Commit a5f06e6

@@ -173,3 +173,78 @@ cylinders_queue = [98, 183, 37, 122, 14, 124, 65, 67]
 C-SCAN will actually go back to 0 once it reaches the end.
 C-LOOK will check to see what the next smallest cylinder is when it reaches the
 max cylinder in the queue.
+
+### Selecting a Disk-Scheduling Algorithm
+
+How the fuck do we choose the best one?
+
+SSTF is common and has natural appeal because it increases performance over FCFS.
+SCAN and C-SCAN perform better for systems that place a heavy load on the disk,
+because they are less likely to cause a starvation problem.
+
+For any particular list of requests, we can define an optimal order of retrieval,
+but the computation needed to find an optimal schedule may not justify the savings
+over SSTF or SCAN.
+
+With any scheduling algorithm performance depends heavily on the number and types of
+requests.
+
+E.g.
+
+If the queue usually has just one outstanding request. Then all scheduling algorithms
+behave the same, because they have only one choice of where to move the disk head;
+they all behave like FCFS scheduling.
+
+Requests for device service can be greatly influenced by the file-allocation method.
+A program reading a contiguously allocated file will generate several requests that
+are close together on disk, resulting in limited head movement.
+
+A linked or indexed file may include blocks that are widely scattered on the disk,
+resulting in greater head movement.
+
+The location of directories and index blocks is also important.
+Since every file must be opened to be used, and opening a file requires searching
+the directory structure, the directories will be accessed frequently.
+
+Suppose that a directory entry is on the first cylinder and a file's data are
+on the final cylinder. In this case, the disk head has to move the entire width
+of the disk.
+
+If the directory entry were on the middle cylinder, the head would have to move
+only one-half the width. Caching the directories and index blocks in main
+memory can also help to reduce disk-arm movement, particularly for read
+requests.
+
+Because of these complexities, the disk-scheduling algorithm should be written
+as a separate module of the operating system, so that it can be replaced
+with a different algorithm if necessary. Either SSTF or LOOK is a reasonable
+choice for the default algorithms.
+
+The scheduling algorithms described here consider only the seek distances.
+
+For modern disks, the rotational latency can be nearly as large as the
+average seek time. It is difficult for the operating system to schedule for
+improved rotational latency because modern disks do not disclose the
+physical location of logical blocks. Disk manufacturers have been alleviating
+this problem by implementing disk-scheduling algorithms in the controller
+hardware built into the disk drive.
+
+If the operating system sends a batch of requests to the controller,
+the controller can queue them and then schedule them to improve both
+the seek time and the rotational latency.
+
+If I/O performance were the only consideration, the operating system would
+gladly turn over the responsibility of disk scheduling to the disk hardware.
+
+In practice the OS may have other constraints on the service order for requests.
+
+Demand paging may take priority over application I/O and writes are more urgent
+than reads if the cache is running out of free pages.
+It may be desirable to guarantee the order of a set of disk writes to make the
+file system robust int he face of a system crash.
+What could happen if the operating system allocated a disk page to a file and
+the application wrote data into that page before the operating system had
+a chance to flush the file system metadata block to disk.
+To accommodate such requirements an operating system may choose to do its own
+disk scheduling and to spoon-feed the requests to the disk controller, one by
+one, for some type of I/O.

@@ -208,6 +208,7 @@ Your answer for each question should be about 150 words. (100 marks total)
 
 1. What are the factors influencing the selection of a disk-scheduling algorithm? (8 marks)
 1. Explain the disadvantage(s) of the SSTF scheduling algorithm. (8 marks)
+  Starvation
 1. Explain the concepts of a bus and a daisy chain. Indicate how these concepts are related. (8 marks)
 1. What are the three reasons that buffering is performed? (6 marks)