On 6 Dec 2005 at 11:35, Roman Zippel wrote:
> Hi,
>
> On Tue, 6 Dec 2005, Ingo Molnar wrote:
>
> > > > I'm thinking about moving the leap second handling to a timer, with the
> > > > new timer system it would be easy to set a timer for e.g. 23:59.59 and
> > > > then set the time. This way it would be gone from the common path and it
> > > > wouldn't matter that much anymore whether it's used or not.
> > >
> > > Will the timer solution guarantee consistent and exact updates?
> >
> > it would still be dependent on system-load situations.
>
> Interrupt-load, actually.
>
> > It's an
> > interesting idea to use a timer for that, but there is no strict
> > synchronization between "get time of day" and "timer execution", so any
> > timer-based leap-second handling would be fundamentally asynchronous. I
> > dont think we want that, leap second handling should be a synchronous
> > property of 'time'.
>
> I'm not really sure what you're talking about. Could you please elaborate
> on "fundamentally asynchronous" and "synchronous property of 'time'"?
It's always the same: A process busily reads time, and it wants to have it smooth
(low jitter, preferrably constant jitter, small time increments):
/*
* This program can be used to calibrate the clock reading jitter of a
* particular CPU and operating system. It first tickles every element
* of an array, in order to force pages into memory, then repeatedly calls
* gettimeofday() and, finally, writes out the time values for later
* analysis. From this you can determine the jitter and if the clock ever
* runs backwards.
*/
#include <sys/time.h>
#include <stdio.h>
#define NBUF 20002
void
main()
{
struct timeval ts, tr;
struct timezone tzp;
long temp, j, i, gtod[NBUF];
gettimeofday(&ts, &tzp);
/*
* Force pages into memory
*/
for (i = 0; i < NBUF; i ++)
gtod[i] = 0;
/*
* Construct gtod array
*/
for (i = 0; i < NBUF; i ++) {
gettimeofday(&tr, &tzp);
gtod[i] = (tr.tv_sec - ts.tv_sec) * 1000000 + tr.tv_usec;
}
/*
* Write out gtod array for later processing with S
*/
for (i = 0; i < NBUF - 2; i++) {
/*
printf("%lu\n", gtod[i]);
*/
gtod[i] = gtod[i + 1] - gtod[i];
printf("%lu\n", gtod[i]);
}
/*
* Sort the gtod array and display deciles
*/
for (i = 0; i < NBUF - 2; i++) {
for (j = 0; j <= i; j++) {
if (gtod[j] > gtod[i]) {
temp = gtod[j];
gtod[j] = gtod[i];
gtod[i] = temp;
}
}
}
fprintf(stderr, "First rank\n");
for (i = 0; i < 10; i++)
fprintf(stderr, "%10ld%10ld\n", i, gtod[i]);
fprintf(stderr, "Last rank\n");
for (i = NBUF - 12; i < NBUF - 2; i++)
fprintf(stderr, "%10ld%10ld\n", i, gtod[i]);
}
(Code taken from some 10 year old xntp source tree)
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