Operating Systems

Start Lecture #28

5.4.3 Disk Arm Scheduling Algorithms

There are three components to disk response time: seek, rotational latency, and transfer time. Disk arm scheduling is concerned with minimizing seek time by reordering the requests.

These algorithms are relevant only if there are several I/O requests pending. For many PCs, the system is so underutilized that there are rarely multiple outstanding I/O requests and hence no scheduling is possible. At the other extreme, many large servers, are I/O bound with significant queues of pending I/O requests. For these systems, effective disk arm scheduling is crucial.

Although disk scheduling algorithms are performed by the OS, they are also sometimes implemented in the electronics on the disk itself. The disks I brought to class were somewhat old so I suspect those didn't implement scheduling, but the then-current operating systems definitely did.

We will study the following algorithms all of which are quite simple.

1. FCFS (First Come First Served).
   The most primitive. One could called this no scheduling, but I wouldn't.
2. Pick.
   Same as FCFS but pick up requests for cylinders that are passed on the way to the next FCFS request.
3. SSTF or SSF (Shortest Seek (Time) First).
   Use the greedy algorithm and go to the closest requested cylinder. This algorithm can starve requests. To prevent starvation, one can periodically enter a FCFS mode, but SSTF would still be unfair. Typically, cylinders in the middle receive better service than do cylinders on both extremes.
4. Scan (Look, Elevator).
   This is the method used by an old fashioned jukebox (remember Happy Days) and by elevators.
   Those jukeboxes stole coins since requesting an already requested song was a nop.
   The disk arm proceeds in one direction picking up all requests until there are no more requests in this direction at which point it goes back the other direction. This favors requests in the middle, but can't starve any requests.
5. N-step Scan.
   This is what the natural implementation of Scan actually does. The idea is that requests are serviced in batches. Specifically, it works as follows.
   • While the disk is servicing a Scan direction, the controller gathers up new requests and sorts them.
   • At the end of the current sweep, the new list becomes the next sweep.
   • Compare this to selfish round robin (SRR) with b≥a=0.
6. C-Scan (C-look, Circular Scan/Look).
   Similar to Scan but only service requests when moving in one direction. Let's assume it services requests when the head is moving from low-numbered cylinders to high-numbered one. When there are no pending requests for a cylinder with number higher than the present head location, the head is sent to the lowest-numbered, requested cylinder. C-Scan doesn't favor any spot on the disk. Indeed, it treats the cylinders as though they were a clock, i.e., after the highest numbered cylinder comes cylinder 0.

Minimizing Rotational Latency

Once the heads are on the correct cylinder, there may be several requests to service. All the systems I know, use Scan based on sector numbers to retrieve these requests. Note that in this case Scan is the same as C-Scan. Why?
Ans: Because the disk rotates in only one direction.

Homework: 24.

Homework: A salesman claimed that their version of Unix was very fast. For example, their disk driver used the elevator algorithm to reorder requests for different cylinders. In addition, the driver queued multiple requests for the same cylinder in sector order. Some hacker bought a version of the OS and tested it with a program that read 10,000 blocks randomly chosen across the disk. The new Unix was not faster that an old one that did FCFS for all requests. What happened?

Track Caching

Often the disk/controller caches (a significant portion of) the entire track whenever it access a block, since the seek and rotational latency penalties have already been paid. In fact modern disks have multi-megabyte caches that hold many recently read blocks. Since modern disks cheat and don't have the same number of blocks on each track, it is better for the disk electronics (and not the OS or controller) to do the caching since the disk is the only part of the system to know the true geometry.

5.4.4 Error Handling

Most disk errors are handled by the device/controller and not the OS itself. That is, disks are manufactured with more sectors than are advertised and spares are used when a bad sector is referenced. Older disks did not do this and the operating system would form a secret file of bad blocks that were never used.

5.5 Clocks (Timers)

5.5.1 Clock Hardware

The hardware is simple. It consists of

• An oscillator that generates a pulse at a know fixed frequency.
• A counter that is decremented at each pulse.
• A register that can be used to reload the counter.
• Electronics that generate an interrupt whenever the counter reaches zero.

The counter reload can be automatic or under OS control. If it is done automatically, the interrupt occurs periodically (the frequency is the oscillator frequency divided by the value in the register).

The value in the register can be set by the operating system and thus this programmable clock can be configured to generate periodic interrupts and any desired frequency (providing that frequency divides the oscillator frequency).

