Performance issue with fiber

[drug]

28.07.2021 17:39, Mathias LANG пишет:
> On Wednesday, 21 July 2021 at 22:51:38 UTC, hanabi1224 wrote:
>> Hi, I'm new to D lang and encounter some performance issues with 
>> fiber, not sure if there's something obviously wrong with my code.
> 
> I took a quick look, and the first problem I saw was that you were using 
> `spawnLinked` but not replacing the scheduler.
> `std.concurrency` uses a global `scheduler` variable to do its job.
> Hence doing:
> ```diff
> - auto scheduler = new FiberScheduler();
> + scheduler = new FiberScheduler();
> ```
> 
> Will ensure that `spawnLinked` works as expected.
> There are a few other things to consider, w.r.t. fibers:
> - Our Fibers are 4 pages (on Linux) by default;
> - We have an extra guard page, because we are a native language, so we 
> can't do the same trick as Go to auto-grow the stack;
> - Spawning fibers *is* expensive and other languages reuse fibers (Yes, 
> Go recycle them);
> 
> The `FiberScheduler` implementation is unfortunately pretty bad: it 
> [does not re-use 
> fibers](https://github.com/dlang/phobos/blob/b48cca57e8ad2dc56872499836bfa1e70e390abb/std/concurrency.d#L1578-L1599). 
> I believe this is the core of the issue.

I profiled the provided example (not `FiberScheduler`) using perf. Both 
dmd and ldc2 gave the same result - `void filterInner(int, int)` took 
~90% of the run time. The time was divided between:
	`int std.concurrency.receiveOnly!(int).receiveOnly()` - 58%
	`void std.concurrency.send!(int).send(std.concurrency.Tid, int)` - 31%

So most of the time is messages passing.

Between the fibers creating took very few time. Perf output contains 
information only of `void std.concurrency.FiberScheduler.create(void 
delegate()).wrap()` which took less than 0.5%. But I wouldn't say that I 
did the profiling ideally so take it with a grain of salt.