setInterval定时和ajax请求

fnSetMarkPoint = function (param) {                
$.ajax({  
    success: function (returnValue) {
        window.setInterval("fnSetMarkPoint()", 5000); 
    }
});

var isRequst = true;

fnSetMarkPoint = function (param) {                
$.ajax({  
    success: function (returnValue) {                          

        if (isRequst) {
            isRequst = false;
            window.setInterval("fnSetMarkPoint()", 5000);
        }
    }
});