Priority Preemptive Scheduling

gantt-chartprocessscheduling

When using Priority Preemptive Scheduling, does a higher priority yield way to a process with a lower priority but with a shorter burst time?

For example, if I had:

    Arrival Time   Burst Time   Priority
P1       0             5           3
P2       2             6           1
P3       3             3           2

Would the Gannt chart look like this?

| P1 | P2 | P3 | P1 |
0    2    8   11   16

Best Answer

Priority Scheduling always selects the process(es) with the highest priority currently ready to run. If there is more than one process having the currently highest priority, you need a second scheduling algorithm to choose among these processes. Non-preemptive Priority Scheduling only selects a new process to run if the running process finished its work or yields (voluntarily) to the scheduler.

Preemptive Priority Scheduling is the same algorithm but if a new process having a higher priority than the currently running process arrives, it gets selected immediately. The new process has not to wait until the currently running process finishes or yields.

In your sample, the Gantt chart for Preemptive Priority Scheduling and 3 being the highest and 1 the lowest priority would look like:

|    P1   |  P3 |        P2      |
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Related Solutions

Node.js – How to run a node.js app as a background service

Copying my own answer from How do I run a Node.js application as its own process?

2015 answer: nearly every Linux distro comes with systemd, which means forever, monit, PM2, etc are no longer necessary - your OS already handles these tasks.

Make a myapp.service file (replacing 'myapp' with your app's name, obviously):

[Unit]
Description=My app

[Service]
ExecStart=/var/www/myapp/app.js
Restart=always
User=nobody
# Note Debian/Ubuntu uses 'nogroup', RHEL/Fedora uses 'nobody'
Group=nogroup
Environment=PATH=/usr/bin:/usr/local/bin
Environment=NODE_ENV=production
WorkingDirectory=/var/www/myapp

[Install]
WantedBy=multi-user.target

Note if you're new to Unix: /var/www/myapp/app.js should have #!/usr/bin/env node on the very first line and have the executable mode turned on chmod +x myapp.js.

Copy your service file into the /etc/systemd/system.

Start it with systemctl start myapp.

Enable it to run on boot with systemctl enable myapp.

See logs with journalctl -u myapp

This is taken from How we deploy node apps on Linux, 2018 edition, which also includes commands to generate an AWS/DigitalOcean/Azure CloudConfig to build Linux/node servers (including the .service file).

Homework – Preemptive Priority Scheduling

Firstly: two processes with the same priority will generally be sorted according to their arrival time: so first come first serve.

Turn around time: generally refers to the amount of time required for the process to produce output. Your course may use a different or simpler definition to make your calculations easier. From what I can see you are probably being asked to compute the amount of time it takes for each process to complete. This is equal to: finish_time - time_first_scheduled_on_CPU (not finish_time - arrival_time).

Average waiting time: Based on the Gantt chart you develop which shows exactly when and for how long a process is on the CPU, you can compute the total amount of time each process was waiting to get on the CPU.

This consists of:

Amount of time between arrival and when it is first scheduled on the CPU
If the process has a longer execution time than a single quantum then you need to sum all of the time spent off the CPU
Add the two quantities together and you have the total waiting time for the process
Repeat steps 1-3 for each process and take the average.

Best Answer

Related Solutions

Node.js – How to run a node.js app as a background service

Homework – Preemptive Priority Scheduling

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