A multithreaded Web server is organized as shown in Fig. 7-21. It takes 500 µsec to accept a request and check the cache. Half the time the file is found in the cache and returned immediately. The other half of the time the module has to block for 9 msec while its disk request is queued and processed. How many modules should the server have to keep the CPU busy all the time (assuming the disk is not a bottleneck)?