Detecting CPU Speed: Difference between revisions

==What is CPU Speed==

There are several different things that could be called "CPU speed":

# how quickly it can execute code (e.g. instructions per second)

# how fast its clock is running (e.g. cycles per second)

How quickly a CPU can execute code is important for determining the CPU's performance. How fast a CPU's clock is running is only useful for specific cases (e.g. calibrating the CPU's TSC to use for measuring time).

There are also several different measurements for these different "CPU speeds":

# best case

# nominal case

# average case

# current case

# worst case

For example, if you look at a modern Core i7 CPU (with turbo-boost, power management and hyper-threading), the best case instructions per second would occur when there's no throttling/power saving at all, only one logical CPU is running (turbo-boost activated and hyper-threading not being used), you're executing simple instructions with no dependencies in a loop that fits in the CPU's "loop buffer", there are no branch mispredictions, and there are no accesses to memory (no data being transferred to/from caches or RAM). The worst case instructions per second would be the exact opposite; and may be several orders of magnitude worse (e.g. a best case of 4 billion instructions per second and a worst case of 100 million instructions per second). The nominal instructions per second is an estimation of "normal" - e.g. the normal average instructions per second you'd expect (note: "nominal cycles per second" is used more often). All of these things are fixed values - a specific CPU always has the same best case, worst case and nominal case, and these values don't change depending on CPU load, which instructions are/were executed, etc.

The current instructions per second is the instructions per second at a specific instant in time and must be somewhere between the best and worst cases. It can't actually be measured, but can be estimated by finding the average instructions per second for a very short period of time. The average case is something that has to be measured. Both the current instructions per second and the average instructions per second depend heavily on the code that was running. For example, the average instructions per second for a series of NOP instructions may be much higher that the average instructions per second for a series of DIV instructions.