On Behalf Of Douglas Paul Gregor Subject: Re: [boost] [function] timing trivia

On Thu, 12 Feb 2004, Matthew Hurd wrote:

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Just completed some timings of the cost of a boost function call using a boost::function<void (void)> and a raw function pointer versus inlining the function's code directly into the source.

For my circumstances (your mileage may vary): Calling via raw function pointer = 314 microseconds +/- 1 Calling via boost function variable = 328 microseconds +/- 1

Only 14 microseconds of difference. Nice work, I was expecting much more. Note: Optimised VC7.1 output, winXP 1a, Athlon 2800+ (2.09GHz)

Looks like VC 7.1 is doing a good job optimizing this. I get asked about the performance of function<> every once in a while; mind if I stick this data in the FAQ?

Please do. Please consider anything I post public domain unless it is obviously embarrassing ;-) Regards, Matt Hurd.

On Behalf Of Matthew Hurd Subject: RE: [boost] [function] timing trivia

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On Behalf Of Douglas Paul Gregor Subject: Re: [boost] [function] timing trivia

On Thu, 12 Feb 2004, Matthew Hurd wrote:

...

Just completed some timings of the cost of a boost function call using a boost::function<void (void)> and a raw function pointer versus inlining the function's code directly into the source.

For my circumstances (your mileage may vary): Calling via raw function pointer = 314 microseconds +/- 1 Calling via boost function variable = 328 microseconds +/- 1

Only 14 microseconds of difference. Nice work, I was expecting much more. Note: Optimised VC7.1 output, winXP 1a, Athlon 2800+ (2.09GHz)

Looks like VC 7.1 is doing a good job optimizing this. I get asked about the performance of function<> every once in a while; mind if I stick this data in the FAQ?

Doug

Doug,

These results are invalid.

I think I am embarrassing myself here... my benchmarking code is reporting the function call overhead in proportion to the size of the loop internal to a function :-( I'm doing something wrong here, it might be the optimizer eliding some code due to it detecting an unused variable.

I'll investigate and report some results I'm more confident in.

Regards,

Matt Hurd.

I've been staring at measurements and double checked many things, but I'm at a loss to explain what I see. The good news is I'm confident the overhead of boost function with a static method call or a plain function for optimised code is within 60 nanoseconds of embedding the code itself for code that is a tight loop irrespective of the size of the loop for my circumstances. Importantly, sometimes boost::function measures faster to the resolution of my high resolution timer. That is, for the optimized case, I measure no consistent discernable abstraction penalty at all. Lemma: boost::function is beautiful What am I measuring? This method, or the equivalent function... static double not_empty() { static double sum; static double i; sum = 0.0; for (i = 0.0; i < MAX_FN_LOOP ; ++i) { sum += i * i; } return sum; } I see some strange behaviour for unoptimised debug code (median of 1000 measurements): Iterations in the "internal loop of the cost in microseconds of function" boost::function<double (void)> -------------------------------------------------------------- 1 0.182 10 0.196 100 0.183 1,000 0.059 10,000 -1.176 100,000 -14.767 1,000,000 +67.110 10,000,000 -1,131.9 Can't explain it. Boost function can't be faster for the 10 million loop by more than a millisecond. I see none of this whacky behaviour with release optimized code. The code is pretty straight forward at the core. double answer1 = 0.0; for (size_t j = 0; j< num_trials; ++j ) { trial.restart(); answer1 += fn(); now = trial.elasped(); timing1[j] = now; } double answer2 = 0.0; for (size_t j = 0; j< num_trials; ++j ) { trial.restart(); static double sum; static double i; sum = 0; for (i = 0; i < MAX_FN_LOOP ; ++i) { sum += i * i; } answer2 += sum; now = trial.elasped(); timing2[j] = now; } Doug, I hope this says something, but the strangeness leaves me uneasy. Regards, Matt Hurd.