XML CO527 Anonymous Questions and Answers Keyword Index

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Keyword reference for scheduling

Question 35 (2011):

Submission reference: IN2149

I am really confused about the questions relating to scheduling in the 2011 past paper (question 1(g)) and the 2009 past paper (question 2(c)). Any suggestions?

Answer 35:

The first thing to do is understand what the diagram is telling you. Specifically, these are traces for some user processes (P1, etc.) the OS and others (FLIH and idle-task). As you should know from the lectures, processes exist in one of several states (runnable, running, blocked). This is what the diagrams are showing, with the state names replaced with "A", "B", etc.

The first part of both questions asks what states are represented by the various letters. To make this possible, some additional information is given, e.g. from the 2011 paper "The events at times "t2", "t4" and "t6" are timer interrupts causing time-slicing context switches.". As can be seen in the diagram, at time "t2" process "P1" changes from state "A" to state "B", with some execution by the FLIH and OS inbetween. Similar occurs at times "t4" and "t6" too. From this, it's reasonable to assume that state "A" is running and state "B" is runnable. That leaves "C" to be blocked (which would make sense). Once you've figured that out, the rest of the question becomes simpler.

For the 2011 paper, part (ii) of the question asks how many processor cores there are. If state "A" is running, then there are at least as many processor cores as processes in this state at any given time (i.e. a certain number of processes are running simultaneously). If you scan through the diagram you'll see that this is 2. Thus, there are two processors.

Part (iii) of that question in the 2011 paper asks "Assuming that at time "t1" a request is made by P3 to read a packet from the network, what occurs at time "t3" (or is likely to have occurred) and why?". Given what you know about process states (and what's said in this part of the question may help in answering the first part), "P3" transitions from running to blocked (presumably whilst it waits for the packet). At "t3", whatever happens causes "P3" to transition from blocked to runnable. Based on that, it's reasonable to infer that the event was probably a packet being received for that particular process, or an error (basically, something that causes "P3" to finish waiting for the packet and continue execution).

Keywords: scheduling