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Definitions:
Process(task): ist a program in execution. e.g: Notepad
Thread: A thread is a single sequence of instructions. A process consists of one or more threads(but only one can execute at a time).

According to the lecture a single core processor can run a single process(task) at a time.Only one thread can execute at a time but the Operating system achieves Multithreading using time slicing(thread context switch). This Thread switching happens frequently enough that the user perceives the threads as running at the same time (but they aren't running parallel!)and it occurs inside the one process.
A Process context switch is similar to thread context switch with a difference that it takes place between processes (example between mediaplayer und notepad) instead between threads.

I'm not sure if this example is valid : taking two processes e.g: Notepad and Mediaplayer on a single core processor. One can play music and write in a Notepad at the same time although the two processes aren't runnin parallely(Process context switching or multitasking).Inside the one process e.g :Mediaplayer one can listen to music and create playlists at the same time although the two threads aren't running parallely (Thread context switch or multithreading)


one core

1st Question : Are my Information above right ?

2nd Question : would an Execution of Threads in a multicore Process look the same inside a one core but with a difference that the threads of different processes can run parallely?.Is multithreading here the process of running multiple threads simultaneously on difference processes or the process of swiching between threads on a one core ? The same Question would be also for Multitasking. How would the Process context switch and thread context switch in this case take place ?

3rd Question: The Professor used the Term single threaded processor. Is this Term an another name for sigle core Processor ?

enter image description here or
enter image description here several threads belonging to the same process can be executed on several CPU cores simultaneously.Time slicing still happens on multicore systems. Say one have Process with 20 Threads running on a quadcore - the OS still has to schedule 21 Threads to run on only 4 cores.

A single-threaded process runs on only one single core at a time. But that doesn't mean it'll run on the same core until it exits. The OS might give him a time slice to run on Core 1 now, pause it, and give it another time slice on Core 2 later

note : I read a lot of books and i googled enough before i decided to ask here.

EDITED

alixander
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Yes, you seem to have a good understanding of this topic (not sure if it is really interesting, though). However, you seem to overthink it. I suggest a simpler way of understanding the way it works on the modern systems (it is really wild west when you start look back, with the idea of light-weighted processess and such, but I will not talk about it).

The process is a shell. It's only purpose in life is to provide environment for threads. Only the threads are really executed, process itself is never executed. A single process can host multiple threads within it, and when it hosts only one thread, one can say process is executed - but it is simply a manner of saying. A CPU can only execute a thread, not a process.

Your professor, as they often do, makes misleading statements. There is no such thing as single-threaded processor. There are single and multicored processors, and those processors can be joined together to provide multi-processor environment. From the application developer perspective, a single CPU with 4 cores does not differ from 4 single-core CPUs. There are differences, of course - but usually not for the application developer.

Multitasking is a laymen term. It can mean whatever one wants it to mean, and better be avoided in non-specific contexts.

I hope I did clarify your confusiuon.

SergeyA
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    I think @alixander is trying to understand how multiprocessing/multithreading works "under the hood" because (s)he learns about it at the university, so only accepting it as an abstraction that simply works is out of question. Please note, that I might be wrong, – mg30rg Oct 01 '15 at 13:25
  • I am eager to clarify more :), I am waiting for questions. – SergeyA Oct 01 '15 at 13:40
  • @SergeyA : Can threads(a,b,c,d) of Process **α** be executed parallely on multiple cores on a multicore Process ? – alixander Oct 01 '15 at 15:39
  • @alixander, what do you mean by multicore process? – SergeyA Oct 01 '15 at 16:08
  • @SergeyA sorry i mistyped! I meant multicore processor :) – alixander Oct 01 '15 at 16:09
  • @alixander, like I said above, processess are never executed. Only the threads are. – SergeyA Oct 01 '15 at 16:15
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    @alixander Again Process.... I take your question as following: can several threads belonging to the same process be executed on several CPU cores simultaneously? The general answer is yes, they can. This is usually the desired outcome and the whole reason to write a multi-threaded application. However, there are techniques which would prevent this from happening. – SergeyA Oct 01 '15 at 16:21
  • "There is no such thing as single-threaded processor." I think he's refering to hyperthreading/simultaneous multi-threading where a single core can run multiple (hyper/hardware) threads at the same time. – ysdx Oct 02 '15 at 07:13
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The answer to your questions is the following:

