CPU confusion

[shireknight]

I'm thinking of upgrading my desktop PC to a multiple core CPU and Iv'e just been trying to get my head round how multiple cores work, to do this I have just speed tested my laptop which is already a dual core.

It's supposed to be 1.9 ghz dual core which I took to mean it has two processors that each run at 1.9ghz giving a total processor power of just under 4ghz which just makes it slightly more powerful than my desktop PC which is a P4 3.4.

However Iv'e just run a speedtest programme called CPU-Z and the results have quite surprised me.

Vista says my processor is a 1.9ghz as I expected however CPU-Z is saying my core speed is only 800mhz? I expected this to say around 3.8mhz and certainly nowhere near 800mhz lol.

Can someone please explain this to me and help me to understand how these multiple cores work? I thought a 2.4ghz quad core was the equivalent of a P4 9.6.

Here's the shocking picture of my PC's stats.

Scary stuff huh?

 

[hp7909]

Cool 'n' Quiet is on. A very useful feature that reduces processor speed when doing less demanding tasks like word processing, surfing, listening to music, even watching (some) videos, etc. This equals to lower power consumption & on a laptop longer battery life. Read more here. Intel's equivalent is SpeedStep.

With newer processors its not just about clock-speeds. & that's because the microarchitecture has been re-designed from scratch making them more efficient at even lower speeds. Compare here

 

[Corasik]

Multiple cores, or multiple CPU's dont "ADD" together. They work side by side.

Each core runs at the same clock speed, so a quad core 2.4 ghz, is still 2.4ghz.

Oversimplifying the truth slightly, but generally programs work in one of two ways, threaded (in which the programs work is divided into sub programs) or not in which case the program does all the work in 1 go.

If you have a threaded program, each thread can be assigned to a separate core, and the workload runs in parallel, speeding up execution. A non threaded program will sit on 1 core, and will not use the other cores at the same time. (Windows might move it between different cores, but as far as performance is concerned its just using 1 core at a time).

So, multi core or multi CPU has two advantages. Threaded applications will recieve a speed boost, how much of a boost depends on how much of the workload can be done in parallel. Sometimes threads are "stalled" waiting for results from other threads, so performance gain is rarely as good as the number of cores x mhz. Its not impossible though, apache web server for example can assign 1 thread to each incoming request, give it a quad core processor and it will service pretty much 4x as many requests per second. The second advantage is multi tasking. Say your a "hydra gamer" and you play 4 copies of your favorite MMORPG. Each copy of the game now has its own processor core to play with, so you will get a much better play experience with a quad, than a single.

But run a program that times itself counting to 1 million, and it will run exactly the same speed on a 2.4Ghz single core, to a 2.4ghz quad core.

Also you can't compare a Core 2 Duo (or Quad) with a P4. P4 were very inefficient processors which on average managed between 1 and 2 instructions per clock cycle. Each core of a Core 2 can manage on average 2-4 instructions per clock cycle, so a 1.6Ghz Core 2 Duo is infact a fair match in raw performance with a 3.2Ghz Pentium 4 for single threaded applications, or even the dual core Pentium D's (Pentium D 900 series). AMD's Turion like in your laptop probably averages 2-3 instructions per clock. Its better than a P4, but not as good as a Core 2.

Think of a multi core CPU as a motorway with 2 lanes, and fill it with trucks carrying coal. If you increase to a 10 lane motorway, you can fit in a lot more coal, and more coal will appear to arrive at the destination faster. But if you measure any 1 specific truck, it still limited to the same top speed.

All that said, as intel and amd are both driving hard down the multi core route, programmers are slowely improving their techniques and increasing the amount of work that can be done in parallel, so use of multicores should only keep getting better.

Finally, as already mentioned, your laptop is running cool and quiet, to extend battery life. If you do anything that needs CPU power, it will jump to 1.9Ghz as thats the CPU's top speed. It will never display 3.8Ghz because that would be untrue. 2x1.9 = 1.9 except on a certain online auction, and some unscrupulous computer dealers who would multiply it by the cores.

 

[shireknight]

Thanks a lot guys and thanks for the simple explanation Corasik your post has really cleared up my confusion about multiple cores