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Interprocess Synchronization
and Communication
Steve Goddard
http://www.cse.unl.edu/~goddard/Courses/CSCE
CSCE 351
Operating System Kernels
Producer/Consumer
Implementation
processProducer varc : char begin loop < produce a character “c” > whilenextIn+1 modn =nextOut do NOOP end while buf[nextIn] :=c nextIn :=nextIn+ 1 modn end loop end Producer
processConsumer varc : char begin loop whilenextIn =nextOut do NOOP end while c :=buf[nextOut] nextOut :=nextOut+1 modn < consume a character “c” > end loop endConsumer
globals buf : array [0..n-1] of char; n-1 nextIn,nextOut :0..n-1 := 0 n-
nextIn nextOut
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processProducer varc : char begin loop < produce a character “c” > whilecount =n do NOOP end while buf[nextIn] :=c nextIn :=nextIn+ 1 modn count :=count + 1 end loop end Producer
processConsumer varc : char begin loop whilecount = 0 do NOOP end while c :=buf[nextOut] nextOut :=nextOut+1 mod n count :=count - 1 < consume a character “c” > end loop endConsumer
globals buf : array [0..n-1] of char; nextIn,nextOut : 0..n-1 := 0 count : integer := 0
n- n-
nextIn nextOut
Producer/Consumer Implementation
with a shared counter
The Critical Section Problem
u One implementation of the shared counter
processProducer
begin
<count := count + 1>
MOV R1, @count
ADD R1, 1
MOV @count, R
end Producer
processConsumer
begin
<count := count - 1>
MOV R2, @count
SUB R2, 1
MOV @count, R
endConsumer
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Message Passing
u Two fundamental communication & synchronization
paradigms
» Shared memory
v Efficient, familiar
v Not always available
v Potentially insecure
» Message passing
v Awkward, less standardized
v Extensible to communication in distributed systems
v Syntax:
send(process : process_id,message : string)
receive(process : process_id, var message : string)
Message Passing Example
Ye Olde Producer/Consumer System
process producer begin loop < produce a char “c” > send(consumer, c) end loop end producer
process consumer begin loop receive(producer,mesg) < consume message “mesg” > end loop end consumer
OS Kernel
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Issues
Synchronization semantics
u When does a send/receive operation terminate?
» Blocking
v sender waits until its message is received
v receiver waits if no message is available
» Non-blocking
v send operation “immediately” returns
v receive operation returns if no message is
available
» Variants
v send()/receive() with timeout
OS Kernel
Sender Receiver
OS Kernel
Sender Receiver
Send message to recvr.
Wait until message is
accepted.
Broadcast message to all
receivers. Wait until
message is accepted by all.
Send message to recvr.
Broadcast message to all
receivers.
Blocking Nonblocking
Explicit
Implicit
Synchronization
Naming
Semantics of Message Passing
send(recvr,mesg)
