Using libevent together with GCD (libdispatch) in Swift

Question

I'm creating a server side app in Swift 3. I've chosen libevent for implementing networking code because it's cross-platform and doesn't suffer from C10k problem. Libevent implements it's own event loop, but I want to keep CFRunLoop and GCD (DispatchQueue.main.after etc) functional as well, so I need to glue them somehow.

This is what I've came up with:

var terminated = false

DispatchQueue.main.after(when: DispatchTime.now() + 3) {
    print("Dispatch works!")
    terminated = true
}

while !terminated {
    switch event_base_loop(eventBase, EVLOOP_NONBLOCK) { // libevent
    case 1:
        break // No events were processed
    case 0:
        print("DEBUG: Libevent processed one or more events")
    default: // -1
        print("Unhandled error in network backend")
        exit(1)
    }
    RunLoop.current().run(mode: RunLoopMode.defaultRunLoopMode,
                          before: Date(timeIntervalSinceNow: 0.01))
}

This works, but introduces a latency of 0.01 sec. While RunLoop is sleeping, libevent won't be able to process events. Lowering this timeout increases CPU usage significantly when the app is idle.

I was also considering using only libevent, but third party libs in the project can use dispatch_async internally, so this can be problematic.

Running libevent's loop in a different thread makes synchronization more complex, is this the only way of solving this latency issue?

LINUX UPDATE. The above code does not work on Linux (2016-07-25-a Swift snapshot), RunLoop.current().run exists with an error. Below is a working Linux version reimplemented with a timer and dispatch_main. It suffers from the same latency issue:

let queue = dispatch_get_main_queue()
let timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue)
let interval = 0.01
let block: () -> () = {
    guard !terminated else {
        print("Quitting")
        exit(0)
    }
    switch server.loop() {
    case 1: break // Just idling
    case 0: break //print("Libevent: processed event(s)")
    default: // -1
        print("Unhandled error in network backend")
        exit(1)
    }
}
block()
let fireTime = dispatch_time(DISPATCH_TIME_NOW, Int64(interval * Double(NSEC_PER_SEC)))
dispatch_source_set_timer(timer, fireTime, UInt64(interval * Double(NSEC_PER_SEC)), UInt64(NSEC_PER_SEC) / 10)
dispatch_source_set_event_handler(timer, block)
dispatch_resume(timer)
dispatch_main()

Answer 1

A quick search of the Open Source Swift Foundation libraries on GitHub reveals that the support in CFRunLoop is (perhaps obviously) implemented differently on different platforms. This means, in essence, that RunLoop and libevent, with respect to cross-platform-ness, are just different ways to achieve the same thing. I can see the thinking behind the thought that libevent is probably better suited to server implementations, since CFRunLoop didn't grow up with that specific goal, but as far as being cross-platform goes, they're both barking up the same tree.

That said, the underlying synchronization primitives used by RunLoop and libevent are inherently private implementation details and, perhaps more importantly, different between platforms. From the source, it looks like RunLoop uses epoll on Linux, as does libevent, but on macOS/iOS/etc, RunLoop is going to use Mach ports as its fundamental primitive, but libevent looks like it's going to use kqueue. You might, with enough effort, be able to make a hybrid RunLoopSource that ties to a libevent source for a given platform, but this would likely be very fragile, and generally ill-advised, for a couple of reasons: Firstly, it would be based on private implementation details of RunLoop that are not part of the public API, and therefore subject to change at any time without notice. Second, assuming you didn't go through and do this for every platform supported by both Swift and libevent, you would have broken the cross-platform-ness of it, which was one of your stated reasons for going with libevent in the first place.

One additional option you might not have considered would be to use GCD by itself, without RunLoops. Look at the docs for dispatch_main. In a server application, there's (typically) nothing special about a "main thread," so dispatching to the "main queue", should be good enough (if needed at all). You can use dispatch "sources" to manage your connections, etc. I can't personally speak to how dispatch sources scale up to the C10K/C100K/etc. level, but they've seemed pretty lightweight and low-overhead in my experience. I also suspect that using GCD like this would likely be the most idiomatic way to write a server application in Swift. I've written up a small example of a GCD-based TCP echo server as part of another answer here.

If you were bound and determined to use both RunLoop and libevent in the same application, it would, as you guessed, be best to give libevent it's own separate thread, but I don't think it's as complex as you might think. You should be able to dispatch_async from libevent callbacks freely, and similarly marshal replies from GCD managed threads to libevent using libevent's multi-threading mechanisms fairly easily (i.e. either by running with locking on, or by marshaling your calls into libevent as events themselves.) Similarly, third party libraries using GCD should not be an issue even if you chose to use libevent's loop structure. GCD manages its own thread pools and would have no way of stepping on libevent's main loop, etc.

You might also consider architecting your application such that it didn't matter what concurrency and connection handling library you used. Then you could swap out libevent, GCD, CFStreams, etc. (or mix and match) depending on what worked best for a given situation or deployment. Choosing a concurrency approach is important, but ideally you wouldn't couple yourself to it so tightly that you couldn't switch if circumstances called for it.

When you have such an architecture, I'm generally a fan of the approach of using the highest level abstraction that gets the job done, and only driving down to lower level abstractions when specific circumstances require it. In this case, that would probably mean using CFStreams and RunLoops to start, and switching out to "bare" GCD or libevent later, if you hit a wall and also determined (through empirical measurement) that it was the transport layer and not the application layer that was the limiting factor. Very few non-trivial applications actually get to the C10K problem in the transport layer; things tend to have to scale "out" at the application layer first, at least for apps more complicated than basic message passing.