blocks|key|325232|text|我的印象是，在x86上，QueryPerformanceCounter()只是在幕后调用rdtsc。我很惊讶它在多核机器上有任何减慢(我从来没有注意到它在我的4核cpu上)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|325233|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I was under the impression that on x86 QueryPerformanceCounter() just called rdtsc under the covers. I'm suprised that it has any slowdown on multi-core machines (I've never noticed it on my 4-core cpu).

blocks|key|328286|text|Windows+QueryPerformanceCounter()具有确定处理器数量的逻辑，并在必要时调用同步逻辑。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|328287|MSDN说，在哪个处理器上调用它并不重要，因此您可能会看到针对这种情况的额外同步代码导致开销。还要记住，它可以调用总线传输，因此您可能会看到总线争用延迟。|328288|如果可能，请尝试使用SetThreadAffinityMask()将其绑定到特定的处理器。否则，您可能只能忍受延迟，或者您可以尝试不同的计时器(例如，查看http://en.wikipedia.org/wiki/High_Precision_Event_Timer)。|offset|length|328289|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/High_Precision_Event_Timer^0|0|0|25|1J|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Q|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@$F|S|G|T|1|U]]|A|$]]|$1|H|3|-4|5|6|7|V|8|@]|9|@]|A|$]]]|I|$J|$5|K|L|M|A|$N|O]]]]

Windows QueryPerformanceCounter() has logic to determine the number of processors and invoke syncronization logic if necessary. It attempts to use the TSC register but for multiprocessor systems this register is not guaranteed to be syncronized between processors (and more importantly can vary greatly due to intelligent downclocking and sleep states). 

MSDN says that it doesn't matter which processor this is called on so you may be seeing extra syncronization code for such a situation cause overhead. Also remember that it can invoke a bus transfer so you may be seeing bus contention delays.

Try using SetThreadAffinityMask() if possible to bind it to a specific processor. Otherwise you might just have to live with the delay or you could try a different timer (for example take a look at <a href="http://en.wikipedia.org/wiki/High_Precision_Event_Timer" rel="noreferrer">http://en.wikipedia.org/wiki/High_Precision_Event_Timer</a>).

blocks|key|3292608|text|我已经用了很长时间了，但是如果内存服务，这个功能没有一个实现，因为内部是由不同的硬件制造商提供的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3292609|这里有一篇来自MSDN的小文章：http://msdn.microsoft.com/ja-jp/library/cc399059.aspx|offset|length|3292610|此外，如果您要查询跨多个CPU的性能(而不是一个CPU上的多个内核)，它将不得不通过总线进行通信，这既较慢，也可能会出现一些阻塞。|3292611|然而，就像我之前说的，已经有很长一段时间了。|3292612|麦克|3292613|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/ja-jp/library/cc399059.aspx^0|0|G|1H|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|U|8|@]|9|@$D|V|E|W|1|X]]|A|$]]|$1|F|3|G|5|6|7|Y|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|Z|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|10|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|M|$N|$5|O|P|Q|A|$R|S]]]]

It's been a long time since I used this much, but if memory serves there isn't one implementation of this function, as the guts are provided by the various hardware manufacturers. 

Here is a small article from MSDN:
<a href="http://msdn.microsoft.com/ja-jp/library/cc399059.aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/ja-jp/library/cc399059.aspx</a>

Also, if you're querying performance across multiple CPUs (as opposed to multiple cores on one CPU), it's going to have to communicate across the bus, which is both slower and could be where you are seeing some blocking. 

However, like I said before it's been quite a while.

