3BChapter 1-Introduction-12B1.3.Risks of Threads 》 In well designed concurrent applications the use of threads is a net performance gain,but threads nevertheless carry some degree of runtime overhead.Context switches-when the scheduler suspends the active thread temporarily so another thread can run-are more frequent in applications with many threads,and have significant costs:saving and restoring execution context,loss of locality,and CPU time spent scheduling threads instead of running them.When threads share data,they must use synchronization mechanisms that can inhibit compiler optimizations,flush or invalidate memory caches,and create synchronization traffic on the shared memory bus.All these factors introduce additional performance costs;Chapter 11 covers techniques for analyzing and reducing these costs.3BChapter 1Ͳ IntroductionͲ12B1.3. Risks of Threads 7 In well designed concurrent applications the use of threads is a net performance gain, but threads nevertheless carry some degree of runtime overhead. Context switches Ͳwhen the scheduler suspends the active thread temporarily so another thread can run Ͳare more frequent in applications with many threads, and have significant costs: saving and restoring execution context, loss of locality, and CPU time spent scheduling threads instead of running them. When threads share data, they must use synchronization mechanisms that can inhibit compiler optimizations, flush or invalidate memory caches, and create synchronization traffic on the shared memory bus. All these factors introduce additional performance costs; Chapter 11 covers techniques for analyzing and reducing these costs.