3.3 Scheduling Goals

Scheduling

• Optimize system resources
• Provide snappy user experience

System Usage

• Many consumer devices (laptops, phones, etc) are idle most of the time
• Servers in data centers often have a backlog of work to do

Performance Goals

• Throughput
• Response time

Throughput

• Measure of work done per time
• An example would be requests per second (RPS) for a webserver
• Typically goes down as we change threads more quickly

Context Switching

• Changing threads incurs a cost
• State has to be saved to memory when pausing a thread
• State has to be loaded from memory when resuming a thread

What impact does the cache hierarchy have on context switching?

Cache Hierarchy

• The implementation of the cache hierarchy makes context switching more expensive
• Another thread will replace many of our cached values

How does this change in multiprocessor systems?

Multiprocessor Systems

• A context switch to another CPU will be much slower
• The thread’s old cache will be inaccessible from the new CPU
• Processor affinity is helpful to keep threads running on the same CPU

Cache Coherence Protocol

• Values that exist in another CPU’s cache are especially slow to access
• This further increases the cost of context switches to another CPU

Response Time

• Time to service a single request
• Typically goes down the faster we change contexts

Control Goals

• Urgency
• Importance
• Resource allocation

Urgency

• How soon a task needs to be completed
• A homework assignment with a due date
• A VOIP packet to send

Importance

• How critical the task is to complete
• Your graduation application

Urgency and Importance

• Orthogonal concepts
• A task can be urgent without being import or important without being urgent

Resource allocation

• How to split resources between users

Scheduling Goals

• Performance
  • Throughput
  • Response Time
• Control
  • Urgency
  • Importance
  • Resource allocation