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Process Synchronization

Reference: S&G, Chapter 6
Concurrent Processes

• several processes between starting and finishing

Concurency

• managing concurrent prcesses that share resources

Mutual Exclusion

• only one process is allowed access to a shared resource at the same time (e.g. printer, processor, variable, data structure, semaphore)

• Example: A train requires exclusive access to the track running through the tunnel. The shared variable is the single track going through the tunnel.

Resource Allocation

• we will focus on shared variables
• printers are actually controlled by one process called the printer server or printer daemon. Modern operating systems do not use mutual exclusion techniques for controlling the printer.

NOTE: S&G, p. 167: ``we do assume that the basic machine-language instructions (the primitive instructions such as load, store, and test) are executed atomically.'' Another way of thinking about this is that the memory hardware enforces mutually exclusive access to a word of memory. Thus, if two processes or processors try to write different values (say 16 and 29) to word OD45E, one or the other of these values will be written first (say 29) and the other will be written second (say 16), but the bits of the two values 16 and 29 will not be scrambled together.

Low-Level Techniques for Mutual Exclusion

1. Turn Off Interrupts
   • in UNIX, if the operating system is about to make changes to its kernel data structures (e.g., create a new process by changing the process table)
   • turn off interrupts; therefore, a process may get more time on the processor
     • process cannot lose the processor because the short term scheduler is run in response to a timer interrrupt

   • change data structure
     • keep the code very short without bugs
     • usually only one or two changes are needed here

   • allow interrupts again
   • used on hercules
   • great for single-processor machines

   Two Problems:

   1. if you need to do a lot of operations in the mutually exclusive segment
   2. multiple processor machine (multiprocessor)
      • Zeus has 12 processors that share the same memory (shared memory architecture)
      • turning off interrupts gives one process exclusive access to its processor but other processes on other processors can still make changes to memory

2. Busy Wait (SpinLock)

• one process ``busy waits'' for another
• used for multiple processor machines

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