What happens if I try to use more cores than I have?

Question

In my sparkconf, i can set the number of cores to use, i have 4 physical, 8 logical on my laptop, what does spark do if I specify a number that was not possible on the machi

Answer 1

Number of cores doesn't describe physical cores but a number of running threads. It means that nothing really strange happens if the number is higher than a number of available cores.

Depending on your setup it can be actually a preferred configuration with value around twice a number of available cores being a commonly recommended setting. Obviously if number is to high your application will spend more time on switching between threads than actual processing.