On posix it is possible to use timespec to calculate accurate time length (like seconds and milliseconds). Unfortunately I need to migrate to windows with Visual Studio compiler. The VS time.h library doesn't declare timespec so I'm looking for other options. As far as could search is it possible to use clock and time_t although I could't check how precise is counting millisecons with clock counting.
What do you do/use for calculating time elapse in a operation (if possible using standards c++ library) ?
The function GetTickCount is usually used for that.
Also a similiar thread: C++ timing, milliseconds since last whole second
Depends on what sort of accuracy you want, my understanding is that clock and time_t are not accurate to the millisecond level. Similarly GetTickCount() is commonly used (MS docs say accurate to 10-15ms) but not sufficiently accurate for many purposes.
I use QueryPerformanceFrequency and QueryPerformanceCounter for accurate timing measurements for performance.
Related
My program frequently calls WINAPI function timeGetTime(), which should be replaced with usage of <chrono> (standard library). What is the fastest standardized way to get system time - in float or int, for my case?
I do not need tracking the date or day time, i only need precise relative ms/seconds value, which always increments. Is there any?
For benchmarking, you likely want std::chrono::high_resolution_clock. It may not be steady - in the sense that it "always increments". The only clock that guarantees steadiness is std::chrono::steady_clock.
The best, steady clock would then be:
using ClockType = std::conditional<
std::chrono::high_resolution_clock::is_steady,
std::chrono::high_resolution_clock,
std::chrono::steady_clock>::type;
Note that high_resolution_clock could itself simply be an alias for steady_clock.
If you need precise relative ms, you're
looking for system-specific
not wallclock
You tagged question with WinApi, I assume, this is Windows-specific.
For Windows that is High Resolution Timer. This technology allows you to precisely calculate relative times (how much time is spent in certain function call, for example).
On Windows, I can call QueryPerformanceCounter to get high resolution data points, but this method call is affected by issues with the BIOS, multi-core CPUs, and some AMD chips. I can call timeBeginPeriod to increase the system clock resolution in Windows down to 1ms (instead of the standard ~15ms) which means that I can use just call timeGetTime and get the time in the clock resolution that I've specified.
So! On OSX/Linux, what C++ clock resolutions should I expect? Can I get 1ms resolution similar to Windows? Since I'm doing real time media, I want this clock resolution to be as low as possible: can I change this value in the kernel (like in Windows with timeBeginPeriod)? This is a high performance application, so I want getting the current time to be a fast function call. And I'd like to know if the clock generally drifts or what weird problems I can expect.
Thanks!
Brett
If you are using C++11 you can use std::chrono::high_resolution_clock which should give you as high a resolution clock as the system offers. To get a millisecond duration you would do
typedef std::chrono::high_resolution_clock my_clock;
my_clock::time_point start = my_clock::now();
// Do stuff
my_clock::time_point end = my_clock::now();
std::chrono::milliseconds ms_duration =
std::chrono::duration_cast<std::chrono::milliseconds>(end - start);
If you aren't using C++11 then the gettimeofday function works in OSX and most linux distributions. It gives you the time since epoch in seconds and microseconds. The resolution is unspecified, but it should give you at least millisecond accuracy on any modern system.
To add to David's answer, if you can't use C++11, Boost's Timer classes can help you.
Here's what I'd need to do:
double now=getdoubletimestampsomehow();
Where getdoubletimestampsomehow() should be a straight-forward, easy to use function returning a double value representing the number of seconds elapsed from a given date. I'd need it to be quite precise, but I don't really need it to be more precise than a few milliseconds. Portability is quite important, if it isn't possible to directly port it anywhere could you please tell me both an unix and a windows way to do it?
Have you looked at Boost and particularly its Date_Time library ? Here is the seconds since epoch example.
You will be hard-pressed to find something more portable, and of higher resolution.
Portable good precision double timestamp in C++?
There is no portable way to get high-precision timestamp (milliseconds) without using 3rd party libraries. Maximum precision you'll get is 1 second, using time/localtime/gmtime.
If you're fine with 3rd party libraries, use either Boost or Qt 4.
both an unix and a windows way to do it?
GetSystemTime on Windows and gettimeofday on linux.
Please note that if you're planning to use timestamps to determine order of some events, then it might be a bad idea. System clock might have very limited precision (10 milliseconds on windows platform), in which case several operations performed consequently can produce same timestamp. So, to determine order of events you would need "logical timestamps" ("vector clock" is one of examples).
On windows platform, there are highly precise functions that can be used to determine how much time has passed since some point in the past (QueryPerformanceCounter), but they aren't connected to timestamps.
C++11 introduced the <chrono> header containing quite a few portable clocks. The highest resolution clock among them is the std::chrono::high_resolution_clock.
It provides the current time as a std::chrono::time_point object which has a time_since_epoch member. This might contain what you want.
Reference:
Prior to the release of the C++11 standard, there was no standard way in which one could accurately measure the execution time of a piece of code. The programmer was forced to use external libraries like Boost, or routines provided by each operating system.
The C++11 chrono header file provides three standard clocks that could be used for timing one’s code:
system_clock - this is the real-time clock used by the system;
high_resolution_clock - this is a clock with the shortest tick period possible on the current system;
steady_clock - this is a monotonic clock that is guaranteed to never be adjusted.
If you want to measure the time taken by a certain piece of code for execution, you should generally use the steady_clock, which is a monotonic clock that is never adjusted by the system. The other two clocks provided by the chrono header can be occasionally adjusted, so the difference between two consecutive time moments, t0 < t1, is not always positive.
Doubles are not precise - therefore you idea for double now=getdoubletimestampsomehow(); falls down at the first hurdle.
