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Let's say I have a 4-core CPU, and I want to run some process in the minimum amount of time. The process is ideally parallelizable, so I can run chunks of it on an infinite number of threads and each thread takes the same amount of time.
Since I have 4 cores, I don't expect any speedup by running more threads than cores, since a single core is only capable of running a single thread at a given moment. I don't know much about hardware, so this is only a guess.
Is there a benefit to running a parallelizable process on more threads than cores? In other words, will my process finish faster, slower, or in about the same amount of time if I run it using 4000 threads rather than 4 threads?
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Each CPU core can have two threads. So a processor with two cores will have four threads. A processor with eight cores will have 16 threads. A processor with 24 cores (yes, those exist), will have 48 threads.
Ideally the total thread count for all the jobs should be the number of cores of the system, except on systems that support hyper-threading, in which it should be twice the number of cores. So if the system doesn't have hyper-threading, there are 8 calculations running, each should run in one thread.
A single CPU core can have up-to 2 threads per core. For example, if a CPU is dual core (i.e., 2 cores) it will have 4 threads. And if a CPU is Octal core (i.e., 8 core) it will have 16 threads and vice-versa.
If your use is for multithreaded enabled production apps, then 8 threads is best. But, if your usage is for gaming, then there is little value in having more than 4 threads. Most games can effectively use only 2-3 threads. For games, single thread performance is more important.
If your threads don't do I/O, synchronization, etc., and there's nothing else running, 1 thread per core will get you the best performance. However that very likely not the case. Adding more threads usually helps, but after some point, they cause some performance degradation.
Not long ago, I was doing performance testing on a 2 quad-core machine running an ASP.NET application on Mono under a pretty decent load. We played with the minimum and maximum number of threads and in the end we found out that for that particular application in that particular configuration the best throughput was somewhere between 36 and 40 threads. Anything outside those boundaries performed worse. Lesson learned? If I were you, I would test with different number of threads until you find the right number for your application.
One thing for sure: 4k threads will take longer. That's a lot of context switches.
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