the results of pgbench are not stable (in tps)

hi,

i try Excuting pgbench in order to get some
performance results of postgreSQL .

my system:
Person Computer with PIII 600,RAM 256M,Maxtor 10G
7200rpm IDE
OS RedHat 7.1 linux,PostgreSQL 7.1.3

However,even the same command (such as "./pgbench -t
10 -c 10") I Excute every time, the results returned
to me are not stable. The defference is obvious and
great. Following are some examples of my test....


bash-2.04$ ./pgbench testdb -t 16 -c 16
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
number of clients: 16
number of transactions per client: 16
number of transactions actually processed: 256/256
tps = 150.468657(including connections establishing)
tps = 174.254568(excluding connections establishing)

bash-2.04$ ./pgbench testdb -t 16 -c 16
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
number of clients: 16
number of transactions per client: 16
number of transactions actually processed: 256/256
tps = 148.327807(including connections establishing)
tps = 171.441123(excluding connections establishing)

bash-2.04$ ./pgbench testdb -t 16 -c 16
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
number of clients: 16
number of transactions per client: 16
number of transactions actually processed: 256/256
tps = 92.120429(including connections establishing)
tps = 100.808873(excluding connections establishing)

bash-2.04$ ./pgbench testdb -t 16 -c 16
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
number of clients: 16
number of transactions per client: 16
number of transactions actually processed: 256/256
tps = 151.239901(including connections establishing)
tps = 177.997803(excluding connections establishing)

bash-2.04$ ./pgbench testdb -t 16 -c 16
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
number of clients: 16
number of transactions per client: 16
number of transactions actually processed: 256/256
tps = 114.787139(including connections establishing)
tps = 128.095623(excluding connections establishing)

bash-2.04$ ./pgbench testdb -t 16 -c 16
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
number of clients: 16
number of transactions per client: 16
number of transactions actually processed: 256/256
tps = 130.486818(including connections establishing)
tps = 149.041969(excluding connections establishing)

bash-2.04$ ./pgbench testdb -t 16 -c 16
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
number of clients: 16
number of transactions per client: 16
number of transactions actually processed: 256/256
tps = 136.926660(including connections establishing)
tps = 156.382960(excluding connections establishing)

bash-2.04$ ./pgbench testdb -t 16 -c 16
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
number of clients: 16
number of transactions per client: 16
number of transactions actually processed: 256/256
tps = 109.489675(including connections establishing)
tps = 121.939660(excluding connections establishing)

......


Can you tell me some reason why the results of Pgbench
on my system are very diverse?
what should i do?
Give me some advice please
(tuning system parameter?tuning postgresql ?)

i started my pgSQL by " postmaster -i -B 128 -S & "

Thanks.

Best regards
Chao