Mike

blocks|key|3292628|text|我知道这个帖子有点老了，但我想补充更多的信息。首先，我确实同意QueryPerformanceCounter可以在某些机器上花费更多时间，但我不确定Ron的答案是否一直是原因。当我在这个问题上做一些研究时，我发现了一个各种各样的网页，上面谈到了QueryPerformanceCounter是如何实现的。例如，Precision+is+not+the+same+as+accuracy告诉我，Windows，HAL更具体地说，会使用不同的计时装置来获取数值。这意味着如果windows使用较慢的计时设备，例如PIT，则获取时间值将需要更多时间。显然，使用PIT可能需要PCI事务，因此这将是一个原因。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3292629|我还找到了另一篇文章：“它是如何工作的:+SQL+Server2008TSC中的定时器输出--+R2+-不变性TSC”给出了类似的描述。实际上，本文讲述了SQLServer如何以最佳方式对事务进行计时。|3292630|然后，我在VMware网站上找到了更多信息，因为我必须与使用虚拟机的客户打交道，而且我发现使用虚拟机进行时间测量还有其他问题。对于感兴趣的人，请参考本文中的VMware文件-+VMware虚拟机中的计时，它还讨论了一些版本的windows如何同步每个TSC。因此，在某些情况下使用QueryPerformanceCounter()是安全的，我认为我们应该尝试一下它的工作原理:+SQL+Server2008+R2中的计时器输出建议我们在调用QueryPerformanceCounter()时查看可能发生的情况。|3292631|entityMap|0|LINK|mutability|MUTABLE|url|http://blogs.msdn.com/b/oldnewthing/archive/2005/09/02/459952.aspx^0|4B|11|0|0|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@$A|Q|B|R|1|S]]|C|$]]|$1|D|3|E|5|6|7|T|8|@]|9|@]|C|$]]|$1|F|3|G|5|6|7|U|8|@]|9|@]|C|$]]|$1|H|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|I|$J|$5|K|L|M|C|$N|O]]]]

I know that this thread is a bit old but I would like to add more info. 
First, I do agree that QueryPerformanceCounter can take more time on certain machines, but I am not sure if Ron's answer is the reason for that all the time.
While I was doing some research on this issue, I found a various web pages that talks about how QueryPerformanceCounter is implemented.
For instance, <a href="http://blogs.msdn.com/b/oldnewthing/archive/2005/09/02/459952.aspx" rel="nofollow noreferrer">Precision is not the same as accuracy</a> tells me that Windows, HAL to be more specific would use different timing device to obtain the value. This means that if windows get to use slower timing device such as PIT, it will take more time to obtain the time value. Obviously, using PIT might require PCI transaction so that would be one reason.

I also found another article: How It Works: Timer Outputs in SQL Server 2008 R2 - Invariant TSC giving similar description. In fact, this article tells how SQLServer would time the transaction in the best way.

Then, I found more information on VMware site because I had to deal with customers who are using VMs and I found that there are other issues with time measurement with VMs. For those who are interested, please refer to VMware paper - Timekeeping in VMware Virtual Machines
In this paper, it also talks about how some versions of windows would synchronize each TSCs. Thus, it would be safe to use QueryPerformanceCounter() in certain situations and I think that we should try something like what How It Works: Timer Outputs in SQL Server 2008 R2 suggested to find what might happen when we call QueryPerformanceCounter()

I'm looking into the exact implications of using QueryPerformanceCounter in our system and am trying to understand it's impact on the application. I can see from running it on my 4-core single cpu machine that it takes around 230ns to run. When I run it on a 24-core 4 cpu xeon it takes around 1.4ms to run. More interestingly on my machine when running it in multiple threads they don't impact each other. But on the multi-cpu machine the threads cause some sort of interaction that causes them to block each other. I'm wondering if there is some shared resource on the bus that they all query? What exactly happens when I call QueryPerformanceCounter and what does it really measure?

What happens when QueryPerformanceCounter is called?

SQL Server

虚拟机

Windows

我正在研究在我们的系统中使用QueryPerformanceCounter的确切含义，并试图了解它对应用程序的影响。我可以看到，在我的4核单cpu机器上运行它大约需要230 to。当我在24核4 cpu xeon上运行它时，大约需要1.4ms。更有趣的是，在我的机器上，当它在多个线程中运行时，它们不会相互影响。但是在多...

问当调用QueryPerformanceCounter时会发生什么？
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