Messages in this thread | | | Date | Thu, 20 Mar 2003 18:50:46 -0800 | From | Joel Becker <> | Subject | Re: Clock monotonic a suggestion |
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On Thu, Mar 20, 2003 at 04:16:13PM -0800, george anzinger wrote: > Define CLOCK_MONOTONIC to be the same as > (gettimeofday() + wall_to_monotonic). > ... > Both clocks will tick at the same rate, even under NTP corrections. > The conversion is a simple (well almost simple) add. > Both clocks will have the same resolution.
The issue for CLOCK_MONOTONIC isn't one of resolution. The issue is one of accuracy. If the monotonic clock is ever allowed to have an offset or a fudge factor, it is broken. Asking the monotonic clock for time must always, without fail, return the exact, accurate time since boot (or whatever sentinal time) in the the units monotonic clock is using. This precludes gettimeofday(). If the system is delayed (udelay() or such) by a driver or something for 10 seconds, then you have this (assume gettimeofday is in seconds for simplicity):
1 gettimeofday = 1000000000 2 driver delays 10s 3 gettimeofday = 1000000000 4 timer notices lag and adjusts 5 gettimeofday = 1000000010
In the usual case, if a program calls gettimeofday() between 3 and 4, the program gets the wrong time. For most programs, this doesn't matter. CLOCK_MONOTONIC is designed for those uses where it absolutely matters. If an application queries CLOCK_MONOTONIC at 3.5, it must return 1000000010, not 1000000000.
Joel --
"A narcissist is someone better looking than you are." - Gore Vidal
Joel Becker Senior Member of Technical Staff Oracle Corporation E-mail: joel.becker@oracle.com Phone: (650) 506-8127 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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