Auto-detect number of cpus

[AlecThomson]

Hey @yymao,

Thanks so much for this awesome tool!

This is just a minor PR to change the number of default threads from the hard-coded 8, and use the number of detected cores instead from os.cpu_count()

[yymao]

Thanks for the PR @AlecThomson! Can you say a bit about the motivation for this change? I limited the default to 8 to ensure the number of requests we send to ADS are not too high within a small period of time, especially given that nowadays many computers comes with a high number of cores.

[AlecThomson]

Hey @yymao, that's a fair concern. My motivation came from using a 12-core laptop and wanting to get the speed boost by default. I haven't DDoS-ed ADS so far.

If thread limit is a concern, I it would probably be better to enforce a global limit rather than a default. For example, we could set a constant like:

MAX_CORES = 16 # Do not allow more than 16 threads to hit ADS
...
if args.parallel:
    threads = args.threads
    if threads is None:
        threads = os.cpu_count() or 1
        # enforce global maximum
        threads = min(threads, MAX_CORES)
    Parallel(n_jobs=threads, prefer="threads")(delayed(update)(key) for key in keys)

Here I've guessed 16 as a maximum. I'm not sure what the most reasonable limit would be...

[yymao]

Thanks @AlecThomson.

Given this conversation, I am not sure if this edit is actually needed. I think capping the threads to MAX_CORES is functionally identical to hard coding MAX_CORES . If the machine has more cores available, we don't want to use them anyway. If the machine has fewer cores available, setting a higher number would still work ok for joblib and the overhead is not significant in this case.

If I set MAX_CORES to 8, then I'll need to create a new option so that, in your specific use case, you can change the MAX_CORES to 12. This doesn't seem to be easier for the user than just changing the threads option.

In addition, there's now a way to set default options via an environmental variable. For example, if someone wants to always use all available cores, they can set:

export ADSTEX_ARGS="--parallel --threads=-1"

Does this make sense?