Why do CPU's have "Turbo frequencies"?

[Stealth2668]

Like the i5-4670k has a normal clock of 3.4Ghz and a turbo of 3.8Ghz. Why not just advertise it as 3.8Ghz since it goes that fast when it needs to? What's so special about the "turbo" range? (3.4 - 3.8 Ghz)

 

[TheChick3n]

3.4GHz takes up less power.
More power is uses when needed instead of having a static speed and wasting power.
When you don't need that much speed like when watching a video or something, it doesn't comsume much power.
It's just like GPUs.
They have an idle speed of 300 -400 MHz That doesn't use too much power. And you are not playing games.
Scrolling 700-800MHz. Meh power
(Boost speed) You need that speed. You're playing games.
Does that make sense to you?
I got a little bit lost myself.

 

[clutchc]

Turbo usually is only available when not all cores are needed or working at full power. The fewer cores needed, the more the remaining cores can be accelerated. So, Turbo is never available when all cores are actively working on threads at their max capability.

 

[ss202sl]

Power saving feature

 

[photonboy]

Also...

It would be very MISLEADING to say CPU A is a "3.4GHz CPU" and CPU B is a "3.8GHz CPU" if the first one has no Turbo feature, and the second one does but can't run all cores at 3.8GHz.

Most laptops can't run all of the cores at the TURBO frequency so the first core might be 2.4GHz, and the other three at 1.8GHz.

So if we compared a 1.8GHz non-Turbo to my theoretical 2.4GHz (one-core Turbo) it sounds like a huge difference (33% faster. Yeah!). If all cores were used on both for say, Handbrake, the difference is actually only 8.3%.

 

[coolcole01]

basically instead of all cores running at the higher speed one core basically safely maxes out to help out with a strenuous task at least thats how intel works why the others stay the rated frequency

 

[Stealth2668]

So when turbo is activated, not all cores are running at 3.8? It may actually be 2 of them running at 3.4 and the others at 3.8 for example? But they can all run at 3.4 at the same time?

 

[coolcole01]

depending on the architecture yes that is how it works with intel i have seen amd have all cores go to the turbo speed

 

[coolcole01]

heres a good video explanation
https://www.youtube.com/watch?v=H4ryOzIZvpQ

 

[Stealth2668]

heres a good video explanation
https://www.youtube.com/watch?v=H4ryOzIZvpQ

Ahh that makes sense. So if my pc runs cool enough and with a safe voltage, you can get all cores to turbo at the same time?

 

[coolcole01]

just depends on the chip

 

[k1114]

There is some false/wrong info and/or mixing up info vs other companies' turbo features so I'm just going to do a detailed explanation; although the vid was good. The following info is about intel's tech as others are different.

Turbo is not really a power saving feature, it's for better efficiency which I understand maybe confused with power saving but this is not so straightforward. Efficiency is using what is available in the best way possible and in this case it's the cpu's tdp. Please note tdp is not power. In nominal speeds it will not fully utilize all its available headroom. Turbo uses this extra headroom to increase performance. However, the cpu can and will go above its tdp during turbo. In some cases turbo does lower power but it can also raise power. As explained in the vid, speedstep is for power saving, it lowers the speed and voltage and lowers the power. But on most desktop cpus, when turbo shifts power around, they also add extra power and thus raise power consumption above stock. They are using the extra tdp headroom and this extra is why turbo doesn't stay on all the time (although it can in many desktops). Similarly most intel cpus have a turbo speed when all cores are active that is above the normal speed. Even if they didn't add this extra, shifting power around would be the same power consumption so again it's not really saving power.

Also mentioned in the vid, their is software that may not fully utilize all cores so turbo can just boost a few cores that are actually being used: more efficiently using the power for better performance and not so much of saving power although finishing the task earlier can get it back to idle earlier and save power. In gaming that's just more fps and more power in the end so it can also depend on the task.

Being activated depends on heat and power and intel has a multi step approach and has a different speeds per active cores. The speed increases will be different on different cpus but for the i5 4670k, turbo bins is 2/3/4/4. So:
With a normal speed of 3.4
4 cores - 3.6
3 cores - 3.7
2 and 1 core - 3.8