Re: Computer operations per second - Question.

[Austin Obyrne <austin.obyrne@hotmail.com>]

On Monday, July 23, 2012 7:10:10 AM UTC+1, Niklas Holsti wrote:
> On 12-07-22 23:20 , Austin Obyrne wrote:
> > I have just completed writing a very strong cryptographic cipher in
> > Ada-95 and I need to try and analyise the time complexity of this
> > cipher i.e the time taken to test a key space of 2560,
> > 000,000,000,000 keys at say ‘n’ operations per second.
> 
> If you are really *analysing* the time complexity (as a big-oh function 
> of problem size), based on the structure of the algorithm, the actual 
> speed of your current computer is irrelevant.
> 
> If you want to compare or verify your analysis with measurements, the 
> normal method is to measure the execution time as a function of problem 
> size and compare the shape of the measured function with the shape of 
> the big-oh complexity function. In other words, to adjust the unknown 
> constants in the big-oh function to fit the measurements.
> 
> > The processor of my home computer is a 2.61 GHz AMD processor and it
> > has 2.87 Gb of RAM.
> >
> > Is it correct for me say that my computer has a capability of 2.61
> > Giga operations per second i.e ‘n’ = 2.61 x 10^6?
> 
> First, a giga is 10^9, not 10^6. Second, it all depends on what you mean 
> by an "operation".
> 
> > Can I assume that an operation is performed every cycle of the
> > computer clock at 2.61 x10^6 operations persecond
> 
> Some basic "operations" are certainly performed at the stated 
> clock-rate, but what the operations are, in terms of your program, 
> depends on many other factors, such as the nature and number of 
> processor cores, the cache size, and the memory access patterns of your 
> program. The number of clock cycles needed to execute even a simple 
> assignment statement such as X := Y can vary from less than one to 
> several thousand, depending on the presence or absence of X and Y in the 
> caches.
> 
> Current PCs are so complex that it is not useful to compare execution 
> speeds of different algorithms when run on different PCs, unless the 
> differences are very large (such as a factor of 5 or more). If you want 
> to compare the actual speed of your encryption method against other 
> methods, you should run all the methods on the same PC (and on the same 
> data, of course).
> 
> -- 
> Niklas Holsti
> Tidorum Ltd
> niklas holsti tidorum fi
>        .      @       .

Thanks a lot Niklas - I suspected as much - it depends on may factors. - Austin O'Byrne.