Cooperative Multitasking

• Threads run until they yield time back to the OS

What are some issues with cooperative multitasking?

Cooperative Multitasking Issues

• One thread may not be as cooperative and hog time from others
• The OS doesn’t know when it will next have CPU time to perform device I/O

Pre-emptive Multitasking

• Allows the OS to take control and switch to another thread

Interrupts

• CPUs generally execute the next instruction
• Devices have the ability to interrupt this flow and redirect execution to an interrupt handler
• Using timer and interrupt handlers, the OS can build pre-emptive multitasking

DoS

• One thread hogs execution preventing the system from running properly

Race Conditions

• If not programmed carefully, threads can lead to undefined and unexpected behavior
• This behavior can sometimes be exploitable by an attacker

Threads

• A sequence of computational instructions run one after another
• Requires OS-level management for runtime data

Thread Switching

• A thread can ask the OS to run another thread by calling yield()
• The OS may take control of the CPU using pre-emptive multitasking

How does the OS determine which thread to switch to?

Scheduling

• OS determines which process to run next
• Decision should be based on what the users of the system need
• Should optimize resource use

Waiting Threads

• Threads may be blocked waiting for I/O
• e.g. a webserver waiting on a disk access

Busy Waiting

• Loop until the task is complete, checking frequently
• Wastes CPU time that could be used by other threads
• OS should implement a better mechanism

Queues

• Run queue - Queue of threads that can be run now
• Wait queue - Queue of threads waiting on something
  • 1 queue per wait reason