Measuring the execution time of some code

DebugHelpers.h

#include <mach/mach_time.h>

void RecordPerformanceMetricks(uint64_t start, uint64_t end);

DebugHelpers.m

#import "DebugHelpers.h"

void RecordPerformanceMetricks(uint64_t start, uint64_t end) {
    dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        ////////////////////////////////////////////////////////////////////
        // https://developer.apple.com/library/mac/#qa/qa1398/_index.html //
        ////////////////////////////////////////////////////////////////////
        static mach_timebase_info_data_t    sTimebaseInfo;
        uint64_t elapsed = end - start;

        // Convert to nanoseconds.

        // If this is the first time we've run, get the timebase.
        // We can use denom == 0 to indicate that sTimebaseInfo is
        // uninitialised because it makes no sense to have a zero
        // denominator is a fraction.

        if ( sTimebaseInfo.denom == 0 ) {
            (void) mach_timebase_info(&sTimebaseInfo);
        }

        // Do the maths. We hope that the multiplication doesn't
        // overflow; the price you pay for working in fixed point.
        uint64_t elapsedNano = elapsed * sTimebaseInfo.numer / sTimebaseInfo.denom;
        /////////////////////////////////////////////////////////////////////
        /////////////////////////////////////////////////////////////////////

        NSLog(@"%lli ms", elapsedNano / 1000000); // I would like milliseconds
    });
}

FFMyClass.m

#import "DebugHelper.h"

@implementation FFMyClass

- (void)someMethod {
    uint64_t drawStart = mach_absolute_time();

    // something goes here...

    uint64_t drawEnd = mach_absolute_time();
    RecordPerformanceMetricks(drawStart, drawEnd);
}

@end

Especially interested: am I right with using dispatch_sync and NSLog? Any other suggestion and improvements?

Thanks.