Scheduling Alternating Sequence of CPU And IO Bursts

Scheduling

Alternating Sequence of CPU And I/O Bursts

Simple Categories of Processes n n CPU-bound process is one that has more and larger CPU bursts (spends most of its time computing). An I/O-bound process spends most of its time waiting for I/O.

Histogram of CPU-burst Times

Computing Environments n Batch n E. g. , supercomputing centers, mainframes/workstations for business computing.

Job Owner Job Name Queu e State #N ds % Time Max Time 427 130 kensong A 3 i dqu e ON HOLD 24 0% 24: 00 427 226 kensong A 3 i dqu e ON HOLD 24 0% 24: 00 460 157 jmehring SCEC_Cyber. Shake_PAS_2 dqu e Queued 14 4 0% 24: 00 460 233 stolbov cocuph. Ggga long RUNNING 8 3% 96: 00 460 234 stolbov cocuph. Ggga long ON HOLD 8 0% 96: 00 462 375 stolbov cocuph. Mgga long ON HOLD 8 0% 96: 00 462 376 stolbov cocuph. Mgga long ON HOLD 8 0% 96: 00 464 535 zhaol SCEC_LAB_TOMO dqu e RUNNING 25 6 02: 05: 07 9% 24: 00 470 342 yujiewu q 192 a_1 dqu e RUNNING 64 02: 43: 06 11 % 24: 00 Time Used 02: 42: 53

n n Interactive Systems (e. g. , Gandalf). Real-Time systems.

CPU Scheduler n n Selects from among the processes in memory (on ready queue), and allocates the CPU to one of them. CPU scheduling decisions may take place when a process: 1. 2. 3. 4. n n Switches from running to waiting state. Switches from running to ready state. Switches from waiting to ready. Terminates. Scheduling under 1 and 4 is nonpreemptive. All other scheduling is preemptive.

Dispatcher n Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: n n switching context switching to user mode jumping to the proper location in the user program to restart that program Dispatch latency – time it takes for the dispatcher to stop one process and start another running.

Possible Scheduling Goals n CPU utilization – keep the CPU as busy as possible n Throughput – # of processes that complete their n Turnaround time – amount of time to execute a particular execution per time unit process

Possible Scheduling Goals (continued) n Waiting time – amount of time a process has been n Response time – amount of time it takes from issuing a waiting in the ready queue command getting response (interactive systems, PCs).

Optimization Criteria n n n Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time

Optimization Criteria: Conflicting n Maximize throughput n n Execute all shortest jobs first. Minimize turnaround time n Turnaround time is increased significantly if long jobs are never executed.

Goals of Scheduling Algorithms n All systems: n n n Batch Systems: n n Fairness Balance (keep all parts of the system busy). Maximize throughput. Minimize turnaround time CPU utilization. Interactive systems: n Response time.

n Real-time systems: n Meeting deadlines

First-Come, First-Served (FCFS) Scheduling n n Allocate CPU to processes based on arrival order. Non-preemptive. Process executes until completes or blocks on I/O or other system resource. n Not a good idea for a timesharing system!

First-Come, First-Served (FCFS) Scheduling Process P 1 P 2 P 3 n Burst Time 24 3 3 Suppose that the processes arrive in the order: P 1 , P 2 , P 3

The Gantt Chart for the schedule is: P 1 0 n n P 2 24 P 3 27 Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 30

FCFS Scheduling (Cont. ) Suppose that the processes arrive in the order P 2 , P 3 , P 1. n The Gantt chart for the schedule is: P 2 0 n n n P 3 3 P 1 6 Waiting time for P 1 = 6; P 2 = 0; P 3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case. 30

n n n One problem with FCFS is that average waiting time can be quite large. Another problem is the Convoy effect. Consider: n 4 processes; n n 1 CPU-bound with CPU burst of 20 units followed by an I/O request requiring 10 units. 3 I/O bound processes with 1 unit of CPU burst followed by 10 units of I/O.

0 19 20 21 22 P 0 P 1 P 2 P 3

0 19 20 21 22 P 0 P 1 P 2 P 3 I/O 1 I/O 2 I/O 3 I/O 4 P 0: 7 P 1: 8 P 2: 9 P 3: 10

I/O Devices Idle CPU Idle 0 19 20 21 22 29 P 0 P 1 P 2 P 3 P 0 I/O 1 I/O 2 I/O 3 I/O 4 P 1: 1 P 2: 2 P 3: 3

I/O Devices Idle CPU Idle 0 19 20 21 22 P 0 P 1 P 2 P 3 I/O 1 I/O 2 I/O Devices Idle 29 32 P 0 I/O 3 I/O 4 49

FCFS Scheduling (Cont. ) n Convoy effect short I/O bound processes behind long CPUbound process. n n n CPU-bound process executes, I/O processes wait in Ready Queue. CPU-bound process completes execution burst and waits on I/O device. IO-Bound processes quickly complete CPU burst and block on I/O device. CPU is idle until I/O completed for CPU-bound process resumes, I/O-bound processes complete I/O request and move to RQ. I/O devices idle while CPU-bound process monopolizes CPU.

Shortest-Job-First (SJR) Scheduling n n Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time. Two schemes: n nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst.

