Re: Parallel Select query performance and shared buffers

Here are some extra information:

- When we increased NUM_BUFFER_PARTITIONS to 1024, this problem is
disappeared for 8 core machines and come back with 16 core machines on
Amazon EC2. Would it be related with PostgreSQL locking mechanism?

- I tried this test with 4 core machines including my personel computer and
some other instances on Amazon EC2, I didn't see this problem with 4 core
machines. I started to see this problem in PostgreSQL when core count is 8
or more.

- Here are the results of "vmstat 1" while running 8 parallel select
count(*). Normally I would expect zero idle time.

procs -----------memory---------- ---swap-- -----io---- --system--
-----cpu-----
r b swpd free buff cache si so bi bo in cs us sy id
wa st
0 0 0 29838640 94000 38954740 0 0 0 0 22 21 0 0
100 0 0
7 2 0 29788416 94000 38954740 0 0 0 0 53922 108490 14
24 60 1 1
5 0 0 29747248 94000 38954740 0 0 0 0 68008 164571 22
48 27 2 1
8 0 0 29725796 94000 38954740 0 0 0 0 43587 150574 28
54 16 1 1
0 0 0 29838328 94000 38954740 0 0 0 0 15584 100459 26
55 18 1 0
0 0 0 29838328 94000 38954740 0 0 0 0 42 15 0 0
100 0 0

- When I run 8 parallel wc command or other scripts, they scale out as
expected and they utilize all cpu. This leads me to think that problem is
related with PostgreSQL instead of OS.