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To: Joe NYC who wrote (53288)	8/30/2001 8:22:01 PM
From: wanna_bmw	Read Replies (1) \| Respond to of 275872

Joe, Re: "a P4 running at 1 GHz would have (IMO) higher IPC than the same processor running at 2 GHz, since a cache miss would stall the 2 GHz CPU for a lot more cycles than 1 GHz CPU. Is my understanding of IPC correct?" IPC stands for Instructions Per Cycle. Assuming that everything fits in the lowest level cache, performance would increase linearly with frequency. Therefore, IPC would be exactly equal for both processors. On the other hand, let's consider the other possibility that all the processor caches are disabled, and all data comes from memory. Let's say, for example, that memory stalls a piece of data for 100ns on the front side bus. That's the equivalent of 100 cycles for a 1GHz processor, and 200 cycles for a 2GHz processor. Therefore, if every instruction required a piece of data from memory, the 2GHz processor would have half the IPC of the 1GHz processor. So those are your two extremes. Either IPC is exactly equal, or IPC gets lower with frequency. In real life, however, there are a lot of things that weigh heavily on the former, rather than the latter. Out-of-order execution allows the processor to work on more data, while accesses go out to memory. Applications usually reside partly in cache, and partly in memory, and therefore, you require data from both (except that it's from cache far more often than from memory). Other things are involved, but most people tend to relate IPC as being constant among all frequencies, since that is a close approximation. But since you are observant, you've realized that IPC innately shrinks as a processor increases in frequency. wanna_bmw