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Calculating the bandwidth of crossbar system

Posted 11 December 2013 - 10:16 AM

Not quite sure which StackExchange community this question would fit it.

I had an midterm last week where I had a question I still don't quite understand, and I hope to get some help here.

So the problem is as follows

Consider 4 processors to be connected to a globally shared memory, consisting of 3 modules. Processor request probability is given as r = 0.7
    
    a) Calculate the bandwidth and expected wait time for a single bus system
    B ) Calculate the bandwidth for crossbar system assuming requests are equally distributed over memory modules
    c) Calculate the bandwidth for crossbar systems assuming M[sub]i[/sub] equally preferred by P[sub]i[/sub] and P[sub]i+1[/sub]


a) The bandwidth can be calculated with `BW = 1-(1-r)p`, where `r=0.7` and `p=4`. Thus resulting in `0,9919`. The `expected wait time` can be calculated by calculating the average number of requests arriving, divided by the bandwidth (`p*r/BW`), which is roughly `2.823`.

B ) The formula for bandwidth calculation in a crossbar system with equally distributed over all memory modules is `BW = m(1-(1-r/m)p)`, which is roughly `1.9636`.

Now for c) is the one I don't really understand what to do. Can someone point me in the right directions here? We once did something where we had a table of distributions, but not quite sure anymore.

Thanks!

EDIT: I obviously posted this in different forums as well. Can't remove the first sentence anymore :-)

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