Suppose thread A goes through a loop 100 times, each time performing (i) one disk I/O operation, taking 10 milliseconds, and then (ii) some computation, taking 1 millisecond. While each 10-millisecond disk operation is in progress, thread A cannot make any use of the processor. Thread B runs for 1 second, purely in the processor, with no I/O. One millisecond of processor time is spent each time the processor switches threads; other than this switching cost, there is no problem with the processor working on thread B during one of thread A’s I/O operations. (The processor and disk drive do not contend for memory access bandwidth, for example.)

(a) Suppose the processor and disk work purely on thread A until its completion, and then the processor switches to thread B and runs all of that thread. What will the total elapsed time be?

(b) Suppose the processor starts out working on thread A, but every time thread A performs a disk operation, the processor switches to B during the operation and then back to A upon the disk operation’s completion. What will the total elapsed time be?