Where are we on this patch? ---------------------------------------------------------------------------
Simon Riggs wrote: > On Tue, 2006-03-21 at 17:47 -0500, Tom Lane wrote: > > > I'm fairly unconvinced about Simon's underlying premise --- that we > > can't make good use of work_mem in sorting after the run building phase > > --- anyway. > > We can make good use of memory, but there does come a point in final > merging where too much is of no further benefit. That point seems to be > at about 256 blocks per tape; patch enclosed for testing. (256 blocks > per tape roughly doubles performance over 32 blocks at that stage). > > That is never the case during run building - more is always better. > > > If we cut back our memory usage > Simon inserts the words: "too far" > > then we'll be forcing a > > significantly more-random access pattern to the temp file(s) during > > merging, because we won't be able to pre-read as much at a time. > > Yes, thats right. > > If we have 512MB of memory that gives us enough for 2000 tapes, yet the > initial runs might only build a few runs. There's just no way that all > 512MB of memory is needed to optimise the performance of reading in a > few tapes at time of final merge. > > I'm suggesting we always keep 2MB per active tape, or the full > allocation, whichever is lower. In the above example that could release > over 500MB of memory, which more importantly can be reused by subsequent > sorts if/when they occur. > > > Enclose two patches: > 1. mergebuffers.patch allows measurement of the effects of different > merge buffer sizes, current default=32 > > 2. reassign2.patch which implements the two kinds of resource > deallocation/reassignment proposed. > > Best Regards, Simon Riggs > [ Attachment, skipping... ] [ Attachment, skipping... ] > > ---------------------------(end of broadcast)--------------------------- > TIP 9: In versions below 8.0, the planner will ignore your desire to > choose an index scan if your joining column's datatypes do not > match -- Bruce Momjian http://candle.pha.pa.us EnterpriseDB http://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---------------------------(end of broadcast)--------------------------- TIP 6: explain analyze is your friend
