On Thu, 2004-10-14 at 16:57 -0700, Josh Berkus wrote: > Simon, > > <lots of good stuff clipped> > > > If you draw a graph of speedup (y) against cache size as a > > % of total database size, the graph looks like an upside-down "L" - i.e. > > the graph rises steeply as you give it more memory, then turns sharply at a > > particular point, after which it flattens out. The "turning point" is the > > "sweet spot" we all seek - the optimum amount of cache memory to allocate - > > but this spot depends upon the worklaod and database size, not on available > > RAM on the system under test. > > Hmmm ... how do you explain, then the "camel hump" nature of the real > performance? That is, when we allocated even a few MB more than the > "optimum" ~190MB, overall performance stated to drop quickly. The result is > that allocating 2x optimum RAM is nearly as bad as allocating too little > (e.g. 8MB). > > The only explanation I've heard of this so far is that there is a significant > loss of efficiency with larger caches. Or do you see the loss of 200MB out > of 3500MB would actually affect the Kernel cache that much? > In a past life there seemed to be a sweet spot around the applications working set. Performance went up until you got just a little larger than the cache needed to hold the working set and then went down. Most of the time a nice looking hump. It seems to have to do with the additional pages not increasing your hit ratio but increasing the amount of work to get a hit in cache. This seemed to be independent of the actual database software being used. (I observed this running Oracle, Informix, Sybase and Ingres.)
> Anyway, one test of your theory that I can run immediately is to run the exact > same workload on a bigger, faster server and see if the desired quantity of > shared_buffers is roughly the same. I'm hoping that you're wrong -- not > because I don't find your argument persuasive, but because if you're right it > leaves us without any reasonable ability to recommend shared_buffer settings. > -- Timothy D. Witham - Chief Technology Officer - [EMAIL PROTECTED] Open Source Development Lab Inc - A non-profit corporation 12725 SW Millikan Way - Suite 400 - Beaverton OR, 97005 (503)-626-2455 x11 (office) (503)-702-2871 (cell) (503)-626-2436 (fax) ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to [EMAIL PROTECTED])