Sleep() becomes less accurate after replacing a PC? (C++)
c++, sleep, timer, windows
Solution
The time resolution on XP is around 10ms - the system basically "ticks" every 10ms. Sleep is not a very good way to do accurate timing for that reason. I'm pretty sure win2000 has the same resolution but if I'm wrong that could be a reason.
You can change that resolution , atleast down to 1ms - see http://technet.microsoft.com/en-us/sysinternals/bb897569.aspx or use this http://www.lucashale.com/timerresolution/ - there's probably a registry key as well(windows media player will change that timer as well, probably only while it's running.
Could be the resolution somehow was altered on your old machine.
Problem
I have a program that was built in C++ (MFC, Visual Studio 6.0) several years ago and has been running on a certain Windows machine for quite some time (more than 5 years). The PC was replaced a month ago (the old one died), and since then the program's timing behavior changed. I need help understanding why. The main functionality of the program is to respond to keystrokes by sending out ON and OFF signals to an external card, with very accurate delay between the ON and the OFF. An example program flow: ``` > wait for keystroke... > ! keystroke occurred > send ON message > wait 150ms > send OFF message ``` Different keystrokes have different waiting periods associated with them, between 20ms and 150ms (a very deterministic time depending on the specific keystroke). The timing is very important. The waiting is executed using simple `Sleep()`. The accuracy of the sleep on the old PC was 1-2ms deviation. I can measure the timing externally to the computer (on the external card), so my measurement of the sleep time is very accurate. Please take into account this machine executed such ON-sleep-OFF cycles thousands of times a day for years, so the accuracy data I have is sound. Since the PC was replaced the timing deviation is more than 10ms. I did not install the previous PC, so it may had some additional software packages installed. Also, I'm ashamed to admit I don't remember whether the previous PC was Windows 2000 or Windows XP. I'm quite sure it was XP, but not 100% (and I can't check now...). The new one is Windows XP. I tried changing the sleeping mechanism to be based on timers, but the accuracy did not improve. Can anything explain this change? Is there a software package that may have been installed on the previous PC that may fix the problem? Is there a best practice to deal with the problem?