[cpeterso]

> 6. Didn't try to convince anyone runtime.GOMAXPROCS(runtime.NumCPU()) was a good idea (it's not)

So what is an appropriate GOMAXPROCS? As someone who has only dabbled in a few Go tutorials, I would imagine that you would want GOMAXPROCS to be NumCPU() (or even greater) so the goroutine thread pool could "fire on all pistons". Why does Go's scheduler default to GOMAXPROCS=1 instead of NumCPU()?

[dlsspy]

Or turning that around, do you think it requires all 8 of my cores to copy data over the network? Do you think the two lines of code + justification text provides sufficient value for this application to distract from the point of it in order to show why someone should override the default behavior?

Do you believe users shouldn't have any control over the number of cores any particular application consumes?

Have you measured the CPU contention of the application and determined that using more cores is worth the overhead of increased overhead of multi-thread exclusions (vs. more simple things happening directly in the scheduler)?

Overall, it has nothing to do with this article and now even more people are going to copy it in more unnecessary places as a cargo-cult "turbo button" for their programs.

If you are going to use an idiom like that, the least you could do is check for the GOMAXPROCS environment variable and only do this as a default when the user hasn't specified otherwise.

[smosher]

In my experience the majority of well-written concurrent programs become I/O-bound on a single processor. It's pointless to add more processors to that, and can only slow you down, and Go programs are far better behaved in the non-parallel case.

At other times you should think about the number of processors you want to occupy. If the objective is to behave like an appliance, then 1:1 schedulers:cpus is not a bad ballpark.

[dlsspy]

Then why isn't that the default?

[smosher]

The default is 1 isn't it? As I pointed out, this will serve the majority of concurrent code.

The best number of processes to use is equal to the parallelism of the solution. Even with highly concurrent problems, this is still most often 1. If you get it wrong performance will suffer. But in practical terms we have more to worry about, and if you're talking to the disk and the network more than you're computing, parallelism will only increase the contention on those resources. The extra processes will consume more CPU without doing any more useful work.

So the default is pretty good.

By the way 1:1 isn't the limit either. Sometimes you will want more. If the problem truly is parallel enough to exceed your CPUs, you may want additional processes anyway. This will keep things up to speed thanks to the host's scheduler which is typically preemptive, unlike Go's. This sometimes works much better if you can pick and choose which routines run on which schedulers, and I'm not sure if Go exposes that.

[dlsspy]

I have no idea what my response was about. As I said, I was pretty sick that day. Sorry you had to type all this stuff to explain to me that I'm a moron. :)