Difference between getIPC() and getExecUsage

[arajeev9]

I am trying to extract CPU counter information. I see 2 APIs:
getIPC() description: Computes average number of retired instructions per core cycle (IPC)
getExecUsage() description: Computes average number of retired instructions per time interval

Are the values correlated as getExecUsage() = c * getIPC() ? Why do we have these 2 different APIs when it appears as if they are providing the same information ?

Thanks

[opcm]

IPC = instructions /cycles
ExecUsage = instructions / time

the number of cycles per time period is not constant because it depends on C-state and P-states.