2 - Scheduling

Question 1

What are the process states?

Answer 1

Running
Runnable
Blocked

Question 2

What are the three DECSystem-10 tables?

Answer 2

JBTQ - Job Table Queue
JBSTS - Job Status Table
QTT - Queue Transfer Table

Question 3

JBTQ Purpose

Answer 3

To store the set of all jobs in three “priority queues” PQ1,2,3

Question 4

What is the run queue?

Answer 4

The concatenation of three “priority queues”

Question 5

What is a doubly linked list?

Answer 5

Each element has a pointer to the next and previous element.
Each PQ uses one.

Allows efficient deletion and insertion

Question 6

Purpose of Job Status Table?

Answer 6

Stores extra info about each job (process)

Question 7

What are the Job Status Bits?

Answer 7

RUN: 1 if runnable
JRQ: 1 if changed state and needs requeue
The wait state code - reason it’s not runnable

Others:
SWP: currently swapped out or trainsit
CMWB: set to swap in
NSWP: Do not swap out

Question 8

Define QRT

Answer 8

Quantum Run TIme left

Time for which the program should be left to run

Question 9

What is QRT measured in?

Answer 9

Jiffies

Linked to frequency,
Denoted by interrupt,
Regular source of interrupts,
Marks end of time slice

Question 10

What happens if QRT exceeded?

Answer 10

in PQ1:
Moved to PQ2 with small qrt

in PQ2:
Moved to PQ3 with large qrt

in PQ3:
Moved to PQ2 with small qrt

Question 11

Purpose of the QTT (Queue Transfer Table)

Answer 11

Denote which queue to move to and what QRT to set when an “event” occurs (eg QRT exceeded)