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CPU Scheduling
* Maximum CPU utilization obtained with
multiprogramming
* CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait
* CPU burst distribution
Alternating Sequence of CPU & I/O Bursts
CPU Scheduler
* Selects from among the processes in memory that are ready to implement, and allocates the CPU to one of them
* CPU scheduling solutions may take place when a process:
1. Switches from running to waiting state
2. Switches from running to ready state
3. Switches from waiting to ready
4. Terminates
* Scheduling under 1 and 4 is nonpreemptive
* All other scheduling is preemptive
Dispatcher
* Dispatcher module gives manage of the CPU to the process selected by the short-term scheduler; this involves:
- switching context
- switching to user mode
- jumping to the proper location in the user program to resume that program
* Dispatch latency – time it takes for the dispatcher to end one process and start another running
Scheduling Criteria
* CPU utilization – keep the CPU as busy as possible
* Throughput – No. of processes that finish their execution per time unit
* Turnaround time – amount of time to implement a particular process
* Waiting time – amount of time a process has been waiting in the ready queue
* Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)
Optimization Criteria
* Max CPU utilization
* Max throughput
* Min turnaround time
* Min waiting time
* Min response time
First-Come, First-Served (FCFS) Scheduling
Process Burst Time
P1 24
P2 3
P3 3
Suppose that the processes enter in the order: P1 , P2 , P3
The Gantt Chart for the schedule is:
* Waiting time for P1 = 0; P2 = 24; P3 = 27
* Average waiting time: (0 + 24 + 27)/3 = 17
* Suppose that the processes arrive in the order
P2 , P3 , P1
* The Gantt chart for the organizer is:
* Waiting time for P1 = 6; P2 = 0; P3 = 3
* Average waiting time: (6 + 0 + 3)/3 = 3
* Much better than previous case
* Convoy effect small process behind large process
Shortest-Job-First (SJF) Scheduling
* Associate with each process the length of its next CPU burst. Use these lengths to organise the process with the shortest time
* Two schemes:
- nonpreemptive – once CPU given to the process it cannot be preempted until finishes its CPU burst
- preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is known as the
Shortest-Remaining-Time-First (SRTF)
* SJF is optimal – gives less average waiting time for a given set of processes
Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
SJF (non-preemptive)
* Average waiting time = (0 + 6 + 3 + 7)/4 =4
Example of Preemptive SJF
Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
* SJF (preemptive)
* Average waiting time = (9 + 1 + 0 +2)/4 = 3
Priority Scheduling
* A priority number (integer) is related with each process
* The CPU is allocated to the process with the highest priority (smallest integer =highest priority)
- Preemptive
- Non preemptive
* SJF is a priority scheduling where priority is the forecasted next CPU burst time
* Problem = Starvation – low priority processes may never implement
* Solution = Aging - as time progresses increase the priority of the process (means Aging increases the priority of the processes so that to terminate in finite amount of time).
Round Robin (RR)
* Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has progressed, 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 blocks of at most q time units at once. No process waits more than (n-1)q
time units.
* Performance
- q large ->FIFO
- q small -> q must be large with respect to context switch, otherwise projecting is too high
Example of RR with Time Quantum = 20
Process Burst Time
P1 53
P2 17
P3 68
P4 24
The Gantt chart is:
* Typically, higher average turnaround than SJF, but better response
Multilevel Queue Scheduling
* Ready queue is partitioned into separate queues:
foreground (interactive)
background (batch)
* Each queue has its own scheduling algorithm
- foreground – RR
- background – FCFS
* Scheduling must be done between the queues
- Fixed priority scheduling; (i.e., serve all from foreground then from background).
Possibility of starvation.
- 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 Scheduling
* A process can move between the various queues; aging can be implemented this way
* Multilevel-feedback-queue scheduler defined by the following parameters:
- number of queues
- scheduling algorithms for each queue
- 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
Thread Scheduling
* Local Scheduling – How the threads library decides which thread to put onto an available LWP
* Global Scheduling – How the kernel decides which kernel thread to run next
Multiple-Processor Scheduling
* CPU scheduling more complex when multiple CPUs are available
* Homogeneous processors within a multiprocessor
* Load sharing
* Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing
Operating System Examples
Windows XP Priorities
Linux Scheduling
* Two algorithms: time-sharing and real-time
* Time-sharing
- Prioritized credit-based – process with most credits is scheduled next
- Credit subtracted when timer interrupt occurs
- When credit = 0, another process chosen
* When all processes have credit = 0, recrediting occurs
- Based on factors including priority and history
* Real-time
- Soft real-time
- Posix.1b compliant – two classes
• FCFS and RR
• Highest priority process always runs first
Java Thread Scheduling
* JVM Uses a Preemptive, Priority-Based Scheduling Algorithm
* FIFO Queue is Used if There Are Multiple Threads With the Same Priority
JVM Schedules a Thread to Run When:
1. The Currently Running Thread Exits the Runnable State
2. A Higher Priority Thread Enters the Runnable State
* Note – the JVM Does Not Specify Whether Threads are Time-Sliced or Not