Bounded buffer problem

producer consumer problem

issues

no data in buffe: consumer has nothing to consume: data race
buffer is full: no space for producer to produce: busy waiting

private vars

in: next pos in buf to be writted
out: next pos in buf to be read

shared variables

buffer
counter: number of items in buffer

waiting

busy: proces keeps checking (if buffer is still full) wasting CPU time
non busy: suspend the process

soution with shared array

PRODUCER: repeat 
		...
		produce an item in nextp 
		...
		while counter = n do no-op;
		buffer[in] := nextp;
		in := in+1 mod n;
		counter := counter+1;
	until false;	
CONSUMER: repeat 
		while counter = 0 do no-op;
		nextc := buffer[out];
		out := out+1 mod n;
		counter := counter-1;

		...
		consume the item in nextc 
		...
	until false;

counter is a shared variable - causes a data race problem

critical sections

guarantee that certain sections of coopeating processes are not interleaved to avoid data race problem

only one process can execute critical section at a time

but there is a data race problem for locking the processes!

hardware locking

disalow interrupts while a shared variable is being modified. only works for one CPU

modern computers have special atomic instructions

CompareAndSwap
testAndSet

CAS(0, a, n): If the value at address a is o, write the value n to address a and then return true; Otherwise, return false, where o and n are integers, and a is the address of a memory location.

1.8 KiB Raw Blame History

Bounded buffer problem

critical sections

hardware locking

CAS based LOCK function

1.8 KiB

Raw Blame History