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Reasons that processes utilize threading
- Programming abstraction. Dividing up work and assigning each division to a unit of execution (a thread) is a natural approach to many problems. Programming patterns that utilize this approach include the reactor, thread-per-connection, and thread pool patterns. Some, however, view threads as an anti-pattern. The inimitable Alan Cox summed this up well with the quote, "threads are for people who can‘t program state machines."
- Blocking I/O. Without threads, blocking I/O halts the whole process. This can be detrimental to both throughput and latency. In a multithreaded process, individual threads may block, waiting on I/O, while other threads make forward progress. Blocking I/O via threads is thus an alternative to asynchronous & non-blocking I/O.
- Memory savings. Threads provide an efficient way to share memory yet utilize multiple units of execution. In this manner they are an alternative to multiple processes.
- Parallelism. In machines with multiple processors, threads provide an efficient way to achieve true parallelism. As each thread receives its own virtualized processor and is an independently schedulable entity, multiple threads may run on multiple processors at the same time, improving a system‘s throughput.
The number of processors and the number of threads
The are two types of processors that we can encounter while doing parallel programming: physical processors and logical processors. The number of logical processors (processors that the operating system and applications can work with) is (usually) greater or equal to the number of physical processors (actual processors on the motherboard). For example, a hyperthreaded processor with 4 physical processors will have 8 logical processors. That means that the operating system can schedule up to 8 threads at the same time, even though on the motherboard you only have 4 processors. Therefore, the maximum number of threads you can create is equal to the number of logical processors your operating system sees. Core duo and core 2 duo are not hyperthreaded, hence the maximum number of threads you can create is 2, since you have 2 physical processors and 2 logical processors.
You can run more threads than you have logical processors. However, unless those threads are performing I/O, using more threads than you have logical processors will generally be less effective than using the number of threads as you have logical processors.
Discuss: Threads configuration based on no. of CPU-cores
Performance test with a process doesn‘t do I/O. From: Optimal number of threads per core.
to be continued ...
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原文地址：http://www.cnblogs.com/william-cheung/p/5771410.html
