CODEWITHSUNDEEP
multithreadingoptimizationconcurrencyperformance
sharptooth
sharptooth

Reputation: 170479

How would a multithreaded program be more energy efficient?

In its Energy-Efficient Software Guidelines Intel suggests that programs are designed multithreaded for better energy efficiency.

I don't get it. Suppose I have a quad core processor that can switch off unused cores. Suppose my code is perfectly parallelizeable (synchronization overhead is negligible).

If I use only one core I burn one core for one hour, if I use four cores I burn four cores for 15 minutes - the same amount of core-hours either way. Where's the saving?

Upvotes: 5

Views: 1962

Answers (6)

Damon
Damon

Reputation: 70106

There are 3 reasons, two of which have already been pointed out:

  1. More overall time means that other (non-CPU) components need to run longer, even if the net calculation for the CPU remains the same
  2. More threads mean more things are done at the same time (because stalls are used for something useful), again the overall real time is reduced.
  3. The CPU power consumption for running the same calculations on one core is not the same. Intel CPUs have a built-in clock boosting for single-core usage (I forgot the marketing buzzword for it). A higher clock means dysproportionally more power consumption and dysproportionally more heat, which again requires the fan to spin faster, too.

So in summary, you consume more power with the CPU and more power for cooling the CPU for a longer time, and you run other components for a longer time, too.

As a 4th reason, one could allege (note that this is only an assumption!) that Intel CPUs are hyperthreaded, and since hyperthreaded cores share some resources, running two threads at once is more efficient than running one thread twice as long.

Upvotes: 2

Jonathan Dickinson
Jonathan Dickinson

Reputation: 9218

I suspect it has to do with a non-linear relation between CPU utilization and power consumption. So if you can spread 100% CPU utilization over 4 CPUs each will have 25% utilization - and say 12% consumption.

This is especially true when dynamic CPU scaling is used according to Wikipedia the power drain of a CPU is P = C(V^2)F. When a CPU is running faster it requires higher voltages - and that 'to the power of 2' becomes crucial. Furthermore the voltage will be a function of F (which means F can be solved for V) giving something like P = C(F^2)F. Thus by spreading the load over 4 CPUs (running at 100% capacity at that frequency) you can mitigate the cost for the same work.

We can make F a function of L (load) at 100% of one core (as it would be in your OS), so:

F = 1000 + L/100 * 500 = 1000 + 5L
p = C((1000 + 5L)^2)(1000 + 5L) = C(1000 + 5L)^3

Now that we can relate load (L) to the power consumption we can see the characteristics of the power consumption given everything on one core:

p = C(1000 + 5L)^3
p = 1000000000 + 15000000L + 75000L^2 + 125L^3

Or spread over 4 cores:

p = 4C(1000 + (5/4)L)^3
p = 4000000000 + 15000000L + 18750.4L^2 + 7.5L^3

Notice the factors in front of the L^2 and L^3.

Upvotes: 6

Hassan Syed
Hassan Syed

Reputation: 20485

A CPU is one part of a computer. It has fans, a motherboard, hard drives, graphics card, RAM etc, lets call this the BASE. If your doing scientific computing (i.e., a compute cluster) you are powering many computers. If you are powering 100's of BASE's anyway, why not allow those BASES to have multiple physical CPU's on them so those CPU's can share the resources of the BASE, physical and logical.

Now INTEL's marketing blurb probably also depends on the fact that these days, each CPU wafer contains multiple cores. Powering multiple physical CPU's is different to powering a single physical cpu with multiple cores.

So if amount of work done per unit of power is the benchmark in question, then modern CPU's performing highly parallel tasks then yes you get more bang for your buck, compared with the previous generation of processors. As not only can you get more cores / cpu, it is also common to get BASE's which can take multiple CPU's.

One may easily assert that one top-end system can now house the processing power of 8-16 singl-cpu single-core CPU's of the past (assuming that in this hypothetical case, that on the new system and the older generation system, each core has the same processing power ).

Upvotes: 3

nakhli
nakhli

Reputation: 4059

You burn 4 times energy with 4 cores but you do 4 times more work too! If, as you said, the synchro is negligible and the work is parallelizable, you'll spend 4 times less time.

Using multiple threads can save energy when you have i/o waits. One thread can wait while other threads can perform other computations; instead of having your application idle.

Upvotes: 3

JanLikar
JanLikar

Reputation: 1306

If a program is multithreaded that doesn't mean that it would use more cores. It just means that more tasks are dealt with in the same time so the overall processor time is shorter.

Upvotes: 2

Marcelo Cantos
Marcelo Cantos

Reputation: 185852

During that one hour, the one core isn't the only thing you keep running.

Upvotes: 4

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