>       I need to check the scalability of a machine with postgresql and Im doing it 
> with pgbench but Im getting values with a variation of a 40% with the same 
> pgbench call...

You might be looking at the effect of the kernel buffer cache. Try run
pgbench several times with same settings. Another point is how many
transactions pgbench runs (-t option). More transactions would give
more statble results. Here is my small script to run pgbench.  I
usually run it 2 or 3 times and take only the last run result.

#! /bin/sh
pgbench -i -s 2 test
for i in 1 2 4 8 16 32 64 128
do
        t=`expr 640 / $i`
        pgbench -t $t -c $i test
        echo "===== sync ======"
        sync;sync;sync;sleep 10
        echo "===== sync done ======"
done

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