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Windows XP thread pool has, by far, the worst performance of the bunch Though the difference between Vista and XP appears small in this graph, in reality, the XP thread pool only provides 1 2 percent of the callback throughput of Vista We will conclude by looking at some scaling numbers We compare the execution time of running N tasks each comprising of C cycles on a single thread versus queuing each of the N tasks to run on the P thread pool threads, where P is the number of processors on the machine Each of the threads will receive N /P tasks and, for each one, run C cycles' worth of sim ulated work In all measurements, we show the CLR 20 SPI and Windows Vista thread pools side-by-side, and, in all cases, prime the pools to ensure we don't measure the cost of lazily allocating the threads In summary, the single threaded case will execute in roughly O(NC) time, while the thread pool case will execute in O(Q + (CNS) / P), where Q is the overhead that results from using the pool (we measure the calls to
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T h r e a d Pool Qu e u e U s e rWo r k Item in our accounting, which means Q is actu
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ally some factor of N) and S is the overhead that results on the thread pool for each item dequeued Sadly, this isn't a constant factor: it depends heav ily on contention to dispatch work items from the shared queue This depends on the size of individual tasks In the Figure 73, the y-axis represents C, and the abscissa represents the "parallel speedup," a term we will become more familiar with in subsequent chapters This is the time to execute on 1 thread divided by the time to exe cute on many threads The numbers were gathered on a 4-core, 2-CPU machine, that is, an 8-way, so we would like to see these values approach 8 We plot 5 different values for N: 8, 1 00, 1 ,000, 1 0,000, and 1 00,000 Before moving on, please note that these numbers are a snapshot in time on one very specific machine Try not to read too much into them, particularly comparing the absolute numbers between the managed the Vista thread pools Focus on the larger picture It is interesting to note the case in which N is 8 We see that the "break even" point occurs when C is around 1 2,500 for the CLR and 25,000 for Windows Vista : in other words, this is when the speedup exceeds 1 0, and, therefore, the parallel version beats the sequential version in terms of execution time In the other cases, the degradation at the low end of
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FI G U R E 73: Para llel speed u p with sim ple work decom position
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the graph is caused by more contention to dispatch work: high values of N with small values of C means the thread pool will have to revisit the shared queue often In fact, the amount of synchronization is some factor of N One useful technique to avoid the synchronization and constant over heads associated with dispatching each new work item is to logically chunk
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work together algorithmically rather than relying on the dynamic partitioning of the thread pool In this example, we could statically parti tion the number of tasks so that each thread receives the same number of disjoint work items, that is, N I P In other words, in pseudo-code, rather than doing the following
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