Flynn's Bottleneck - maximum speedup 2

Question

According to Flynn's Bottleneck, the speedup due to instruction level parallelism (ILP) can be at best 2. Why is it so?

Answer 1

That version of Flynn's Bottleneck originates in Detection and Parallel Execution of Independent Instructions where the authors empirically conclude that ILP for most programs is less than 2. That was 1970 technology and that was an empirical conclusion. You can contrast it with Fisher's Optimism which said there was lots of ILP out there and proposed trace scheduling and VLIW to exploit it.

So the literal answer to your question is because that's what they measured within basic blocks back then.

The ILP less than 2 meaning isn't really used anymore because superscalars and better compilers have blown past the number 2. So instead, over time Flynn's Bottleneck has come to mean You cannot retire more than you fetch which stems from his earlier paper Some Computer Organizations and Their Effectiveness.

The execution bandwidth of a system is usually referred to as being the maximum number of operations that can be performed per unit time by the execution area. Notice that due to bottlenecks in issuing instructions, for example, the execution bandwidth is usually substantially in excess of the maximum performance of a system.