Cooperative pool deadlock when calling into an opaque subsystem

[ole (Ole Begemann)]

@gwendal.roue

I made another test, this time with a semaphore that allows more parallel readers than there are CPU cores.

There is still no deadlock, as predicted by @Joe_Groff, but we do have "too many" threads. This is not the classic and pathological thread explosion - user is still in control. But we don't exhibit the gracious adaptation of the number of CPU cores that would be satisfying to see

Yes, as long as you stay under GCD's max thread limit (64 on macOS in my tests) you should be fine in terms of no deadlock.

There are OS-specific APIs to query the number of CPU cores. On macOS you can do it like this (APIs on Linux etc. will probably be different):

import Darwin

var size: size_t = 4
var cpuCount: Int32 = 0
let result = sysctlbyname("hw.logicalcpu", &cpuCount, &size, nil, 0)
if result == 0 {
    print(cpuCount) // prints 10 on my machine (Apple M1 Pro)
}

Look at the sysctl(3) man page for more info.