In class we have studied the linkage between request service time and the concept of load average. Let's explore that issue in more detail.
1 2 3 4 5 6 7 8 9 10 P1 +---+---+CCC+CCC+---+---+---+---+---+---+ P2 +---+---+---+RRR+CCC+CCC+CCC+---+---+---+so the total load average is 0.6.
+---+---+---+---+---+---+CCC+---+---+---+ +CCC+CCC+CCC+---+---+---+RRR+CCC+---+---+ +RRR+RRR+RRR+CCC+CCC+---+RRR+RRR+CCC+CCC+will suffice, where C=Computing, R=Ready, -=Waiting. Note that Ready must be followed by Computing; there is no transition from Ready to Waiting without Computing. Note also that only one process can be "Computing" at a time.
+---+---+---+---+---+---+CCC+---+---+---+ +CCC+CCC+CCC+---+---+---+CCC+---+---+---+ +CCC+CCC+---+---+---+---+RRR+CCC+CCC+---+The new load average = 10/10 = 1.0 Note that it is not precisely 1/2 of the original load average because of the small sample.
+---+???+---+???+---+???+---+???+---+???+---+???+RRR+???+RRR+???+RRR+???+RRR+ +CCC+???+CCC+???+CCC+???+CCC+???+CCC+???+CCC+???+RRR+???+RRR+???+RRR+???+RRR+ +RRR+???+RRR+???+RRR+???+RRR+???+RRR+???+RRR+???+CCC+???+CCC+???+CCC+???+CCC+In this case, the first phase gets stretched out and doesn't complete, with the result that the end of the original computation doesn't fit into 10 cycles. The load average (from this instance's point of view only) = 24/10=2.4. Note that it is not precisely twice because of the small sample.
For a specific interval of time T, split total time T = toverhead + tservice, where toverhead is the amount of time spent serving the operating system, while tservice is the total time spent serving clients. Then tservice/T is the proportion of time spent working on providing service, while toverhead/T is the proportion of time spent on operating system issues.
Then note that this is exactly the same situation as having multiple virtual servers, counting the operating system itself as a "virtual service". The equation W = L/λ only applies to the amount of time spent on our service. During overhead time, all instances of our service have to wait, whether they were computing, ready, or idle. Overhead is "blind" to what we happen to be doing, so it increases all times equally.
Thus the wait time W must be adjusted for overhead, so that on average, W = L/λ/(toverhead/T).