Hi all,
Say if I have a processor like this which says # cores = 4, # threads = 4 and without Hyper-threading support. Does that mean I can run 4 simultaneous program/process (since a core is capable of running only one thread)? Or does that mean I can run 4 x 4 = 16 program/process simultaneously? My guess is the first one. From my digging, if no Hyper-threading, there will be only 1 thread (process) per core. Correct me if I am wrong.
That's basically correct, with the obvious qualifier that most operating systems let you execute far more tasks simultaneously than there are cores or threads, which they accomplish by interleaving the executing of instructions.
A system with hyperthreading generally has twice as many hardware threads as physical cores.
The term thread is generally used as a description of an operating system concept that has the potential to execute independently of other threads. Whether it does so depends on whether it is stuck waiting for some event (disk or screen I/O, message queue), or if there are enough physical CPUs (hyperthreaded or not) to allow it run in the face of other non-waiting threads.
Hyperthreading is a CPU vendor term that means a single core, that can multiplex its attention between two computations. The easy way to think about a hyperthreaded core is as if you had two real CPUs, both slightly slower than what the manufacture says the core can actually do.
A thread differs from a process. A process can have many threads. A thread is a sequence of commands that have a certain order. A logical core can execute on sequence of commands. The operating system distributes all the threads to all the logical cores available, and if there are more threads than cores, threads are processed in a fast cue, and the core switches from one to another very fast.
It will look like all the threads run simultaneously, when actually the OS distributes CPU time among them.
Having multiple cores gives the advantage that less concurrent threads will be placed on one single core, less switching between threads = greater speed.
Hyper-threading creates 2 logical cores on 1 physical core, and makes switching between threads much faster.
Basically this is up to the OS. A thread is a high-level construct holding a instruction pointer, and where the OS places a threads execution on a suitable logical processor. So with 4 cores you can basically execute 4 instructions in parallell. Where as a thread simply contains information about what instructions to execute and the instructions placement in memory.
An application normally uses a single process during execution and the OS switches between processes to give all processes "equal" process time. When an application deploys multiple threads the processes allocates more than one slot for execution but shares memory between threads.
Normally you make a difference between concurrent and parallell execution. Where parallell execution is when you actually physically execute instructions of more than one logical processor and concurrent execution is the the frequent switching of a single logical processor giving the apperence of parallell execution.