CPU Instructions per second

Question

I was given this question on my study guide for a test I have coming up. I don't understand how to tackle this problem. My test is today and I would appreciate some help.

If a CPU issues one memory request every instruction and the computer runs at 200 MIPS, about how many CPUs will it take to saturate a 400-MHZ bus? Assume a memory reference requires one bus cycle. Now repeat this problem for a system in which caching is used and the caches have a 90% hit rate. Finally, what cache hit rate would be needed to allow 32 CPUs to share a bus without overloading it?

Answer 1

I am trying to guide you to figuring out the answers... hopefully this helps.

how many CPUs will it take to saturate a 400-MHZ bus?

Computing the number of processors is simple division. You know what the bus speed is and the number of operations per processor. This is pretty much the obvious answer.

Now repeat this problem for a system in which caching is used and the caches have a 90% hit rate.

OK, so here you would figure out how many of the instructions are hitting the bus for each processor , and use that figure instead of using the 200 MIPS number. Same division as above.

Finally, what cache hit rate would be needed to allow 32 CPUs to share a bus without overloading it?

This is basically the reverse of the above... you know that bus rate, and the desired nuumber of processors. Figure out how many instructions are reaching the bus for each processor (division), and then use that number to figure out each processor's cache hit ratio (also division).