An embodiment of the invention provides a method to minimize blocking overhead in upcall based M.times.N threads. The method includes: setting a limit for the number of outstanding unbound user threads that has blocked and has invoked a blocking upcall mechanism on a particular virtual processor at any point in time; after reaching the limit, if the user threads still blocks, then temporarily disabling the upcall mechanism for these user threads; and re-enabling the upcall mechanism for the user threads during the next context switch time. In another embodiment, an apparatus implemented in a computer to minimize blocking overhead in upcall based M.times.N threads, includes: a virtual processor configured to track the number of blocked unbound threads that are blocked; and an M.times.N user space scheduler configured to disable a block upcall handler and an unblock upcall handler for the blocked unbound threads if the number of blocked unbound threads has reached a limit value.