即使在多核环境中，也有std :: chrono线程安全性保证吗? [英] Is there any std::chrono thread safety guarantee even with multicore context?

问题描述

首先，我假设调用std :: chrono的任何函数都保证是线程安全的(没有不确定的行为或竞争条件，或者如果从不同的线程调用则不会有任何危险).我说得对吗?

First, I'm assuming that calling any function of std::chrono is guaranteed to be thread-safe (no undefined behaviour or race conditions or anything dangerous if called from different threads). Am I correct?

接下来，例如在Windows上，存在一个与多核处理器有关的众所周知的问题，该问题迫使与时间相关的系统的一些实现，以允许强制特定内核获取任何时间信息

Next, for example on windows there is a well known problem related to multi-core processors that force some implementations of time related systems to allow forcing a specific core to get any time information.

我想知道的是:

1. 在标准中使用std :: chrono，是否可以保证不会出现思考问题?
2. 还是实现定义了
3. 或者是否明确缺乏保证，暗示您最好在Windows上始终从同一内核获取时间?

推荐答案

是的，从不同线程调用 some_clock :: now()应该是线程安全的.

Yes, calls to some_clock::now() from different threads should be thread safe.

关于您在 QueryPerformanceCounter 中提到的特定问题，仅仅是Windows API在某些平台上暴露了硬件问题.其他操作系统可能会也可能不会将此硬件问题暴露给用户代码.

As regards the specific issue you mention with QueryPerformanceCounter, it is just that the Windows API exposes a hardware issue on some platforms. Other OSes may or may not expose this hardware issue to user code.

就C ++标准而言，如果时钟声称是稳定时钟"，则它绝不能向后退，因此，如果同一线程上有两次读取，则第二个绝不能返回早于该值的值.首先，即使操作系统将线程切换到其他处理器.

As far as the C++ standard is concerned, if the clock claims to be a "steady clock" then it must never go backwards, so if there are two reads on the same thread, the second must never return a value earlier than the first, even if the OS switches the thread to a different processor.

对于非稳定时钟(例如，在许多系统上为)，由于外部代理可以随时更改时钟，因此无法保证.

For non-steady clocks (such as std::chrono::system_clock on many systems), there is no guarantee about this, since an external agent could change the clock arbitrarily anyway.

使用我对C ++ 11线程库的实现(包括 std::chrono 的东西)的实现要注意确保稳定的时钟确实是稳定的.这样做确实要比对 QueryPerformanceCounter 的原始调用要多得多，以确保同步，但是不再将线程固定到CPU 0(它曾经这样做).我希望其他实现也可以解决此问题.

With my implementation of the C++11 thread library (including the std::chrono stuff) the implementation takes care to ensure that the steady clocks are indeed steady. This does impose a cost over and above a raw call to QueryPerformanceCounter to ensure the synchronization, but no longer pins the thread to CPU 0 (which it used to do). I would expect other implementations to have workarounds for this issue too.

稳定时钟的要求在20.11.3 [time.clock.req](C ++ 11标准)中

The requirements for a steady clock are in 20.11.3 [time.clock.req] (C++11 standard)

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