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L27 - Additional Reference ECS 154B Computer Architecture...

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1 ECS 154B Computer Architecture II Winter 2008 Multiprocessor Synchronization Adapted from slides by Mary Jane Irwin, Penn State 2 Additional Reference Some information and more details provided in Parallel Computer Architecture by David Culler, Jaswinder Pal Singh, and Anoop Gupta Sections 5.3, 5.5, 6.3 Available in library reserves 3 Process Synchronization Processes must coordinate when working on a common task Mutual Exclusion (semaphores) Provides access to only one process Binary value For example, access to a shared resource Barrier or Event Synchronization Ensures all processes reach the same point before proceeding Possibly multiple values For example, compute partial sums before continuing Provided through hardware support and variables 4 Main Actions for Synchronization Acquire: how to obtain a synchronization element Bus provides serialization of access attempts Hardware must provide some atomic operations (actions that are guaranteed to complete before subsequent operations occur) Wait: how a process waits for the signal to continue Polling or busy waiting: process continually checks for access Blocking: process stalls until signaled to continue Release: how processes know to continue Notify one process explicitly Notify many processes implicitly
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2 5 Mutual Exclusion Hardware locks possible – Only a limited number possible in hardware Require bus signal lines Can’t be changed after system is built – Not flexible to process workload requirements Waste for workloads that don’t need as many locks Insufficient for workloads than need many locks; have to use software locks anyway
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