about multiprocessingmassdata

Hi list!

i have a table which has 8500000 rows records. i write a java program to update these records.
i use 100 threads to update the records. For example, thread-1 update 1~85000 records; thread-2 update 85001~170000 and so on.
The update sql's aim is remove the space in the column and it is simple: update poi set py=replace(py,' ','') where id=?;

By the program log, i find the database of processing data speed so slow, per thread updating 1000 rows need take 260s.

BTW: The PG Server is running on a PC. The PC's total memory is 2G and CPU is "Intel(R) Core(TM)2 Duo E7500 2.93GHz".
When the program's running, CPU just be used 1% and Memory left 112MB.

Is the PC configuration too low cause the problem ?

Please help ~~

Thanks for any tips,

superman0920