iPhone / Objective C: How to log a methods execution time exactly in milliseconds?

Question

Is there any way to determine how long a method needs for execution? In seconds would be not accurately enough - in milliseconds would be better.

Answer 1

OK, if your objective is to find out what you can fix to make it faster, that's a little different goal. Measuring the time that functions take is a good way to find out if what you did made a difference, but to find out what to do you need a different technique. This is what I recommend, and I know you can do it on iPhones.

Answer 2

I use this:

clock_t start, end;
double elapsed;
start = clock();

//Start code to time

//End code to time

end = clock();
elapsed = ((double) (end - start)) / CLOCKS_PER_SEC;
NSLog(@"Time: %f",elapsed);

But I'm not sure about CLOCKS_PER_SEC on the iPhone. You might want to leave it off.

Answer 3

For fine-grained timing on OS X, you should use mach_absolute_time( ) declared in <mach/mach_time.h>:

#include <mach/mach_time.h>
#include <stdint.h>

// Do some stuff to setup for timing
const uint64_t startTime = mach_absolute_time();
// Do some stuff that you want to time
const uint64_t endTime = mach_absolute_time();

// Time elapsed in Mach time units.
const uint64_t elapsedMTU = endTime - startTime;

// Get information for converting from MTU to nanoseconds
mach_timebase_info_data_t info;
if (mach_timebase_info(&info))
   handleErrorConditionIfYoureBeingCareful();

// Get elapsed time in nanoseconds:
const double elapsedNS = (double)elapsedMTU * (double)info.numer / (double)info.denom;

Of course the usual caveats about fine-grained measurements apply; you're probably best off invoking the routine under test many times, and averaging/taking a minimum/some other form of processing.

Additionally, please note that you may find it more useful to profile your application running using a tool like Shark. This won't give you exact timing information, but it will tell you what percentage of the application's time is being spent where, which is often more useful (but not always).

Answer 4

NSDate *methodStart = [NSDate date];

/* ... Do whatever you need to do ... */

NSDate *methodFinish = [NSDate date];
NSTimeInterval executionTime = [methodFinish timeIntervalSinceDate:methodStart];

Easy to use and has sub-millisecond precision.

Answer 5

Since you want to optimize time moving from one page to another in a UIWebView, does it not mean you really are looking to optimize the Javascript used in loading these pages?

To that end, I'd look at a WebKit profiler like that talked about here:

http://www.alertdebugging.com/2009/04/29/building-a-better-javascript-profiler-with-webkit/

Another approach would be to start at a high level, and think how you can design the web pages in question to minimize load times using AJAX style page loading instead of refreshing the whole webview each time.