Threading challenge: calculate fib(45) while spinning
Steven Schveighoffer
schveiguy at gmail.com
Fri Oct 15 13:52:19 UTC 2021
On 10/14/21 11:35 PM, jfondren wrote:
> The book, "The Go Programming Language" has this simple goroutine example:
>
> ```go
> func main() {
> go spinner(100 * time.Millisecond)
> const n = 45
> fibN := fib(n) // slow
> fmt.Printf("\rFibonacci(%d) = %d\n", n, fibN)
> }
>
> func spinner(delay time.Duration) {
> for {
> for _, r := range `-\|/` {
> fmt.Printf("\r%c", r)
> time.Sleep(delay)
> }
> }
> }
>
> func fib(x int) int {
> if x < 2 {
> return x
> }
> return fib(x-1) + fib(x-2)
> }
> ```
>
> Attempt #1, with std.concurrency:
>
> ```d
> import std.concurrency : spawn;
> import core.thread : Thread;
> import std.stdio : writefln, writef, stdout;
> import std.datetime : msecs, Duration;
>
> void main() @safe {
> (() @trusted { spawn(&spinner, 100.msecs); })();
> const n = 45;
> const fibN = fib(n); // slow
> writefln!"\rFibonacci(%d) = %d"(n, fibN);
> }
>
> void spinner(Duration delay) @safe {
> (() @trusted { Thread.getThis.isDaemon(true); })();
> while (true) {
> foreach (char c; `-\|/`) {
> writef!"\r%c"(c);
> (() @trusted { stdout.flush; })();
> (() @trusted { Thread.sleep(delay); })();
> }
> }
> }
>
> int fib(int x) pure @safe @nogc {
> if (x < 2)
> return x;
> return fib(x - 1) + fib(x - 2);
> }
> ```
>
> This version has two problems:
>
> 1. a race condition with `isDaemon`: if `main()` ends before
> `isDaemon(true)` is called, then the program never ends because the
> kill-non-daemon-threads module destructor is called while the new thread
> isn't a daemon thread.
You can also just spawn a thread directly with `Thread`, which I believe
allows you to set the daemon-ness from `main`.
>
> 2. it crashes about 10% of the time on exit (in dmd, gdc, and ldc).
> valgrind on a gdc build complains about "Conditional jump or move
> depends on uninitialised value(s)" early on.
The crash is likely because you are using D i/o utilities, and the
runtime is shut down. Technically it shouldn't cause a problem, but
possibly there are things that are needed deep inside `writef`.
If you switch to `printf`, it will probably work.
-Steve
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