Why compute a float in cudaEventElapsedTime() instead of a long integer?

Accelerated Computing CUDA CUDA Programming and Performance
1

Anyone care to explain why the event stream elapsed time function computes a float instead of a 64-bit integer?

[indent][font=“Courier New”]cudaError_t cudaEventElapsedTime (float* ms, cudaEvent_t start, cudaEvent_t end)[/font]
[font=“Georgia”]Computes the elapsed time between two events (in milliseconds with a resolution of around 0.5 microseconds). If
either event has not been recorded yet, this function returns cudaErrorInvalidValue. If either event has been recorded
with a non-zero stream, the result is undefined.[/font][/indent]

2

I’m guessing they use float for convenience. Most GPU functions take a few ms, so it gives you something human readable without further conversion.

But imo, that’s moot because the event interfaces is too cumbersome to use in the 1st place. I still use CPU clocks.

3

Yeah… using CPU clocks to time GPU performance is pretty useless. Resolution isn’t high enough, there’s OS scheduler and PCIe interference, etc.

4

It works fine for my purposes. I’m using QueryPerformanceCounter on Windows and clock_gettime(CLOCK_REALTIME, &t) on Linux, which both give have a resolution of at least 10^-5s. I still use CUDA profiler if I want to know the time without launch overhead.

Actually, I take back about the event interface being too cumbersome. That’s also moot because the SDK has a timer wrapper class, which I totally forgot about.

5

If you ever send me code that uses cutil, I helpfully ignore it completely.

Basically if you are timing what is happening on the card you need to be using events because otherwise context switches will ruin your results.

6

I was just curious because floats are hardly a great representation for time duration and conversion but, as you say, I bet they thought this would be more convenient.

I consider it a bug and hope someone will deprecate it (unless there is something I’m missing).

Perhaps the cudaEvent_t is not entirely opaque and there is an unofficial way to get an integral timestamp?