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I have been reading lately about system architecture and the topic of multi-threading has not been covered in detail with latest improvements in technology. I did my part of search, but could not find answers for the following:

The questions have are

1) Is multi-threading dependent on the system architecuture (CPU). do all CPU (single core) support multi-threading? If it does not, what happens to multi-threaded applications when run on those machines

It is cited here that

Intel CPUs support multithreading, but only two threads per CPU.
AMD CPUs do not support multithreading and AMD often sites Microsoft's 
recommendations to turn off Hyperthreading on Intel CPUs when running applications 
like peoplesoft and Exchange.

2) so what does it mean it say only two threads per CPU here. At any given time, CPU (single core) can process only thread. and the other thread is waiting to be processed correct?

3) how is it different from an application that spawns, say, 10 threads and waiting for them to be executed. If the CPU at the most can tackle only two threads, shouldn't programmer keep that fact in consideration when writing multi-threaded applications.

Even with multi-core processors (say quad-core) at the most 8 threads can be queued, but only 4 threads can be processed at the same time.

P.S: I have a read a little about hyper-threading but I am not sure if that is relevant here and if all processors support hyper-threading

brain storm
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2 Answers2

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1) It depends on the operating system more than anything. Even for single core architectures, multi-threading can be supported, but the threads are not executing in parallel - The OS will context-switch between them.

2) Intel usually supports two-way hardware threading ( also called simultaneous multi-threading), where each thread is allocated a pipeline. So if you have a process with two threads they can both execute on the same core simultaneously.

3) See 1. Basically the operating system is going to allocate as many threads as it can to hardware before it plans to context-switch between the threads it couldn't allocate. This process is dependent on the OS's scheduler, and you can read about the Linux one to get a good idea of what's going on.

Edit: Hypethreading is basically the hardware threading feature I mentioned.

Albert Myers
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  • for point 2) that you answered: process with two-threads can be executed on same core, but not at the same time correct? – eagertoLearn Jan 16 '14 at 19:10
  • No, they can actually execute simultaneously. It's similar to how superscalar processors work, but a pipeline can be dedicated to a thread. So for each CPU cycle, the CPU can issue and retire instructions for two different threads. – Albert Myers Jan 16 '14 at 19:12
  • wow!didnt know that a single core can handle two threads simultaneously. but this one says it does not: http://stackoverflow.com/questions/10183675/can-2-threads-run-concurrently-or-just-1-given-the-possibility-of-hyperthread?rq=1 – eagertoLearn Jan 16 '14 at 19:14
  • I think the confusion is that Linux recognizes each "hardware thread" as a _logical_ cpu, and that's what showing up here. – Albert Myers Jan 16 '14 at 19:17
  • do you have any online references that would go in detail. most of them I read limit with single-cores.. – brain storm Jan 16 '14 at 19:19
  • Can you expand on what you are saying? I don't understand when you say "limit with single-cores". If you want a description of hardware (SMT) threads this works pretty well: http://en.wikipedia.org/wiki/Simultaneous_multithreading – Albert Myers Jan 16 '14 at 19:25
  • do compilers and JVM have any part? I mean are compilers are different for machines that support multi-threading and machines that does not? – brain storm Jan 16 '14 at 19:33
  • As far as compilers are concerned, it depends. At the instruction level, you have to deal with the classic atomic operation problem associated with concurrency. There are often explicit 'lock' instructions in the instruction set and even more sophisticated instructions dealing with memory fences. As far as writing multi-threaded software you can go the classic approach with something like pthreads or use compiler directives through OpenMP, both of which are C/C++. The latter is easier to do but you lose some control over synchronization. You might try both as they have their benefits. – Albert Myers Jan 16 '14 at 19:43
  • I'm not very familiar with Java, so I can't comment on it. I think Java does offer some concurrency libraries though. – Albert Myers Jan 16 '14 at 19:44
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In your question CPU means core.

1) It does. I believe memory access on ARMs is in words, so write to char is not atomic Also memory ordering differs Modern OSes (anything but DOS) support context switching: while one thread executes, others wait. Total number of threads in all Windows processes is about 1000. Common time quant (time to load CPU) is 1-10 ms. One core multithreading don't improve computational power but allows asynchronous tasks. For example GUI doesn't freeze during network activity. One threads waits net, another one responds to user activity.

2) Yes

3) It is common practice to spawn number of threads equal to number of (virtual) cores, ie number of cores in system for AMD and twice for Intel. It is true only for computational threads. Web server threads usually wait net and don't load CPU a lot, so it is better to spawn thousands of threads.

Hyperthreading is cool for tasks that wait RAM. While one thread waits data another one executes. For math it usually not increase performance. It is good for work with data that is not cache-friendly: lists, trees, hash tables that don't fit into cache.

Aleksandr Pakhomov
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