问题
Can someone shed some light on it?
An i7 processor can run 8 threads but I am pretty sure we can create more than 8 threads in a JAVA or C++ program(not sure though). I have an i5 processor and while studying concurrency I have created 10 threads for assignments. I am just trying to understand how Core rating of CPU is related to threads.
回答1:
The thread you are refering to is called a software thread; and you can create as many software threads as you need, as long as your operating system allows it. Each software thread, or code snippet, can run concurrently from the others.
For each core, there is at least one hardware thread to which the operating system can assign a software thread. If you have 8 cores, for example, then you have a hardware thread pool of capacity 8. You can map tens or hundreds of software threads to this 8-slot pool, where only 8 threads are actually running on hardware at the same time, i.e. in parallel.
Software threads are like people sharing the same computer. Each one can use this computer up to some time, not necessarily have his task completed, then give it up to another.
Hardware threads are like people having a computer for each of them. All of them can proceed with their tasks at the same time.
Note: For i7, there are two hardware threads (so called hyper-threading) in each core. So you can have up to 16 threads running in parallel.
回答2:
There are already a couple of good answers talking about the hardware side of things, but there isn't much talk about the software side of things.
The essential fact that I believe you're missing is that not all threads have to be executing all the time. When you have thousands of threads on an 8 core machine, only a few of them are actually running at any given time. The others are sitting around doing nothing until some processor time becomes free. This has huge advantages because threads might be waiting on other resources, too. For example, if I have one thread trying to read a file from disk, then there's no reason for it to be taking up CPU time while it's waiting for the hard disk data to load into RAM. Another example is when the thread is waiting for a response from some other machine (such as a web request over the internet). When you have more threads than your processor can handle at once, the operating system and/or runtime (It depends on the OS and runtime implementation.) are responsible for deciding which threads should get available processor time. This kind of set up lets you maximize your machine's productivity because CPU cycles are doing something useful almost all the time.
回答3:
A "thread" is a software abstraction which defines a single, self-consistent path of execution through the program: in most modern systems, the number of threads is basically only limited by memory. However, only a comparatively small number of threads can be run simultaneously by the CPU. Broadly speaking, the "core count" is how many threads the CPU can run truly in parallel: if there are more threads that want to run than there are cores available, the operating system will use time-slicing of some sort to let all the threads get some time to execute.
There are a whole bunch of terms which are thrown around when it comes to "cores:"
- Processor count: the number of physical CPU chips on a system's motherboard. This used to be the only number that mattered, until CPUs with multiple cores became available.
- Logical core count: the number of threads that the system hardware can run in parallel
- Physical core count: the number of copies of the CPU execution hardware that the system has -- this is not always equal to the logical core count, due to features like SMT ("simultaneous multithreading") which use a single piece of hardware to run multiple threads in parallel
- Module count: Recent (Bulldozer-derived) AMD processors have used an architecture which is a hybrid between SMT and the standard one-logical-core-per-physical-core model. On these CPUs, there is a separate copy of the integer execution hardware for each logical core, but two logical cores share a floating-point unit and the fetch-and-decode frontend; AMD calls the unit containing two logical cores a module.
It should also be briefly mentioned that GPUs, graphics cards, have enormous core counts and run huge numbers (thousands) of threads in parallel. The trade-off is that GPU cores have very little memory and often a substantially restricted programming model.
回答4:
Threads are handled by the OS's scheduler. The number of cores in a CPU determine how many threads it can run at the same time.
Note that threads are constantly switched in and out by the scheduler to give the "illusion" that everything is running at the same time.
More here, if you're interested.
回答5:
no, no, no... Your I7 has eight execution threads and can run 8 threads at once.
1000 threads or more can be waiting for processor time.
calling thread.sleep moves a thread off the execution core and back into memory where it waits until woken.
来源：https://stackoverflow.com/questions/18264592/what-is-the-relationship-between-threads-in-a-java-or-a-c-program-and-number