问题
Can two processes simultaneously run on one CPU core, which has hyper threading? I learn from the Internet. But, I do not see a clear straight answer.
Edit: Thanks for discussion and sharing! My purse to post my question here is not to discuss about parallel computing. It will be too big to be discussed here. I just want to know if a multithread application can benefit more from hyper threading than a multi process application. After further reading, I have following as my learning notes.
1) A Hyper-Threading Technology enabled CPU Core has two set of CPU state and Interrupt Logic. Meanwhile, it has only one set of Execution Units and Cache. (I have not study what is pipeline yet)
2) Multi threading benefits from Hyper Threading only if there is latency happen in some executed thread. I think this point can exactly map to the common reason for why and when software programmer use multi thread. If the multi thread application has been optimized. It may not gain any benefit from Hypter threading.
3) If the CPU state maps to process state, I believe Marc is correct that multiple process application can even benefit more from hyper threading technology.
4) When CPU vendor says "thread", it looks like their "thread" is different from thread that I know as a java programmer?
回答1:
No, a hyperthreaded CPU core still only has a single execution pipeline. Even though it appears as two CPUs to the overlying OS, there's still only ever one instruction being executed at any given time.
Hyperthreading was intended to allow the CPU to continue executing one thread while another thread was stalled waiting for a resource or other operation to complete, without leaving too many stages of the pipeline empty and useless. This goes back to the Pentium 4 days, with its absurdly long pipeline - a stall was essentially catastrophic for efficiency and throughput, and hyperthreading allowed Intel to keep the cpu busy doing other things while it cleaned up from the stall.
回答2:
While Marc B's answer is pretty much the definitive summary of how HT works, I just want to make a small contribution by linking this article, which should clear up a lot of things about HT: http://software.intel.com/en-us/articles/performance-insights-to-intel-hyper-threading-technology/
回答3:
Short answer, yes.
A single core cpu(a processor), can run 2 or more threads simultaneously. These threads may belong to the one program, or they may belong different programs and thus processes. This type of multithreading is called Simultaneous MultiThreading(SMT).
Information that claims cpu core can execute only one instruction at any given time is also not true. Modern CPUs exploit Instruction Level Parallelism(ILP) by duplicating pipeline resources(e.g 2 ALUs instead of 1). This type of pipeline is called "superscalar" pipeline.
Wikipedia page of Simultaneous Multithreading:
Simultaneous multithreading
来源：https://stackoverflow.com/questions/8916723/can-two-processes-simultaneously-run-on-one-cpu-core