run a function with a limited amount of concurrency.

queue.js

var sys   = require('sys'),
exec  = require('child_process').exec;
//  fs    = require('fs');
 //   ecsv  = require('ecsv')

function Queue () {
	var waiting = [];
	this.max = -1;
	this.current = 0;
//run checks if we're under the max processors, and runs if there is room.
	this.run = function () {
		if (this.max < 0 || this.current < this.max) {
   		this.current = this.current + 1;
   		p = waiting.pop()
   		if ('function' === typeof p){
   			p();
   		}
		}
	}
//start starts a process or puts it in the queue
	this.start = function (f) {
	   sys.print(this.current);
		waiting.push(f);
		this.run();
	}
//the callback has to call finish so that the Queue knows it can run something else.

	this.finish = function() {
	   sys.print('!');
	   this.current = this.current - 1;
	   this.run();
	}
}

//ecsv.each('../pub/data/crime_incident_data.csv', function(crime) {
q = new Queue ();
q.max = 250;
for (i = 0 ; i < 1000; i ++) {
	q.start (function (){
	  exec('wc -c "foo"', function(err, stdout, stderr) {
   	 console.log('woot'); 
   	 q.finish();
  	});
  	});
}