User concurrency thresholding: where do I look?

[Josh Berkus <josh(at)agliodbs(dot)com>]

Folks,

I've run into this a number of times with various PostgreSQL users, so we 
tested it at Sun.  What seems to be happening is that at some specific number 
of connections average throughput drops 30% and response time quadruples or 
worse.  The amount seems to vary per machine; I've seen it as variously 95, 
1050, 1700 or 2800 connections.  Tinkering with postgresql.conf parameters 
doesn't seem to affect this threshold.

As an example of this behavior:

Users	Txn/User  Resp. Time
50	105.38	0.01
100	113.05	0.01
150	114.05	0.01
200	113.51	0.01
250	113.38	0.01
300	112.14	0.01
350	112.26	0.01
400	111.43	0.01
450	110.72	0.01
500	110.44	0.01
550	109.36	0.01
600	107.01	0.02
650	105.71	0.02
700	106.95	0.02
750	107.69	0.02
800	106.78	0.02
850	108.59	0.02
900	106.03	0.02
950	106.13	0.02
1000	64.58	0.15
1050	52.32	0.23
1100	49.79	0.25

Tinkering with shared_buffers has had no effect on this threholding (the above 
was with 3gb to 6gb of shared_buffers).   Any ideas on where we should look 
for the source of the bottleneck?

-- 
Josh Berkus
PostgreSQL @ Sun
San Francisco