The solution for us has been twofold: upgrade to the newest PG version available at the time while we waited for our new Opteron-based DB hardware to arrive. Andreas Am Freitag, den 03.11.2006, 13:29 +0000 schrieb Richard Huxton: > Cosimo Streppone wrote: > > Richard Huxton wrote: > > > >> creimer@xxxxxxxxxxxxxx wrote: > >>> > >>> The average context switching for this server as vmstat shows is 1 > >>> but when the problem occurs it goes to 250000. > >> > >> You'll tend to see it when you have multiple clients and most queries > >> can use RAM rather than disk I/O. My understanding of what happens is > >> that PG requests data from RAM - it's not in cache so the process gets > >> suspended to wait. The next process does the same, with the same > >> result. You end up with lots of processes all fighting over what > >> data is in the cache and no-one gets much work done. > > > > Does this happen also with 8.0, or is specific to 8.1 ? > > All versions suffer to a degree - they just push the old Xeon in the > wrong way. However, more recent versions *should* be better than older > versions. I believe some work was put in to prevent contention on > various locks which should reduce context-switching across the board. > > > I seem to have the same exact behaviour for an OLTP-loaded 8.0.1 server > > upgrade from 8.0.1 - the most recent is 8.0.9 iirc > > > when I raise `shared_buffers' from 8192 to 40000. > > I would expect an increase in tps/concurrent clients, but I see an average > > performance below a certain threshold of users, and when concurrent users > > get above that level, performance starts to drop, no matter what I do. > > Are you seeing a jump in context-switching in top? You'll know when you > do - it's a *large* jump. That's the key diagnosis. Otherwise it might > simply be your configuration settings aren't ideal for that workload. > > > Server logs and io/vm statistics seem to indicate that there is little > > or no disk activity but machine loads increases to 7.0/8.0. > > After some minutes, the problem goes away, and performance returns > > to acceptable levels. > > That sounds like it. Query time increases across the board as all the > clients fail to get any data back. > > > When the load increases, *random* database queries show this "slowness", > > even if they are perfectly planned and indexed. > > > > Is there anything we can do? > > Well, the client I saw it with just bought a dual-opteron server and > used their quad-Xeon for something else. However, I do remember that 8.1 > seemed better than 7.4 before they switched. Part of that might just > have been better query-planning and other efficiences though. >
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