asp.net server side button click and ajax request call simultaneously

I have an asp.net ImageButton control on my page.

"Onclick" (server side event) of that

I am calling a method that will export data to excel

and "OnClientClick" of that

I am (javascript/jquery) blocking UI and calling an ajax request in intervals.

This ajax request will check whether exporting process is completed or not by calling a webmethod (This webmethod in turn is reading session variable).

Actually I wanted to display percentage 开发者_JAVA百科execution completed of that process. But I observed in firebug that unless Server Side event is processed all the ajax requests are not served.

How can I achieve displaying percentage completion or any suggestions?