94.574 Elements of Computer Systems

SCE 574

214

Semaphores: Observations

äWhen S.count >=0: the number of processes that can execute wait(S) without being blocked = S.count

äWhen S.count<0: the number of processes waiting on S is = |S.count|

äAtomicity and mutual exclusion: no 2 processes can be in wait(S) and signal(S) (on the same S) at the same time (even with multiple CPUs)

äThis is a critical section problem: critical sections are blocks of code defining wait(S) and signal(S)

äHence: the critical sections here are very short: typically 10 instructions

äSolutions:

äuniprocessor: disable interrupts during these operations (ie: for a very short period). This does not work on a multiprocessor machine.

ämultiprocessor: use previous software or hardware schemes. The amount of busy waiting should be small.