Q1: On a single core processor two tasks can't run parallelly in the form of executing two (processor) instructions at the same time, the only possible way of multithreading is time-slicing realized by the task-scheduler (of the OS), so in that case you are approximately right. I would complete your view on the subject with the fact, that nowadays almost none of the applications are single-threaded. I don't know if notepad uses multiple threads, but I'm pretty sure, media player is multithreaded, and the task scheduler schedules time slices between threads not processes. (Fun fact: a single-threaded .NET application already runs 4-5 threads.)

Q2: Task scheduler on any system tries to spread the load between available cores, so time slices will work most likely how you displayed above, but if a process executes an additional thread, it will be executed on the core with the least load over it. Multiple cores also mean, multiple (processor) instructions can and will be executed at the same time.

Q3: In practice multithreaded processor and multicore processor means something very similar, but not the same. You see for example Intel Core i3/i5/i7 CPUs are equipped with an internal pseudo-task-scheduler, which doubles the number of virtual cores by scheduling the execution of two threads on the same core, so for example my i5 system is 2 cored but 4 threaded.

mg30rg
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  • Do not agree with 'almost none of the applications are single-threaded'. There is a very popular school of thought that backend processess are better be single-threaded, for various reasons. – SergeyA Oct 01 '15 at 13:40
  • @SergeyA Well, I was into driver development and nowadays I'm working on a Windows service based backend solution, and I never seen any complex single threaded backend software. (Please note I'm only working on Windows architecture which is more of a multithreaded then a multiprocess OS.) Can you please clarify what school are you talking about? – mg30rg Oct 01 '15 at 14:07
  • without giving too much of my background, which I hate to do, I can't. You can just trust me that last 3 years I've been dealing with very complex, performance-crytical, single-thread applications. – SergeyA Oct 01 '15 at 14:10
  • I believe you. I wasn't critical about there are single threaded backend processes, I was doubtful about the "very popular school" part. But, as I said my experiences are limited to Windows systems on Intel (and compatible) architectures, so I might be mistaken. – mg30rg Oct 01 '15 at 14:16
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    'very' popular is of course subjective. You can replace it with 'popular within the limited circles' – SergeyA Oct 01 '15 at 14:19
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    @SergeyA I'd love to see some links to the reasoning behind this. Unless they mean single threaded but using multiple processes, there are two slam dunk reasons why this is a terrible idea. 1) It makes *all* code performance critical, even code that almost never executes (such as exceptional error handling). 2) It causes a complete loss of service should any code block in a way that can't be avoided, for example, due to an unexpected page fault. (If this is so obscure there isn't even a web page explaining it, the circles are limited indeed!) – David Schwartz Oct 01 '15 at 17:01
  • @DavidSchwartz, you didn't have to write the long comment, you nailed it in the second sentence. Single-threaded, multiple-processess. – SergeyA Oct 01 '15 at 17:09
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your most of concepts seem valid with non standard terms. here is explanation of what are threads and process and then multithreading

process is running instance of program is true

when there were no thread then resources were only distributed among processes.

Now processes have threads so resources are distributed to threads but isolation is same of process means two processes still need IPC to communicate with each other. You can say multithreading as lightweight processes which can be scheduled by operating system. multit-hreading is an extension of multi-tasking so if there is one core and two processes: one with two threads and one with 4 threads, the contention of accessing core is between 6 threads not 2 processes. for thread switch and process switch see thread context switch vs process context switch

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