[ubuntu-uk] regarding for solution

Mark Harrison Mark at ascentium.co.uk
Wed May 16 17:18:10 BST 2007


fatma oymak wrote:
> Dear all,
>
> I have one problem....I couldnt find right answer...do you have any idea? 
> please please let me know
>
> many thanks
> fatma
>
>
> "........Consider the behaviour of two machines in a distributed system. 
> Both have clocks that are supposed to tick 1000 times per millisecond. One 
> of them ticks 990 times per millisecond. The other ticks 1015  times per 
> millisecond. If the system designer want to guarantee that clocks of these 
> two machines never differ by more than 5 seconds, how often must be clocks 
> be re-synchronized? Supposing that all machines in this distributed systems 
> come from same manufacturer and the maximum drift rate is specified as 1.0%, 
> how often must the clocks of this system must be re-synchonized  if the 
> system designers want to guarantee that clocks of these two machines never 
> differ by more than 5 seconds? Describe in steps how you get your 
> result.......
>   
Part 1: At least every 200 seconds - the difference between them gets 
.025 seconds (25 milliseconds) bigger each "tick", so it will take 200 
seconds for them to be 5 seconds apart (at which point the "fast" clock 
will read 202.02 is, and the "slow" clock will read 197.04 ish)

Part 2: Consider the "pathalogical case" of variance, where one 
over-ticks by 1% and one under-ticks by one percent, so once is at 990 
ms and the other at 1110 ms - hence the divergence is 20ms per tick... 
so it takes 250 ticks to diverge by five seconds. The third clock is 
irrelevant, since it has to be somewhere either at one of these 
extremes, or between them, so will never read further apart than the two 
pathalogical outliers.

As an implementation issue, assuming that this can somehow be made 
Ubuntu-related, I'd sync each every minute with a crontab job :-)

Mark



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