Why parallelism has nothing to do with efficiency?

[pls-kick-me]

Efficiency is the (often measurable) ability to avoid wasting materials, energy, effort, money, and time in doing something or in producing a desired result.
Parallelism is increasing the resources we use to solve a task. It has nothing to do with efficiency.

Why parallelism has nothing to do with efficiency? You wrote above that it reduces time. This is exactly an increase in efficiency.

==============================================================
Alternative 3
...
Wow! On a busy night where the bartender works non-stop for a few hours, you calculate that they now
only use just over 20 seconds on an order. You’ve basically eliminated all the waiting. Your theoretical
throughput is now 240 beers per hour.

Why 240? Maybe 180?

However, you realize that you didn’t actually accomplish 240 beers an hour, since orders come
somewhat erratically and not evenly spaced over time. Sometimes, the bartender is busy with a new
order, preventing them from topping up and serving beers that are finished almost immediately. In
real life, the throughput is only 180 beers an hour.

You mean ~120 beers an hour?

[cfsamson]

Hi @pls-kick-me! Thanks for sumbitting questions and corrections.

Why parallelism has nothing to do with efficiency? You wrote above that it reduces time. This is exactly an increase in efficiency.

So, for something to be efficient, it's not enough to look at the absolute reduction in time. Fast is not the same as efficient. Efficiency has to be measured relative to something, and often it's measured against how much resources you use to achieve your goal. Remember that in this context, CPU-time is a resource, so if you use 10 seconds on a unit of work on one core, but use 5 seconds on two cores, you still use the same amount of CPU-time.

If you spread the work on two cores and somehow got it done in 4 seconds, it would be more efficient, but that's usually not the case unless you also alter the algorithm you use.

Why 240? Maybe 180?
You mean ~120 beers an hour?

That's very observant of you :) You're right, I did the calculations based on an example that used slightly shorter time for pouring the beers, but somehow failed to get the updated numbers for the calculations themselves correctly in the book. It's already noted in the errata, and I'm sorry for that. It's a rather embarrassing mistake to make, to be honest.