a request to flush WAL up to a particular > LSN value, when maybe some of the records before that haven't been fully > transferred into the buffers yet? The best idea I've thought of so far > is shared/exclusive locks on the individual WAL buffer pages, with the > rather unusual behavior that writers of the page would take shared lock > and only the reader (he who has to dump to disk) would take exclusive > lock. But maybe there's a better way. Currently I don't believe that > dumping a WAL buffer (WALWriteLock) blocks insertion of new WAL data, > and it would be nice to preserve that property. Yeh, that's just what we'd discussed previously: http://markmail.org/message/gectqy3yzvjs2hru#query:Reworking%20WAL% 20locking+page:1+mid:gectqy3yzvjs2hru+state:results Are you thinking of doing this for 8.4? :-) -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support - Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