Others have mentioned other possibilities. I would explore those.
I am curious if there is a build-in function in C++ for measuring the execution time?
I am using Windows at the moment. In Linux it's pretty easy...
The best way on Windows, as far as I know, is to use QueryPerformanceCounter and QueryPerformanceFrequency.
QueryPerformanceCounter(LARGE_INTEGER*) places the performance counter's value into the LARGE_INTEGER passed.
QueryPerformanceFrequency(LARGE_INTEGER*) places the frequency the performance counter is incremented into the LARGE_INTEGER passed.
You can then find the execution time by recording the counter as execution starts, and then recording the counter when execution finishes. Subtract the start from the end to get the counter's change, then divide by the frequency to get the time in seconds.
LARGE_INTEGER start, finish, freq;
QueryPerformanceFrequency(&freq);
QueryPerformanceCounter(&start);
// Do something
QueryPerformanceCounter(&finish);
std::cout << "Execution took "
<< ((finish.QuadPart - start.QuadPart) / (double)freq.QuadPart) << std::endl;
It's pretty easy under Windows too - in fact it's the same function on both std::clock, defined in <ctime>
You can use the Windows API Function GetTickCount() and compare the values at start and end. Resolution is in the 16 ms ballpark. If for some reason you need more fine-grained timings, you'll need to look at QueryPerformanceCounter.
C++ has no built-in functions for high-granularity measuring code execution time, you have to resort to platform-specific code. For Windows try QueryPerformanceCounter: http://msdn.microsoft.com/en-us/library/ms644904(VS.85).aspx
The functions you should use depend on the resolution of timer you need. Some of them give 10ms resolutions. Those functions are easier to use. Others require more work, but give much higher resolution (and might cause you some headaches in some environments. Your dev machine might work fine, though).
http://www.geisswerks.com/ryan/FAQS/timing.html
This articles mentions:
timeGetTime
RDTSC (a processor feature, not an OS feature)
QueryPerformanceCounter
C++ works on many platforms. Why not use something that also works on many platforms, such as the Boost libraries.
Look at the documentation for the Boost Timer Library
I believe that it is a header-only library, which means that it is simple to setup and use...
I essentially want to reconstruct the getTickCount() windows function so I can use it in basic C++ without any non standard libraries or even the STL. (So it complies with the libraries supplied with the Android NDK)
I have looked at
clock()
localtime
time
But I'm still unsure whether it is possible to replicate the getTickCount windows function with the time library.
Can anyone point me in the right direction as to how to do this or even if its possible?
An overview of what I want to do:
I want to be able to calculate how long an application has been "doing" a certain function.
So for example I want to be able to calculate how long the application has been trying to register with a server
I am trying to port it from windows to run on the linux based Android, here is the windows code:
int TimeoutTimer::GetSpentTime() const
{
if (m_On)
{
if (m_Freq>1)
{
unsigned int now;
QueryPerformanceCounter((int*)&now);
return (int)((1000*(now-m_Start))/m_Freq);
}
else
{
return (GetTickCount()-(int)m_Start);
}
}
return -1;
}
On Android NDK you can use the POSIX clock_gettime() call, which is part of libc. This function is where various Android timer calls end up.
For example, java.lang.System.nanoTime() is implemented with:
struct timespec now;
clock_gettime(CLOCK_MONOTONIC, &now);
return (u8)now.tv_sec*1000000000LL + now.tv_nsec;
This example uses the monotonic clock, which is what you want when computing durations. Unlike the wall clock (available through gettimeofday()), it won't skip forward or backward when the device's clock is changed by the network provider.
The Linux man page for clock_gettime() describes the other clocks that may be available, such as the per-thread elapsed CPU time.
clock() works very similarly to Windows's GetTickCount(). The units may be different. GetTickCount() returns milliseconds. clock() returns CLOCKS_PER_SEC ticks per second. Both have a max that will rollover (for Windows, that's about 49.7 days).
GetTickCount() starts at zero when the OS starts. From the docs, it looks like clock() starts when the process does. Thus you can compare times between processes with GetTickCount(), but you probably can't do that with clock().
If you're trying to compute how long something has been happening, within a single process, and you're not worried about rollover:
const clock_t start = clock();
// do stuff here
clock_t now = clock();
clock_t delta = now - start;
double seconds_elapsed = static_cast<double>(delta) / CLOCKS_PER_SEC;
Clarification: There seems to be uncertainty in whether clock() returns elapsed wall time or processor time. The first several references I checked say wall time. For example:
Returns the number of clock ticks elapsed since the program was launched.
which admittedly is a little vague. MSDN is more explicit:
The elapsed wall-clock time since the start of the process....
User darron convinced me to dig deeper, so I found a draft copy of the C standard (ISO/IEC 9899:TC2), and it says:
... returns the implementation’s best approximation to the processor time used ...
I believe every implementation I've ever used gives wall-clock time (which I suppose is an approximation to the processor time used).
Conclusion: If you're trying to time so code so you can benchmark various optimizations, then my answer is appropriate. If you're trying to implement a timeout based on actual wall-clock time, then you have to check your local implementation of clock() or use another function that is documented to give elapsed wall-clock time.
Update: With C++11, there is also the portion of the standard library, which provides a variety of clocks and types to capture times and durations. While standardized and widely available, it's not clear if the Android NDK fully supports yet.
This is platform dependent so you just have to write a wrapper and implement the specifics for each platform.
It's not possible. The C++ standard and, as consequence the standard library, know nothing about processors or 'ticks'. This may or may not change in C++0x with the threading support but at least for now, it's not possible.
Do you have access to a vblank interrupt function (or hblank) on the Android? If so, increment a global, volatile var there for a timer.