Shortest-Job-First (SJR) Scheduling preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF). SJF is optimal – gives minimum average waiting time for a given set of processes. n n

Example of Non-Preemptive SJF Process Arrival Time Burst Time P 1 P 2 P 3 P 4 7 4 1 4 0. 0 2. 0 4. 0 5. 0 P 1 0 3 P 3 7 P 2 8 P 4 12 16

Example of Non-Preemptive SJF n Average waiting time = (0 + 6 + 3 + 7)/4 = 4 P 1 0 3 P 3 7 P 2 8 P 4 12 16

Preemptive Shortest Job First n n If a job arrives at the queue with a burst time less than that of the running process, the running process is preempted. Decision only made when a new process enters the queue.

Preemptive SJF Process P 1 P 2 P 3 P 4 0. 0 2. 0 4. 0 5. 0 Arrival Time 7 4 1 4 Burst Time

Process P 1 P 2 P 3 P 4 Arrival Time 0. 0 2. 0 4. 0 5. 0 Burst Time 7 4 1 4 5 11 P 1 0 2 4 7 16

Process Arrival Time P 1 P 2 0. 0 2. 0 Burst Time 7 4 P 2 Remainder = 4. P 1 Remainder = 5. Result: P 1 preempted at time 2. P 1 0 P 2 2 P 3 4 P 2 5 P 4 7 P 1 11 16

Process Arrival Time P 1 P 2 P 3 0. 0 2. 0 4. 0 P 1: 5 P 2: 2 P 3: 1 P 1 0 7 4 1 P 2 Preempted. P 3 completes at time 5. P 2 2 Burst Time P 3 4 P 2 5 P 4 7 P 1 11 16

Process Arrival Time P 1 P 2 P 4 P 1: 5 P 2: 2 P 4: 4 0. 0 2. 0 5. 0 7 4 4 Burst Time

P 1 0 P 2 2 P 3 4 P 2 5 P 4 7 P 2 Completes at time 7. P 1: Remaining time of 5. P 4: Remaining time of 4. P 1 11 16

n n n Process P 1 P 2 P 3 P 4 P 1 0 Arrival Time 0. 0 2. 0 4. 0 5. 0 P 2 2 P 3 4 P 2 5 Burst Time 7 4 1 4 P 1 11 7 P 1: Waits from time 2 to time 11 = 9 P 2: Waits from time 4 to time 5 = 1 P 3: No waiting = 0 P 4: Waits from time 5 to time 7 = 2 Average wait = 12/4 = 3. 16

Determining Length of Next CPU Burst n n Can only estimate the length. Estimate made based on some sort of statistic of historical behavior. n n n Assume BL 2 == BL 1 (Next same as last). Take mean of last n burst lengths. Exponential average of previous bursts.

Homework #2 n Please answer the following question from Chapter 5. n 5. 2, 5. 4, 5. 5, 5. 6, 5. 7, 5. 8, 5. 10, and 5. 13. For problem 5. 8, it should read as follows. n “. . . Implementing a multi-level feedback queue. . . . ”

Scheduling in Batch Systems Three level scheduling

Memory Scheduler n Decisions based on for example: n n Time since swapped out. Amount of CPU time allocated so far. How large. How important.

Admission Scheduler n n Based on “degree of multiprogramming” Process mix.

Priority Scheduling n n A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest integer highest priority). n Preemptive n nonpreemptive SJF is a priority scheduling where priority is the predicted next CPU burst time. Problem Starvation – low priority processes may never execute.

Priority Scheduling n n Solution Aging – as time progresses increase the priority of the process. Unix has mechanism for user to lower their priority through the nice system call.

Scheduling for Interactive Systems: Round Robin (RR) n Each process gets a small unit of CPU time (time quantum), usually 10 -100 milliseconds. n n After this time has elapsed, the process is preempted and added to the end of the ready queue. If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. n No process waits more than (n-1)q time units.

Scheduling for Interactive Systems: Round Robin (RR) n Performance n q large FIFO n q small High overhead: Must be large with respect to context switch, otherwise overhead is too high.

Scheduling in Interactive Systems n Round Robin Scheduling n a) list of runnable processes n b) list of runnable processes after B uses up its quantum

n How long should the quantum be? n quantum too short n Assume switch time = 5 ms and quantum = 20 ms: Wasted time = 5/(5+20) = 20% n quantum too long: e. g. , switch time = 5 ms, quantum = 200 ms: Wasted time = 5/(5+200) = approx. 2% but if have 100 processes, response time for 200 th is pretty bad. This is the quantum Linux uses.

Time Quantum and Context Switch Time

Multilevel Queue Scheduling

Multilevel Queue Ready queue is partitioned into separate queues (e. g. , ): foreground (interactive) background (batch) n n Each queue has its own scheduling algorithm, foreground – RR background – FCFS Processes do not change queues

Multilevel Queue n Scheduling must also be done between the queues. n Fixed priority scheduling; (i. e. , serve all from foreground then from background). Possibility of starvation. n Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i. e. , 80% to foreground in RR 20% to background in FCFS

Multilevel Feedback Queue n n n A process can move between the various queues; aging can be implemented this way. Higher priority queues can preempt lower priority queues. Multilevel-feedback-queue scheduler defined by the following parameters: n number of queues n scheduling algorithms for each queue

Multilevel Feedback Queue n n n method used to determine when to upgrade a process method used to determine when to demote a process method used to determine which queue a process will enter when that process needs service

Example of Multilevel Feedback Queue n Three queues: n n n Q 0 – time quantum 8 milliseconds Q 1 – time quantum 16 milliseconds Q 2 – FCFS

Example of Multilevel Feedback Queue n Scheduling n n A new job enters queue Q 0 which is served RR. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q 1. At Q 1 job is again served RR and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2.

Multilevel Feedback Queues