Kernel release status [LWN.net]

Kernel release status

Posted Feb 21, 2003 6:07 UTC (Fri) by Ross (subscriber, #4065)
In reply to: Kernel release status by zooko
Parent article: Kernel release status

Yes, you are right that gettimeofday() can jump back if settimeofday() is used by the root user to change the time. Hmm... the root user, who is trusted with the ability to do anything, can break the monotonic guarantee. That's not surprising. The root user can break just about everything.

And I'm not saying you can't do time synchronization. You can. Getting the time right at boot is a start. Using NTP or something similar which doesn't change the time but changes the rate of time adjustment is another.

Of course if you absolutely don't trust it you can do it yourself. Just set up a timer and increment a variable yourself. Or use a separate process or thread doing usleep() or select(). It can't go backwards unless the system can somehow take back signals or scheduling time.

But don't forget, root can attach to your process with ptrace() and modify your variable. ;)

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monotic time

Posted Feb 23, 2003 22:45 UTC (Sun) by giraffedata (subscriber, #1954) [Link]

Like most things in a computer, gettimeofday() is monotonic or not depending on what level you look at. If you look at it above the system administration policy level, it can be monotonic. Just establish a policy of not turning back the clock.

But any level below that, it is not monotonic. Ordinary system calls are provided to set the clock back. It is thus normal. And customary. And necessary.

We need to be able write programs that work in spite of system administration policy.

Within the kernel, nobody every uses the real time clock to measure passage of time. They use the 'jiffies' clock, which cannot be turned back at any level higher than "modifying the operating system." We need that at user level.

BTW, Daylight Savings Time is _not_ an issue here. It has always been possible to get absolute time (absolute time, whether it follows the UTC standard or not, doesn't vary according to where the sun appears or local custom).

monotic time

Posted Feb 27, 2003 6:56 UTC (Thu) by hpreg (guest, #9842) [Link]

"Within the kernel, nobody every uses the real time clock to measure passage of time. They use the 'jiffies' clock, which cannot be turned back at any level higher than 'modifying the operating system.' We need that at user level."

You already have it. And it is Posix.

man 2 times

monotonic time

Posted Feb 27, 2003 18:54 UTC (Thu) by zooko (subscriber, #2589) [Link]

Thanks! That's useful.

I still have two challenges to deal with, but the fundamental one of the clock running backward can be fixed by using this. Thanks again.

(Those two challenges are (a) communicating between processes e.g. IPC or storing persistent state between invocations, and (b) mapping to programmer-meaningful elapsed time by dividing by "clocks per second".)

Regards,

Zooko