Strange performance response for high load times

Hi All,

We are having a reasonably powerful machine for supporting about 20
databases but in total they're not more then 4GB in size.

The machine is 2 processor 8 core and 8 Gig or ram so I would expect that PG
should cache the whole db into memory. Well actually it doesn't.

What is more strange that a query that under zero load is running under
100ms during high load times it can take up to 15 seconds !!
What on earth can make such difference ?

here are the key config options that I set up :
# - Memory -

shared_buffers = 170000 # min 16 or
max_connections*2, 8KB each
temp_buffers = 21000 # min 100, 8KB each
#max_prepared_transactions = 5 # can be 0 or more
# note: increasing max_prepared_transactions costs ~600 bytes of shared
memory
# per transaction slot, plus lock space (see max_locks_per_transaction).
work_mem = 1048576 # min 64, size in KB
maintenance_work_mem = 1048576 # min 1024, size in KB
#max_stack_depth = 2048 # min 100, size in KB

# - Free Space Map -

max_fsm_pages = 524298 # min max_fsm_relations*16, 6 bytes
each
max_fsm_relations = 32768 # min 100, ~70 bytes each

# - Kernel Resource Usage -

max_files_per_process = 4000 # min 25
#preload_libraries = ''

any ideas ?

cheers,
Peter