[ntp:questions] Re: Clock drift problems

Bill Unruh unruh at string.physics.ubc.ca
Tue Jan 20 21:45:01 UTC 2004

et472 at FreeNet.Carleton.CA (Michael Black) writes:

]Christopher Browne (cbbrowne at acm.org) writes:
]> Oops! allancady at yahoo.com (Allan Cady) was seen spray-painting on a wall:
]>[stuff deleted] 
]>> The explanation I'm given about why the clock is losing time so
]>> badly in the first place (about 15 minutes a week), is that it
]>> happens when we do our weekly backups to DVD-ROM; something is
]>> locking out the hardware interrupt that makes the clock work.  Is
]>> this "normal"?  They claim it's nothing to do with Linux, that it
]>> would happen with Windows too.  I've never seen anything like this
]>> happen on Windows... DOS maybe, but that was 15 years ago.  This is
]>> a Dell PowerEdge 1600 machine, less than a year old.
]> Yeah, this is something of a "known issue."  When the system bus gets
]> taken over by DMA, that certainly can block clocks' access.  Various
]> Unixes have suffered from similar things over the years, although it
]> is usually just that the clock gets jittery, not that it outright
]> dies.
]Well, it's really because time is kept with the real time clock.
]In the original IBM PC, you had to set the time and date every time
]you turned on the computer.  Soon after, you could get boards that
]kept time, running even when the computer was turned off.  When the AT
]came along, the clock was built into the computer.  But, this clock was
]only read when turning on the computer (unless one deliberately read it at
]other times).  However accurate that clock might be set (and however well
]it keeps the proper time) unless it's being read regularly, the real time
]clock will drift away from it.  And the longer the RTC runs without
]reading the hardware clock, the greater the accumulated error.

Your terminology could cause trouble. What you call the real time clock
is usually called the system clock. What you call the hardware clock is
usually called the real time clock. As you can see, confusion can occur.

More information about the questions mailing list