CSI 400/500 Operating Systems Spring 2009

Lecture #15 – Synchronization with Semaphores

Monday, March 30th, 2009

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Definitions

• Concurrency

• Synchronization

• Critical Section

• Running multiple processes at the same time

• Timing use of resources so processes appear to use them at same time (virtually), but don’t interfere with each other

• Part of process where accesses shared information

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Why important?

• Consider payroll database

• Raises and vacation pay process runs Friday night

• Paychecks run Friday night

• Difference if paycheck process checks pay rate of employee before and after raise processed

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Simple solution

• Read/write flags

• Only one process can update a record at a time

• Multiple processes can read

• Does this help our case?

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Next solution: access lock

• Before an update, process obtains lock

• No other process can access (either read or write) while lock is held

• Does this completely solve our situation?

• Downside

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Deadlock

• SERIOUS downside of using locks• Two processes: one holds lock on resource

that other needs• Each holds until needed lock released• Since both suspended, locks never released and

processes never run

• We’ll discuss ways to detect and prevent deadlock in Lecture 17

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Locks v Kernels

• Some systems create subKernels around critical sections to handle reserving and accessing shared data

• Not preferred – why?

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More definitions

• Mutual Exclusivity

• Semaphore

• Only one process in a shared critical section at a time; others wait

• Data type that controls singular access to a shared resource

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Parts of a Semaphore

• Value

• Hold or Set or Wait

• Release or Signal

• Value of the semaphore (depends upon type)

• Request for resource

• Called P in text

• Suspends process until resource is available

• Done with resource

• Called V in text

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Types of Semaphores

• Binary

• Counting

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How Binary Semaphore Works

• Value starts at 1

• Hold checks value:• If 0, waits

• Once 1, decrements

• Release increments value

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How Counting Semaphore Works

• Value starts at appropriate value• 0 for empty source, limit for full

• Set checks value:• If 0, waits

• Once 1, decrements

• Release increments value

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Semaphore Example #1 – Consumer/Producer Problem

• Like shared data buffers, where any process adds data to it, reads data, or removes data• Slightly more complex than classic consumer/producer,

as there is a non-destructive consume option (read)

• Can’t read or remove if nothing there

• Action of removing or adding requires exclusive access

• How do this?

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Semaphore Example #2 – Readers/Writer Problem

• Multiple readers can access same resource, but only one writer at a time

• Writer has exclusive access

• How handle this?

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Semaphore Considerations

• To avoid potential deadlock situations by indefinite semaphore hold, interrupts temporarily disabled within Set and Signal routines

• Modern OS uses assembly instruction TS (test and set) to accommodate

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Handling Semaphore Wait

• Within Set routine

• Most Operating systems put process in Suspend state, freeing processor for other work

• Signal causes interrupt that Readies all processes suspended on that semaphore• First one scheduled executes

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Active Release

• Some just-in-time operating systems use active Release routine

• Informs suspended process that semaphore is now available

• Only applies to binary semaphores