Operating Systems

1. Lab 2 extended on week. Due 1 april.
2. Office hour thursday (25 mar) moved to 3:30-4:30.
3. I did not have internet access while away and just came in via red eye so have not read email.

**4.1.2: Multiprogramming with fixed partitions

Two goals of multiprogramming are to improve CPU utilization, by overlapping CPU and I/O, and to permit short jobs to finish quickly.

• This scheme was used by IBM for system 360 OS/MFT (multiprogramming with a fixed number of tasks).
• Can have a single input queue instead of one for each partition.
  • So that if there are no big jobs can use big partition for little jobs.
  • But I don't think IBM did this.
  • Can think of the input queue(s) as the ready list(s) with a scheduling policy of FCFS in each partition.
• The partition boundaries are not movable (must reboot to move a job).
  • MFT can have large internal fragmentation, i.e., wasted space inside a region
• Each process has a single ``segment'' (we will discuss segments later)
  • No sharing between process.
  • No dynamic address translation.
  • At load time must ``establish addressability''.
    • i.e. must set a base register to the location at which the process was loaded (the bottom of the partition).
    • The base register is part of the programmer visible register set.
    • This is an example of address translation during load time.
    • Also called relocation.
• Storage keys are adequate for protection (IBM method).
• Alternative protection method is base/limit registers.
• An advantage of base/limit is that it is easier to move a job.
• But MFT didn't move jobs so this disadvantage of storage keys is moot.
• Tanenbaum says a job was ``run until it terminates. This must be wrong as that would mean monoprogramming.
• He probably means that jobs not swapped out and each queue is FCFS without preemption.

4.1.3: Modeling Multiprogramming

• Consider a job that is unable to compute (i.e., it is waiting for I/O) a fraction p of the time.
• Then, with monoprogramming, the CPU utilization is 1-p.
• Note that p is often > .5 so CPU utilization is poor.
• But, if the probability that a job is waiting for I/O is p and n jobs are in memory, then the probability that all n are waiting for I/O is approximately p^n.
• So, with a multiprogramming level (MPL) of n, the CPU utilization is approximately 1-p^n.
• If p=.5 and n=4, then 1-p^n = 15/16, which is much better than 1/2, which would occur for monoprogramming (n=1).
• This is a crude model, but it is correct that increasing MPL does increase CPU utilization up to a point.
• The limitation is memory, which is why we discuss it here instead of process management. That is, we must have many jobs loaded at once, which means we must have enough memory for them. There are other issues as well and we will discuss them.
• Some of the CPU utilization is time spent in the OS executing context switches so the gains are not a great as the crude model predicts.

Homework: 1, 2 (typo in book; figure 4.21 seems irrelevant).

4.1.4: Analysis of Multiprogramming System Performance

Skipped

4.1.5: Relocation and Protection

Relocation was discussed as part of linker lab and at the beginning of this chapter. When done dynamically, a simple method is to have a base register whose value is added to every address by the hardware.

Similarly a limit register is checked by the hardware to be sure that the address (before the base register is added) is not bigger than the size of the program.

The base and limit register are set by the OS when the job starts.

4.2: Swapping

Moving the entire processes between disk and memory is called swapping.

Multiprogramming with Variable Partitions

• IBM OS/MVT (multiprogramming with a varying number of tasks).
• Also early PDP-10 OS.
• Job still has only one segment (as with MFT). That is, the virtual address is contiguous.
• The physical address is also contiguous.
• The job can be of any size up to the size of the machine and the job size can change with time.
• A single ready list.
• A job can move (might be swapped back in a different place).
• This is dynamic address translation (during run time).
• Must perform an addition on every memory reference (i.e. on every address translation) to add the start address of the partition.
• Called a DAT (dynamic address translation) box by IBM.
• Eliminates internal fragmentation.
  • Find a region the exact right size (leave a hole for the remainder).
  • Not quite true, can't get a piece with 10A755 bytes. Would get say 10A760. But internal fragmentation is much reduced compared to MFT. Indeed, we say that internal fragmentation has been eliminated.
• Introduces external fragmentation, i.e., holes outside any region.
• What do you do if no hole is big enough for the request?
  • Can compactify
    • Transition from bar 3 to bar 4 in diagram below.
    • This is expensive.
    • Not suitable for real time (MIT ping pong).
  • Can swap out one process to bring in another, e.g., bars 5-6 and 6-7 in the diagram.
• There are more processes than holes. Why?
  • Because next to a process there might be a process or a hole but next to a hole there must be a process
  • So can have ``runs'' of processes but not of holes
  • If after a process one is equally likely to have a process or a hole, you get about twice as many processes as holes.
• Base and limit registers are used.
  • Storage keys not good since compactifying or moving would require changing many keys.
  • Storage keys might need a fine granularity to permit the boundaries to move by small amounts (to reduce internal fragmentation). Hence many keys would need to be changed.

Homework: 3