Questions On Proportional Share Thread Scheduling printable pdf download

Questions On Proportional Share Thread Scheduling

CS 3430/5430

Lecture 5

Name:_____________

1) Decay usage schedulers (e.g., Mac OS X / MS Windows) dynamically adjust

priorities of waiting threads higher, and running threads lower.

Mac OS X

Thread priority:

“base priority” - recent usage

When adjust priorities?

when a thread changes state

Adjust running thread usage by:

old usage + current burst * current load

Adjust waiting thread usage by:

old usage * (5/8)

, where n is # of 1/8 sec.

“exponential decay”

a) Why are priorities updated when a thread changes state and not continually?

b) The values (5/8) and 1/8 sec. are empirical parameter defaults that can be

tuned by the system administrator.

What would be the general effect of increasing from (5/8) to (7/8)?

What would be the general effect of decreasing from 1/8 second to 1/16 second?

c) In Microsoft Windows, a thread coming out of a wait state has its priority elevated from the base priority

depending on what it was waiting. Why would waiting on a disk be a small boost, but waiting on input from the

keyboard be a large boost?

d) In Microsoft Windows, a thread using the processor has its priority reduced linearly downward toward the

base priority. To avoid starving a runnable thread with base priority, it periodically boosts the priority of any

thread not dispatched otherwise. Why does the Mac OS X scheduler not have a similar starvation problem?

2. Proportional-Share scheduling - allow the user (or system) to specify the proportion of CPU time each thread

should be allocated. Three main types of proportional-share scheduling:

a) weighted round-robin scheduling (WRR) - round-robin so threads with higher weights getting longer-time

slices

Consider three threads we want to run with the following proportions. If we want to run all three in 32 ms, how

big of time slices would each be allocated?

Thread

Proportion of CPU

CPU Time-Slice (ms)

Draw the Gantt chart for this schedule

Lecture 5 Page 1

Questions On Proportional Share Thread Scheduling