On Jun 4, 2009, at 5:39 AM, harpreet_si...@oxyent.com wrote:
I am facing an issue in use of mach_absolute_time().
mach_absolute_time() gives drift even when NTP timing update is on.
Please see my code below:
#import <Cocoa/Cocoa.h>
#import <mach/mach_time.h>
nt main(int argc, char *argv[])
{
uint64_t nowTime = mach_absolute_time();
uint64_t drift = 0;
for (int i=0;i<1000;i++)
{
sleep(5);
uint64_t current_Time = mach_absolute_time();
uint64_t differenceTime = (current_Time - nowTime)*1E-6;
drift = differenceTime - (i+1)*5000;
NSLog(@"Elapsed Time =%lld",differenceTime);
NSLog(@"drift =%lld",drift);
}
return 1;
//return NSApplicationMain(argc, (const char **) argv);
}
Expected Results:
I expected to see zero drift in the print. After several runs, the
drift
kept on increasing
Your expectation is wrong. sleep() will sleep for 5000 milliseconds.
But (1) there's no guarantee that your thread gets to run at the
5001st millisecond, and (2) your timing and printing code itself takes
non-zero time.
If you really want to run every 5000 milliseconds with no drift,
you'll need to use a more careful sleep mechanism (starting with
nanosleep() plus accounting for the non-sleep time). A proper timer
like NSTimer or CFRunLoopTimer might also work; they won't drift, but
their jitter might be too high for your purposes.
--
Greg Parker gpar...@apple.com Runtime Wrangler
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