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POWaitable timer spotty precision
 

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    <p>I'm using a version of the following code in my actual production code. It sets a waitable timer for some time in the future and then has one worker thread waiting for it to fire:</p> <pre><code>//Create timer hWTimer = ::CreateWaitableTimer(NULL, FALSE, NULL); //Number of minutes to wait int nMinutesToWait = 5; //Predict when it should fire COleDateTime dtWhen = COleDateTime::GetCurrentTime() + COleDateTimeSpan(0, 0, nMinutesToWait, 0); //Set timer SetWaitableTimerSecondsFromNow(hWTimer, nMinutesToWait * 60); //Waiting part if(::WaitForSingleObject(hWTimer, INFINITE) == WAIT_OBJECT_0) { //Timer has fired, compare when _TRACE(L"Time scheduled for: %s, time fired: %s", dtWhen.Format(), COleDateTime::GetCurrentTime().Format()); } BOOL SetWaitableTimerSecondsFromNow(HANDLE hWTimer, double fSecs2Wait) { //INFO: I'm setting the timer to a relative time to ensure that it // doesn't fire in case the system clock is adjusted. LARGE_INTEGER li; li.QuadPart = -10000000LL * (LONGLONG)fSecs2Wait; //Convert to 100 nanosecond intervals (MUST BE NEGATIVE) if(li.QuadPart &gt;= 0) return FALSE; return ::SetWaitableTimer(hWTimer, &amp;li, 0, NULL, NULL, FALSE); } </code></pre> <p>It works well and in most circumstances the time when the timer needs to fire and when it actually fires is a 100% match. But very rarely, the timer fires 10-15 seconds earlier for just a 5 minute wait. It also seems like this issue mostly happens on Windows XP.</p> <p>Does anyone have any idea what's going on here and how to mitigate it?</p>
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      1. USzar
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    1. COThis is unlikely the source of your issue, but are you sure this like does what you want it to? `-10000000LL * (LONGLONG)fSecs2Wait;` If you ask for, say, "5.1 seconds" it will only wait for 5 seconds because you truncate to `long long` before the multiply. Seems like you'd rather want to do `(LONGLONG)(-10000000. * fsecs2Wait);`
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      1. USJoe Z
    2. CO@JoeZ: OK, good point. But it wouldn't round it up to 10-15 seconds. I saw those differences in the log file.
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    3. COIs it possible that the system was so busy at that time (high CPU usage, for example, or bogged down with task switching) that the waiting thread didn't get its timeslice quickly?
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      1. USJim Mischel
 

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