start = 0
        keep_going = true

        while keep_going

            # use threads to contain memory consumption
            thread = Thread.new do |t|

                @search_result_page_num = start/10
                puts "google page #{start/10}"
                page = a.get("http://www.google.com/search?safe=off&start=#{start}&sa=N&q=#{search_str}")

                page.links.each do |link|
                    #do some stuff
                end

                keep_going = false if page.body.include?('we have omitted some') || page.body.include?('did not match any documents')

                t.join
                Thread.exit
            end

            # wait for thread
            while thread.alive?
                sleep 1
            end

            start += 10
            sleep 5

        end