usleep in Swifter degrades overtime

Question

I have the following server:

import SwiftUI
import Swifter

@main
struct MacInputServer: App {
    let server: HttpServer;

    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }

    func sleep(req: HttpRequest) -> HttpResponse {
        usleep(100_000);
        return .ok(.text(""))
    }

    init() {
        server = HttpServer()
        server["/sleep"] = sleep
        try! server.start(999, forceIPv4: true, priority: DispatchQoS.QoSClass.userInteractive);
    }
}

I'm running while true; do time curl localhost:999/sleep; done to test it. The first ~100 requests finish in about 120ms as expected. But after that it starts hanging for up to 10 seconds:

curl localhost:999/sleep  0.01s user 0.01s system 11% cpu 0.118 total
curl localhost:999/sleep  0.01s user 0.01s system 0% cpu 10.121 total
curl localhost:999/sleep  0.01s user 0.01s system 0% cpu 5.134 total
curl localhost:999/sleep  0.00s user 0.01s system 0% cpu 5.902 total

Why?

Answer 1

Well looks like it's swifter's fault - in HttpServerIO.swift it dispatches the requests to global queue like this:

DispatchQueue.global(qos: priority).async

Replacing it with

DispatchQueue.init(label: "httpserver", qos: DispatchQoS.userInteractive)

Fixed the problem. Obviously editing source code of a 3rd party package stinks, and the header suggests that this file was last updated in 2016. I've moved to vapor and it's miles better than swifter.