Async usage of multiple PhantomJS instances with NodeJS

run_multiple_phantomjs_instances.js

var phantom = require('phantom');
var async = require('async');

var pagesToCall = [
    ['http://www.google.com', 8000],
    ['http://www.allthingsd.com', 8001],
    ['http://www.wired.com', 8002],
    ['http://www.mashable.com', 8003],
    ['http://www.stackoverflow.com', 8004]
];

function callPage(pageToCall) {

    console.log(new Date().getTime() + ': Started page ' + pageToCall[0]);
    console.log(new Date().getTime() + ': port:' + pageToCall[1]);

    phantom.create({port: pageToCall[1]}, function(ph) {

        ph.createPage(function(page) {

            page.open(pageToCall[0], function(status) {

                console.log(new Date().getTime() + ': Opened page? %s', status);

                page.set('viewportSize', {
                    width: 1280,
                    height: 800
                });
                page.set('settings.userAgent', 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_5) AppleWebKit/537.1 (KHTML, like Gecko) Chrome/21.0.1180.89 Safari/537.1');

                var filename = pageToCall[0].replace('http:\/\/', '') + '.png';
                page.render('./screenshots/' + filename, function() {
                    
                    console.log(new Date().getTime() + ': Wrote page ' + pageToCall[0]);
                    page.close();
                    ph.exit();

                });


            });

        });

    });

}

//Main
async.each(pagesToCall, callPage, function (e) {
    if (e) console.log(e);
});

Multiple PhantomJSw with node, Every phantomjs will send http request . Memory and bandwidth is a bottleneck, how do you optimize. What is the biggest numbers of child phantomjs process.