how to clean the memory ?

Program: parsing multiple urls, at first time it`s very 
fast, but when i use 'top' utility in lunux i see that 
memory size value increasing. It`s working for about 15 
minutes, then slowly and alowly...and when program uses 
for about 90mb it stops...
i use postback, not more than 30 childs at one time

why so? how to clean the memory ?

thanks
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