I'm hoping that some of you can shed some light on this.

I've been trying to peg the "sweet spot" for shared memory using OSDL's 
equipment.   With Jan's new ARC patch, I was expecting that the desired 
amount of shared_buffers to be greatly increased.  This has not turned out to 
be the case.

The first test series was using OSDL's DBT2 (OLTP) test, with 150 
"warehouses".   All tests were run on a 4-way Pentium III 700mhz 3.8GB RAM 
system hooked up to a rather high-end storage device (14 spindles).    Tests 
were on PostgreSQL 8.0b3, Linux 2.6.7.

Here's a top-level summary:

shared_buffers          % RAM   NOTPM20*
1000                            0.2%            1287
23000                   5%              1507
46000                   10%             1481
69000                   15%             1382
92000                   20%             1375
115000                  25%             1380
138000                  30%             1344

* = New Order Transactions Per Minute, last 20 Minutes
     Higher is better.  The maximum possible is 1800.

As you can see, the "sweet spot" appears to be between 5% and 10% of RAM, 
which is if anything *lower* than recommendations for 7.4!   

This result is so surprising that I want people to take a look at it and tell 
me if there's something wrong with the tests or some bottlenecking factor 
that I've not seen.

in order above:

Please note that many of the Graphs in these reports are broken.  For one 
thing, some aren't recorded (flat lines) and the CPU usage graph has 
mislabeled lines.


Josh Berkus
Aglio Database Solutions
San Francisco

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