A multi-core processor is a single computing component with two or more independent actual central processing units (called "cores"), which are the units that read and execute program instructions.
If a multithreaded application runs on a multi-core processor, how many CPUs will is use? For example, if the machine is capable of dual core execution, then 2 CPUs will be used, if my understanding is correct. Within these two CPUs, multiple threads will be executed and do the context switching.
If a Mulithreaded application runs on multi-core processor, how many CPU it will use, for example if the machine is capable of doing the dualcore, then 2 CPU will be used is my understanding is correct, and within these two CPU multiple thread will be executed and do the context switching.
The JVM really doesn't deal directly with processors. It uses the native thread capabilities of the operating system which uses the processors that are exposed by the operating system and hardware. In Java there is a Runtime.availableProcessors() method but this in a only a few places by the JVM code.
To the JVM or any other application running on a computer, the multiple cores typically seem the same as multiple processors if that's how the OS exposes them. This means that the distinction between physical processors versus multiple cores in a single processor is completely hidden from the Java programmer.
There are single core CPUs then there are CPUs with multiple cores which share certain internal components but the OS sees them and schedules them as multiple processors. Multiple cores are most likely seen to the OS as multiple CPUs -- there is no distinction. Then there are the virtual processors often called hyperthreading which share the same processor core (and the associated processing circuitry) but have multiple execution pipelines. These are also (usually) seen by the OS as multiple processors.
Specifically, in the OP's example, you have a single processor with two cores, in linux cat'ing /proc/cpuinfo will show 2 processors and in Java the Runtime.availableProcessors() will return 2. It will also return 2 if you have 2 physical processors also will most likely if you have a single core with dual hyperthreading pipelines depending on the OS kernel.
As to how many processors the JVM will actually be using, this depends again on the native thread code. This said, if the JVM is running on a single CPU with two cores, and the cores are not in use by other applications or the OS, then the JVM's thread will most likely be scheduled to run on them concurrently.
By default you can utilize all processors. One processor can run virtually as many threads as possible at the same time (virtually means that physically there's always just one thread which is running). How many is possible depends on the operating system resource limitations and the used threading framework.
It doesn't matter from software point of view, if the cores are on one die, and there's one CPU socket with a multi-core CPU, or there are more CPU sockets. The OS and JVM will see the collection of the cores. (This brings in an interesting aspect though: data exchange between such cores which are on the same die and those which are in different sockets are not uniform).
Thread schedulers (talking about both the OS's and the virtual machine's) often tend to shuffle and move threads from one core to another throughout scheduler time. That can hurt performance, there are techniques to tie a thread to a certain core (thread affinity).
How much cpu resources your application (lets assume long running task) will really consume depends on how much percentage you need your cpu. Application can be network, memory, harddisk or cpu bound and a few others.
If the cpu has to wait for any other resource such as memory or network it will remain idle or be assigned to other threads.
Example:
If your application is only cpu bound (won't consume much memory) and you run a long task with as many threads as cores (physical or virtual with hyperthreading) you will get almost 100% usage of the free resources that are not used by other running threads (os, programms).
Depending on the program you can tell in which state your application is from the cpu/memory/network consumption and you can analyse the performance.
From programmer's point of view, a core is a processor. Method Runtime.availableProcessors() shows the number of cores. However, from manufacturer's point of view, multi-core processor is similar to ordinary processor, so they decided to leave the name "processor", probably making a marketing mistake.
JVM. So JVM will be responsible for actual CPU utilization. - PM 77-1