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What is different between a program executing simultaneously and another executing concurrently on single core processor?

Darshan Makwana
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    It sounds synonymous; what makes you think they would be different? – B. Clay Shannon-B. Crow Raven Dec 08 '15 at 18:19
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    Do you mean [Concurrent vs Parallel](http://stackoverflow.com/questions/1897993/difference-between-concurrent-programming-and-parallel-programming)? – Yuval Itzchakov Dec 08 '15 at 18:19
  • As i read java multithreading two thread executes simultaneouly but when in multithreading uses context switching than only they executes one bye one... Is it simultaneous ? – Darshan Makwana Dec 08 '15 at 18:22
  • Whether the JVM is able to execute 2 threads simultaneously depends on how many CPU cores are available to the VM. The maximum number of concurrent threads that can execute in any system is the number of CPU cores available. If there are more threads/tasks than cores then the OS will schedule each task according to it's priority scheme. – JJF Dec 08 '15 at 18:28

2 Answers2

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Concurrent execution is overlapping, simultaneous is at the same time. This might involve understanding of OS and hardware scheduling.

You can perfectly see the difference in example of multicore processor with instructions pipelining.

Simultaneous: different cores executing at the same time Concurrent: Instructions can overlap over processing units by each part (FETCH, DECODE, EXECUTE, MEMORY WRITE, ETC)

Pipeline also concurrent

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Multi-core Processing

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Iliiaz Akhmedov
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  • I think an explanation of how what appears to be simultaneous or concurrent execution of a set of programs on a single core processor is implemented might help him and a comparison of that type of system to what really is simultaneous execution of multiple programs on a multi-core system. – JJF Dec 08 '15 at 18:25
  • Frm this we can say that every simultaneous execution is concurrent but reverse is not true? – Darshan Makwana Dec 08 '15 at 18:26
  • @DarshanMakwana This concepts might overlap, it really depends what operating system, scheduling, hardware, language, framework you are talking about. – Iliiaz Akhmedov Dec 08 '15 at 18:28
  • @DarshanMakwana I make no distinction between those two words. – JJF Dec 08 '15 at 18:30
  • In single core processor can two thread execute simultaneously or they require time slicing to execute ? – Darshan Makwana Dec 08 '15 at 18:42
  • In the core of OS or hardware CPU? on CPU in a core 2 instructions can overlap, but obviously not run at the same time in comp units – Iliiaz Akhmedov Dec 08 '15 at 18:44
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Two simultaneous process must be concurrent, but two concurrent process may not be simultaneous. Two Simultaneous process must be executed at sime time. But two concurrent process may execute at different times.

Wai Ha Lee
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sohan nohemy
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  • Just going by the dictionary definitions of the words they are synonymous. I don't make the same distinction you do. – JJF Dec 08 '15 at 18:33