5.5.2 Clock Software

As we have just seen, the clock hardware simply generates a periodic interrupt, called the clock interrupt, at a set frequency. Using this interrupt, the OS software can accomplish a number of important tasks.

1. Time of day (TOD).
   The basic idea is to increment a counter each clock tick (i.e., each interrupt). The simplest solution is to initialize this counter at boot time to the number of ticks since a fixed date (Unix traditionally uses midnight, 1 January 1970). Thus the counter always contains the number of ticks since that date and hence the current date and time is easily calculated. Two problems.

   1. From what value is the counter initialized?
   2. What about overflow?

   Three methods are used for initialization. The system can contact one or more know time sources (see the Wikipedia entry for NTP), the human operator can type in the date and time, or the system can have a battery-powered, backup clock. The last two methods only give an approximate time.

   Overflow is a real problem if a 32-bit counter is used. In this case two counters are kept, the low-order and the high-order. Only the low order is incremented each tick; the high order is incremented whenever the low order overflows. That is, a counter with more bits is simulated.

2. Time quantum for Round Robbin scheduling. The system decrements a counter at each tick. The quantum expires when the counter reaches zero. The counter is loaded when the scheduler runs a process (i.e., changes the state of the process from ready to running). This is what I (and I would guess you) did for the (processor) scheduling lab.

3. Accounting.
   At each tick, bump a counter in the process table entry for the currently running process.

4. 

   Alarm system call and system alarms.
   Users can request a signal at some future time (the Unix alarm system call). The system also on occasion needs to schedule some of its own activities to occur at specific times in the future (e.g., exercise a network time out).

   The conceptually simplest solution is to have one timer for each event. Instead, we simulate many timers with just one using the data structure on the right with one node for each event.

   • The first entry in each node is the time after the preceding event that this event's alarm is to ring.
   • For example, if the time is zero, this event occurs at the same time as the previous event.
   • The second entry in the node is a pointer to the action to perform.
   • At each tick, the system decrements next-signal.
   • When next-signal goes to zero, we process the first entry on the list and any others immediately following with a time of zero (which means they are to be simultaneous with this alarm). We then set next-signal to the value in the next alarm.

5. Profiling.
   The objective is to obtain a histogram giving how much time was spent in each software module of a given user program.

   The program is logically divided into blocks of say 1KB and a counter is associated with each block. At each tick the profiled code checks the program counter and bumps the appropriate counter.

   After the program is run, a (user-mode) utility program can determine the software module associated with each 1K block and present the fraction of execution time spent in each module.

   If we use a finer granularity (say 10B instead of 1KB), we get increased accuracy but more memory overhead.

Homework: 28.

5.6 User Interfaces: Keyboard, Mouse, Monitor

5.6.2 Input Software

Keyboard Software

At each key press and key release a scan code is written into the keyboard controller and the computer is interrupted. By remembering which keys have been depressed and not released the software can determine Cntl-A, Shift-B, etc.

There are two fundamental modes of input, traditionally called raw and cooked in Unix and now sometimes call noncanonical and canonical in POSIX. In raw mode the application sees every character the user types. Indeed, raw mode is character oriented. All the OS does is convert the keyboard scan codes to characters and and pass these characters to the application. For example

1. down-cntl down-x up-x up-cntl is converted to cntl-x
2. down-cntl up-cntl down-x up-x is converted to x
3. down-cntl down-x up-cntl up-x is converted to cntl-x (I just tried it to be sure).
4. down-x down-cntl up-x up-cntl is converted to x

Cooked mode is line oriented. The OS delivers lines to the application program after cooking them as follows.

• Special characters are interpreted as editing characters (erase-previous-character, erase-previous-word, kill-line, etc).
• Erased characters are not seen by the application but are erased by the keyboard driver.
• Also needed is an escape character so that the editing characters can be passed to the application if desired.
• The cooked characters must be echoed (what should one do if the application is also generating output at this time?)

The (possibly cooked) characters must be buffered until the application issues a read (and an end-of-line EOL has been received for cooked mode).

Mouse Software

Whenever the mouse is moved or a button is pressed, it sends a message to the computer consisting of Δx, Δy, and the status of the buttons. That is all the hardware does. Issues such as double click vs. two clicks are all handled by the software.

5.6.3 Output Software

Text Windows

In the beginning these were essentially typewriters (called glass ttys) and therefore simply received a stream of characters. Soon after, they accepted commands (called escape sequences) that would position the cursor, insert and delete characters, etc.

5.10 Summary

Read.

The End: Good luck on the final