More on Semaphores Andy Wang Operating Systems COP

The Pigeon Network Scenario l Pigeons are good message carriers l Reasonably reliable l

Here is the Story… l There are two towns—Mars and Venus l Mars has

Here is the Story… l Based on experience l Whenever both town send messages

Developing the Solution Can we map it to some solved problems? l Standard synchronization

Step 1: Visualization l Identify l Shared resources l Scope of shared resources flying

Step 1: Visualization l Identify l Shared resource: flying path l Scope of the

Simplest Implementation semaphore flying. Path = 1; P(flying. Path); // send the message V(flying.

Simplest Implementation + Simple + Fair - Not efficient l Only one pigeon can

Allowing Multiple Venusians in Transit l Resources and scopes: flying path (global), a counter

Allowing Multiple Venusians in Transit semaphore flying. Path = 1; int Venusians. In. Transit

Allowing Multiple Venusians in Transit Revised semaphore flying. Path = 1; semaphore Venusian. Lock

Allowing Multiple Venusians in Transit Revised semaphore flying. Path = 1; P(flying. Path); //

Allowing Multiple Venusians in Transit Revised Again semaphore flying. Path = 1; semaphore Venusian.

Allowing Multiple Venusians in Transit Revised Again Efficient for Venusians l Not fair for

Allowing Multiple Venusians and Martians in Transit semaphore flying. Path = 1; semaphore Martian.

Allowing Multiple Venusians and Martians in Transit Both sides can transmit efficiently l One

Proving the Correctness l Safety (mutual exclusion): l At most one thread is in

Proving the Correctness l Liveness (progress): l If more than one thread is interested

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More on Semaphores Andy Wang Operating Systems COP 4610 / CGS 5765

The Pigeon Network Scenario l Pigeons are good message carriers l Reasonably reliable l Relatively fast for less developed rural areas l Can sense magnetic field lines

Here is the Story… l There are two towns—Mars and Venus l Mars has all male pigeons l Venus has all female pigeons l Each town delivers messages to the other l By sending a pigeon through the shared flying path l And waiting for the same pigeon to fly back as an acknowledgement

Here is the Story… l Based on experience l Whenever both town send messages simultaneously, the reliability drops significantly l Pigeons of opposite genders decide to take discursions l Goals of a pigeon network: l Efficiency l Fairness

Developing the Solution Can we map it to some solved problems? l Standard synchronization problems: l l Bounded buffer (producers and consumers) l Fairness (readers and writers) l Resource allocation (dining philosopher) l Pigeon network is under the reader-writer category

Step 1: Visualization l Identify l Shared resources l Scope of shared resources flying path Mars Venus

Step 1: Visualization l Identify l Shared resource: flying path l Scope of the shared resource: global flying path Mars Venus

Simplest Implementation semaphore flying. Path = 1; P(flying. Path); // send the message V(flying. Path); flying path Mars Venus

Simplest Implementation semaphore flying. Path = 1; P(flying. Path); // send the message V(flying. Path); flying path something you don’t see everyday… Mars Venus

Simplest Implementation semaphore flying. Path = 1; P(flying. Path); // send the message V(flying. Path); flying path Mars Venus

Simplest Implementation + Simple + Fair - Not efficient l Only one pigeon can fly at a time l Venusians love to fly in groups…

Allowing Multiple Venusians in Transit l Resources and scopes: flying path (global), a counter for Venusians in transit (Venus) flying path Mars Venus

Allowing Multiple Venusians in Transit semaphore flying. Path = 1; int Venusians. In. Transit = 0; P(flying. Path); // send the message V(flying. Path); ++Venusians. In. Transit; if (Venusians. In. Transit == 1) { P(flying. Path); } // send the message flying path Mars --Venusians. In. Transit; if (Venusians. In. Transit == 0) { V(flying. Path); } Venus

Allowing Multiple Venusians in Transit semaphore flying. Path = 1; int Venusians. In. Transit = 0; P(flying. Path); // send the message V(flying. Path); Sure. Date? ++Venusians. In. Transit; if (Venusians. In. Transit == 1) { P(flying. Path); } // send the message flying path Mars --Venusians. In. Transit; if (Venusians. In. Transit == 0) { V(flying. Path); } Venus

Allowing Multiple Venusians in Transit Revised semaphore flying. Path = 1; semaphore Venusian. Lock = 1; int Venusians. In. Transit = 0; P(flying. Path); // send the message V(flying. Path); P(Venusian. Lock); ++Venusians. In. Transit; if (Venusians. In. Transit == 1) { P(flying. Path); } // send the message flying path Mars --Venusians. In. Transit; if (Venusians. In. Transit == 0) { V(flying. Path); } V(Venusian. Lock); Venus

Allowing Multiple Venusians in Transit Revised semaphore flying. Path = 1; P(flying. Path); // send the message V(flying. Path); Wait! semaphore Venusian. Lock = 1; int Venusians. In. Transit = 0; P(Venusian. Lock); ++Venusians. In. Transit; if (Venusians. In. Transit == 1) { P(flying. Path); } // send the message flying path Mars --Venusians. In. Transit; if (Venusians. In. Transit == 0) { V(flying. Path); } V(Venusian. Lock); Venus

Allowing Multiple Venusians in Transit Revised Again semaphore flying. Path = 1; semaphore Venusian. Lock = 1; int Venusians. In. Transit = 0; P(flying. Path); // send the message V(flying. Path); P(Venusian. Lock); ++Venusians. In. Transit; if (Venusians. In. Transit P(flying. Path); } V(Venusian. Lock); // send the message P(Venusian. Lock); --Venusians. In. Transit; if (Venusians. In. Transit V(flying. Path); } V(Venusian. Lock); flying path Mars Venus == 1) { == 0) {

Allowing Multiple Venusians in Transit Revised Again Efficient for Venusians l Not fair for Marsians l

Allowing Multiple Venusians and Martians in Transit semaphore flying. Path = 1; semaphore Martian. Lock = 1; int Martians. In. Transit = 0; P(Martian. Lock); ++Martians. In. Transit; if (Martians. In. Transit == 1) { P(flying. Path); } V(Martian. Lock); // send the message P(Martian. Lock); --Martians. In. Transit; if (Martians. In. Transit == 0) { V(flying. Path); } V(Martian. Lock); semaphore Venusian. Lock = 1; int Venusians. In. Transit = 0; P(Venusian. Lock); ++Venusians. In. Transit; if (Venusians. In. Transit P(flying. Path); } V(Venusian. Lock); // send the message P(Venusian. Lock); --Venusians. In. Transit; if (Venusians. In. Transit V(flying. Path); } V(Venusian. Lock); == 1) { == 0) {

Allowing Multiple Venusians and Martians in Transit Both sides can transmit efficiently l One side can completely block off the traffic from the other side l Fair, since both sides are equally selfish… l Fortunately, pigeons do sleep l Daily cycles will break the starvation… l

Proving the Correctness l Safety (mutual exclusion): l At most one thread is in the critical section at any time l All necessary locks are acquired at the entrance of critical section l All shared resources are protected

Proving the Correctness l Liveness (progress): l If more than one thread is interested in executing the critical section, some processes will eventually do so (deadlock free) l Fairness (bounded waiting): l Every thread that wants to execute the critical section will eventually do so (no starvation)