Homework: Give an argument favoring a large quantum; give an argument favoring a small quantum.

Process	CPU Time	Creation Time
P1	20	0
P2	3	3
P3	2	5

Homework: (Remind me to discuss this last one in class next time): Consider the set of processes in the table below. When does each process finish if RR scheduling is used with q=1, if q=2, if q=3, if q=100. First assume (unrealistically) that context switch time is zero. Then assume it is .1. Each process performs no I/O (i.e., no process ever blocks). All times are in milliseconds. The CPU time is the total time required for the process (excluding any context switch time). The creation time is the time when the process is created. So P1 is created when the problem begins and P3 is created 5 milliseconds later. If two processes have equal priority (in RR this means if they both enter the ready state at the same cycle), we give priority (in RR this means place first on the queue) to the process with the earliest creation time. If they also have the same creation time, then we give priority to the process with the lower number.

Processor Sharing (PS, **, PS, PS)

Merge the ready and running states and permit all ready jobs to be run at once. However, the processor slows down so that when n jobs are running at once, each progresses at a speed 1/n as fast as it would if it were running alone.

• Clearly impossible as stated due to the overhead of process switching.
• Of theoretical interest (easy to analyze).
• Approximated by RR when the quantum is small. Make sure you understand this last point. For example, consider the last homework assignment (with zero context switch time) and consider q=1, q=.1, q=.01, etc.

Homework: 34.

Variants of Round Robin

• State dependent RR
  • Same as RR but q is varied dynamically depending on the state of the system.
  • Favor processes holding important resources.
    • For example, non-swappable memory.
    • Perhaps this should be considered medium term scheduling since you probably do not recalculate q each time.
• External priorities: RR but a user can pay more and get bigger q. That is one process can be given a higher priority than another. But this is not an absolute priority: the lower priority (i.e., less important) process does get to run, but not as much as the higher priority process.

Priority Scheduling

Each job is assigned a priority (externally, perhaps by charging more for higher priority) and the highest priority ready job is run.

• Similar to ``External priorities'' above
• If many processes have the highest priority, use RR among them.
• Can easily starve processes (see aging below for fix).
• Can have the priorities changed dynamically to favor processes holding important resources (similar to state dependent RR).
• Many policies can be thought of as priority scheduling in which we run the job with the highest priority (with different notions of priority for different policies).

Priority aging

As a job is waiting, raise its priority so eventually it will have the maximum priority.

• This prevents starvation (assuming all jobs terminate or the policy is preemptive).
• There may be many processes with the maximum priority.
• If so, can use FIFO among those with max priority (risks starvation if a job doesn't terminate) or can use RR.
• Can apply priority aging to many policies, in particular to priority scheduling described above.

Selfish RR (SRR, **, SRR, **)

• Preemptive.
• Perhaps it should be called ``snobbish RR''.
• ``Accepted processes'' run RR.
• Accepted process have their priority increase at rate b>=0.
• A new process starts at priority 0; its priority increases at rate a>=0.
• A new process becomes an accepted process when its priority reaches that of an accepted process (or when there are no accepted processes).
• Once a process is accepted it remains accepted until it terminates.
• Note that at any time all accepted processes have same priority.
• If b>=a, get FCFS.
• If b=0, get RR.
• If a>b>0, it is interesting.

Shortest Job First (SPN, SJF, SJF, SJF)

Sort jobs by total execution time needed and run the shortest first.

• Nonpreemptive
• First consider a static situation where all jobs are available in the beginning, we know how long each one takes to run, and we implement ``run-to-completion'', also called 11uniprogrammed'' (i.e., we don't even switch to another process on I/O). In this situation, SJF has the shortest average waiting time
  • Assume you have a schedule with a long job right before a short job.
  • Consider swapping the two jobs.
  • This decreases the wait for the short by the length of the long job and increases the wait of the long job by the length of the short job.
  • This decreases the total waiting time for these two.
  • Hence decreases the total waiting for all jobs and hence decreases the average waiting time as well.
  • Hence, whenever a long job is right before a short job, we can swap them and decrease the average waiting time.
  • Thus the lowest average waiting time occurs when there are no short jobs right before long jobs.
  • This is SJF.
• In the more realistic case where the scheduler switches to a new process when the currently running process blocks (say for I/O), we should call the policy shortest next-CPU-burst first.
• The difficulty is predicting the future (i.e., knowing in advance the time required for the job or next-CPU-burst).
• This is an example of priority scheduling.

Homework: 39, 40.

Preemptive Shortest Job First (PSPN, SRT, PSJF/SRTF, --)

Preemptive version of above

• Permit a process that enters the ready list to preempt the running process if the time for the new process (or for its next burst) is less than the remaining time for the running process (or for its current burst).
• It will never happen that a process in the ready list will require less time than the remaining time for the currently running process. Why?
  Ans: When the process joined the ready list it would have started running if the current process had more time remaining. Since that didn't happen the current job had less time remaining and now it has even less.
• Can starve processs that require a long burst.
  • This is fixed by the standard technique.
  • What is that technique?
    Ans: Priority aging.
• Another example of priority scheduling.

Highest Penalty Ratio Next (HPRN, HRN, **, **)