The information in this article applies to Windows NT, Win2K/XP. There is no specific Win32 API that retrieves the CPU usage. An undocumented API,
NtQuerySystemInformation in ntdll.dll, would help us retrieve the CPU usage. However, CPU usage can be retrieved by using performance counters. Since PDH.dll (Performance Data Helper) is not distributed with the Visual Studio, and not everyone has this file, I decided to do it without the help of PDH.dll.
The CPU usage counter is of type
PERF_100NSEC_TIMER_INV which has the following calculation:
X - CounterData
Y - 100NsTime
Time base - 100Ns
where the denominator (Y) represents the total elapsed time of the sample interval and the numerator (X) represents the time during the interval when the monitored components were inactive.
CCpuUsage class has a method called
GetCpuUsage which runs through the performance objects and counters and retrieves the CPU usage. Since the CPU usage can be determined by two samplings, the first call to
GetCpuUsage() returns 0, and all calls thereafter returns the CPU usage.
On Windows NT, CPU usage counter is '% Total processor time' whose index is 240 under 'System' object whose index is 2. However, in Win2K/XP, Microsoft moved that counter to '% processor time' whose index is 6 under '_Total' instance of 'Processor' object whose index is 238. Read 'INFO: Percent Total Performance Counter Changes on Windows 2000' (Q259390) in MSDN.
There is no difference between WinNT and Win2K/XP in the performance counters for getting CPU usage for a specific process. The counter '% processor time' whose index is 6 under the object 'Process' whose index is 230.
int main(int argc, char* argv)
printf("SystemWide Cpu Usage "
"Explorer cpu usage "
"Cpu Usage for processID 0\n");
int SystemWideCpuUsage = usageA.GetCpuUsage();
int ProcessCpuUsageByName = usageB.GetCpuUsage("explorer");
int ProcessCpuUsageByID = usageC.GetCpuUsage(processID);
- 6-May-2003 - Fixed bug when looking for an instance. The bug occurred because the instances in the performance counters are in Unicode. Added a sample function to get CPU usage for a specific process.
- 12-Jun-2003 - Fixed bug in
- 22-Jun-2003 - Defined structure alignment to be 8 (by using
#pragma directive). Other sizes cause the function to return 100% CPU usage. (Thanks to Scolver tip.)
- 12-Feb-2004 - Enabled performance counters of perfOS.dll (which holds processor counters) automatically for Win2K/XP.
- 20-May-2004 - Retrieving only the specified counter data (instead of retrieving all counters and iterating till finding the specified object) (for more information, refer to OscarTV comments).
- 03-Jun-2004 - Enabled performance counters of perfProc.dll (which holds process counters) automatically for Win2K/XP.
- 06-Jun-2004 - Wrapped the code into a class in order to enable getting CPU usage for different processes at the same iteration.
- 18-Nov-2004 - Extended variable from 8 bytes to 32 bytes.
- 03-Feb-2005 - Enabled getting performance counters by process ID.
- 09-Feb-2005 - Returns 0 when the specified process-ID doesn't exist.
More of my articles using performance counters