This one had me scratching my head for a long time. Apparently, the CodeGear / Embarcadero RAD Studio IDEs (I've tested 2007 and 2010) exhibit this behavior - it calls timeBeginPeriod to change system wide timer resolution to 1ms when it starts and timeEndPeriod when it quits.
What's the problem then?
For starters, this means that simple code such as:
will behave very differently with and without the IDE running. You may end up polling a lot slower than you expected when you deploy your applications and may cause DoSomething() to not get called in-time. The evil is in the fact that when you are developing the application, DoSomething() always gets called as you would have expected. But when you deploy your application (or hands it over to the testers for testing), you'd soon realize something is amiss. Everyone knows Windows is not a real-time OS, so no one would expect Sleep(1) to actually sleep for 1ms. But while developing the application, you had found that it is actually quite close.
Well, surprise, surprise! Without the IDE running, Sleep(1) would actually wait for 15.625ms by default - that's more than 15 times slower than what you were expecting.
The Sleep Function documentation from MSDN really doesn't do a good job at explaining the Sleep function. In DOS, I would've expected the system ticks to be at a default of 15.7ms. But I had expected Windows starting from Windows 95 to have a default system tick of 1ms. I was wrong (well, not really, see my comment #1).
Regardless, this is a serious problem with all of RAD Studio's IDEs. I am sure hardly anyone knows about this and at one point or another, you would've been bitten by this, even if you didn't know it - except that your application failed on the day of demo at your client's site. Just your luck again.
Microsoft's Visual Studio IDEs (tested 6, 2005, 2008) don't do this.
Also, it's never a good idea to change the system tick resolution - from MSDN, "(The timeBeginPeriod) function affects a global Windows setting. Windows uses the lowest value (that is, highest resolution) requested by any process. Setting a higher resolution can improve the accuracy of time-out intervals in wait functions. However, it can also reduce overall system performance, because the thread scheduler switches tasks more often. High resolutions can also prevent the CPU power management system from entering power-saving modes. Setting a higher resolution does not improve the accuracy of the high-resolution performance counter."
Perhaps the OS should have simply fixed it at 1ms. To allow processes to change system timer resolution that affects global system settings does not make much sense in a multitasking environment.