为什么在模拟器上运行时 mach_wait_until 迟到？

Question

我正在使用下面的代码来使用 mach_wait_until() 等待指定的时间段（以纳秒为单位）。

private func startTimerAndResume(){

    let idealNanos: UInt64 = 1250130250 //1.25 seconds

    let deadline = CFAbsoluteTime(mach_absolute_time() + (timeUnitsFor(nanos: idealNanos))/100)

    let x = mach_absolute_time()

    mach_wait_until(UInt64(deadline))

    let y = mach_absolute_time()


    var timeBaseInfo = mach_timebase_info_data_t()
    mach_timebase_info(&timeBaseInfo)
    let elapsedNanos = (y - x) * UInt64(timeBaseInfo.numer) / UInt64(timeBaseInfo.denom);

    print("deadline (aka mach-abs-time + timeUnitsFor()) = \(deadline)")
    print("(mach-abs-y)-(mach-abs-x) = \(y-x)")
    print("error in time units = \((y-x)-(timeUnitsFor(nanos: idealNanos))/100)")
    print("elapsed nanos actual = ", elapsedNanos)
    print("elapsed nanos ideal = ", idealNanos)
    print("error in nanoseconds = \(elapsedNanos - idealNanos)")
}


private func timeUnitsFor(nanos: UInt64)-> UInt64{

    var timeBaseInfo = mach_timebase_info_data_t()
    mach_timebase_info(&timeBaseInfo)

    let numer: UInt64 = UInt64(timeBaseInfo.numer)
    let denom: UInt64 = UInt64(timeBaseInfo.denom)

    //elapsed time in nanoseconds = timUnits * (numer / denom) ... therefore ->
    let timeUnits: UInt64 = (nanos*denom/numer)*(UInt64(100))//multiply by 100 to preserve decimal before truncation caused by UInt64() conversion
    print("timeUnits = \((timeUnits)/100) for target nanos \(nanos) when numer = \(numer) and denom = \(denom)")
    return timeUnits
}

当我运行在我的实际 iPhone 设备上执行此操作时，错误通常在 1 毫秒左右，输出是这样的：

timeUnits = 30003126 for target nanos 1250130250 when numer = 125 and denom = 3

deadline (aka mach-abs-time + timeUnitsFor()) = 4025277628801.0

(mach-abs-y)-(mach-abs-x) = 30027213

timeUnits = 30003126 for target nanos 1250130250 when numer = 125 and denom = 3

error in time units = 24087

elapsed nanos actual = 1251133875

elapsed nanos ideal = 1250130250

error in nanoseconds = 1003625

然而，当我运行在我的模拟器上执行此操作时，计时器始终延迟 70 到 74 毫秒，这是输出：

timeUnits = 1250130250 for target nanos 1250130250 when numer = 1 and denom = 1

deadline (aka mach-abs-time + timeUnitsFor()) = 691695760744956.0

(mach-abs-y)-(mach-abs-x) = 1322288698

timeUnits = 1250130250 for target nanos 1250130250 when numer = 1 and denom = 1

error in time units = 72158448

elapsed nanos actual = 1322288698

elapsed nanos ideal = 1250130250

error in nanoseconds = 72158448

想知道为什么模拟器每次都晚70到74毫秒。我做的纳秒和马赫时间单位之间的转换是错误的吗？谢谢

Answer 1

模拟器不是用于时钟周期精确测试的时钟周期精确手臂模拟器。它是 Intel 的 iOS 版本，与主机的其余部分共享内核 OS。

您的计时器可能在模拟器中合并，而它们不在设备上。根据您的主机配置和运行时版本，模拟器进程可能运行在 Utility QoS 层。

所以，您看到时间差异的原因是以下一个或多个原因：

英特尔与 ARM
不同的内核版本（主机内核与配对 iOS 内核版本）
与您系统上的其他进程争用
由于不同的 QoS 层导致定时器合并

您不应该依赖于在特定 window 中取回控制权，因为这不在 API 合同中。将在该时间过后的某个时间安排您。

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