How many threads should I use to render an image?

Question

I've seen very similar questions about, but none seem to offer any scientific evaluation. My problem is that I want to optimize the number of threads being used in my Java thread pool for a fractal rendering application.

By splitting the image into sub-images, I am able to render concurrently as each sub-image is completely independent.

So my thoughts are to make a fixed thread pool based on the number of available threads in the processor(s). Each thread can then render one or more sub-images. If I do it this way, what ratio of threads-to-sub-images should I use? Is there a better way of splitting the load?

If only I had lots of computers with different numbers of cores, I could find out for myself with an experiment. Unfortunately I don't have this luxury =[

Is your sub-image rendering all done with the CPU, or are there also disk accesses to consider? — Gilbert Le Blanc, Mar 14 '13 at 13:05
There is no disk access. As for the 'entirely CPU' part, I'm now about to extend my algorithm to utilize the GPU as well. — Chris Watts, Mar 14 '13 at 13:19
http://stackoverflow.com/questions/13834692/threads-configuration-based-on-no-of-cpu-cores — assylias, Mar 16 '13 at 13:50

Philipp · Answer 1 · 2013-03-16T12:52:27.877

It is usually ideal to have one thread per processor, under the premise that no other CPU-heavy applications are running on the system at the moment.

The rationale behind this is that switching between threads is a expensive operation (in relation), so the best way to use a multicore CPU efficiently is to avoid thread switching while still giving the CPU enough threads so that each core is utilized.

The number of CPU cores of the system can be obtained with

Runtime.getRuntime().availableProcessors();

On a system with hyper threading, this will give you the number of virtual CPUs, not physical ones. When you run one thread per virtual core instead of one per physical core, each thread might take a bit longer to complete, but overall they get more work done in the same time because the internal resources of the CPU can be used more efficiently. So you should run one per virtual core, unless it's important to you that each individual thread finishes as fast as possible (in that case you might consider to use single-threading, because multicore intel CPUs which execute a single thread overclock the core it is running on and switch it around between the cores so that the others can cool down).

For an application which you are going to ship to end-users, you might consider to use this value as a default, but allow them to change it, because the user might want to run your program in parallel with other CPU-intense applications. In that case they might want to reduce the number of threads spawned by your application to not slow down the others.

Sounds good, but will this really work out considering Java chooses where to hand the threads to so there's no guarantee there will be an even distribution across all threads? Furthermore, the scheduler can randomly suspend any of these threads as it pleases, so at any given moment, maybe only a few of the threads will be running. — Chris Watts, Mar 14 '13 at 13:23

How many threads should I use to render an image?

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