mach_wait_until() Strange Behavior on iPad - ios

Question

I created a simple project to test out the functionality of mach_wait_until(). This code gives me an accurate printout of how precise the 1 second delay is. The console printout is virtually identical and extremely precise on both the iOS Simulator and on my iPad Air 2. However, on my iPad there is a HUGE delay, where the same 1 second delay takes about 100 seconds! And to add to the weirdness of it, the printout in the console says it only takes 1 second (with extremely low jitter and/or lag).
How can this be? Is there some timing conversion that I need to do for a physical iOS device when using mach_wait_until()?
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
playNoteTest()
}
var start = mach_absolute_time()
var end = mach_absolute_time()
func playNoteTest() {
let when = mach_absolute_time() + 1000000000
self.start = mach_absolute_time()
mach_wait_until(when)
self.end = mach_absolute_time()
let timeDelta = (self.end - self.start)
let newTimeDelta = Double(timeDelta) / 1000000000.0
print("Delta Time = \(newTimeDelta)")
playNoteTest()
}
}

Answer 1

mach_absolute_time units are CPU dependent. You need to multiply by a device-specific constant in order to get real-world units. It is discussed in this Tech Q&A from Apple.
Here is some playground code that demonstrates the idea:
import PlaygroundSupport
import Foundation
PlaygroundPage.current.needsIndefiniteExecution = true
class TimeBase {
static let NANOS_PER_USEC: UInt64 = 1000
static let NANOS_PER_MILLISEC: UInt64 = 1000 * NANOS_PER_USEC
static let NANOS_PER_SEC: UInt64 = 1000 * NANOS_PER_MILLISEC
static var timebaseInfo: mach_timebase_info! = {
var tb = mach_timebase_info(numer: 0, denom: 0)
let status = mach_timebase_info(&tb)
if status == KERN_SUCCESS {
return tb
} else {
return nil
}
}()
static func toNanos(abs:UInt64) -> UInt64 {
return (abs * UInt64(timebaseInfo.numer)) / UInt64(timebaseInfo.denom)
}
static func toAbs(nanos:UInt64) -> UInt64 {
return (nanos * UInt64(timebaseInfo.denom)) / UInt64(timebaseInfo.numer)
}
}
let duration = TimeBase.toAbs(nanos: 10 * TimeBase.NANOS_PER_SEC)
DispatchQueue.global(qos: .userInitiated).async {
print("Start")
let start = mach_absolute_time()
mach_wait_until(start+duration)
let stop = mach_absolute_time()
let elapsed = stop-start
let elapsedNanos = TimeBase.toNanos(abs: elapsed)
let elapsedSecs = elapsedNanos/TimeBase.NANOS_PER_SEC
print("Elapsed nanoseconds = \(elapsedNanos)")
print("Elapsed seconds = \(elapsedSecs)")
}