Question

• Who do you think is “right": the SEDA paper or the “Events are a Bad Idea" paper and why? Justify your answer.

• What is the difference between Mesa and Hoare Monitor semantics?
  • Which is simpler from the perspective of the application programmer?
  • Which is simpler from the perspective of the OS developer? Why?

Answer 1

Q1.

The paper 'Events are a bad idea' by Rob von Behren, Jeremy Condit and Eric Brewer is right because:

• Events tends to complicates the control flow
• It is more feasible to use Threads than fixing an issue with Events
• For complex parts and issues, event based systems fail to provide whereas it is gracefully handled by threads
• Similar or even higher performance level can be achieved by threads
• Threads simpler programming model gives advantage over Events.

Q2.

Mesa scheduling,

• Signaler keeps the lock and CPU, while the signaled thread is simply put on the ready queue and will run later.
• In Mesa scheduled condition must be rechecked after being awoken from a Wait() operation i.e. they need a while loop around the execution of Wait().

Hoare scheduling,

• Signaler gives the lock and CPU to the signaled thread which begins running until it releases the lock, at which point the signaler regains the lock and CPU.
• Hoare scheduled monitors the condition is check is not needed after being awoken, since they know that the code after the Wait() is executed immediately after the Signal() i.e. they do not need a while loop around the execution of Wait().