Add more constructors

[torkleyy]

• Use the number of cpus as pool size
• Allows sharing the thread pool

[kvark]

Why don't you want to simply remove the constructor that accepts a number of threads? I don't see many consumers of this one, and they can work around by creating ThreadPool themselves.

[torkleyy]

As @nchash wrote

Maybe add a Planner::new, taking a usize and Planner::new_num_cpus. There are use cases where you don't need an Arc and you need to set the number of threads (for example to benchmark performance with and without multithreading).

And from the user's perspective it isn't obvious that the way you get a Planner with a set number of threads is call Planner::from_pool.

Additionally, doing it that way is non breaking and easier to understand what it does from the function name.

And there's not really a disadvantage of having one more function.

[kvark]

Well, I don't agree with @nchashch proposal here. I think new() should be dumb simple and good for everyone, so having no parameters is great.

There are use cases where you don't need an Arc and you need to set the number of threads (for example to benchmark performance with and without multithreading).

The non-multithreaded mod is a bigger task - #72 , we don't need to cater for it since we don't yet know how it is going to work.

And from the user's perspective it isn't obvious that the way you get a Planner with a set number of threads is call Planner::from_pool.

As I said, a regular user would not care. An advanced user would figure from_pool out.

Additionally, doing it that way is non breaking ...

Don't worry about breaking too much. We are on the verge of 0.8, which is fine to break API a little.

... and easier to understand what it does from the function name.

The easiest thing to understand is new() without parameters, and that's what I'm arguing for.

And there's not really a disadvantage of having one more function.

Sure thing, as long as you keep the most used case unencumbered. So, it's fine to have an extra with_num_threads constructor, but not fine to have usize parameter for new().

[torkleyy]

@kvark Good idea, did that

[kvark]

Thanks @torkleyy ! Last things - make CI happy and squash the commits please.