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[CPUs] How does multicore work?

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May 4, 2008 2:39:40 PM

Ok so i have had much debate recently with some pro's, who tell me the multicore cpu's ghz rating stays constant over multiple cores, they tell me that, say a 2.5ghz dual core is 2.5ghz no matter how many cores are used or how hard they are used ( not counting overclocking) i for some reason thought a 2.5ghz dual core had two 2.5ghz cores. i would really like to know if i have this wrong so i can wipe the whole debate from my head and get on with some real work. lol. the debate started because some people are selling cpu's (eg 2.5ghz) at twice or four times the ghz rating (eg 5ghz or 10ghz max) i wondered if they were technically right to sell them like that?

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a c 203 à CPUs
a b K Overclocking
May 4, 2008 2:51:08 PM

seriouslycgi said:
because some people are selling cpu's (eg 2.5ghz) at twice or four times the ghz rating (eg 5ghz or 10ghz max)
Who are those "some people" ??


May 4, 2008 2:59:25 PM

Most honest people say a dual core clocked at 2.4 ghz is just that - 2.4 ghz, but i dont think its technically false advertising to say 4.8
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May 4, 2008 3:03:45 PM

WR2 said:
Who are those "some people" ??

ebay people
May 4, 2008 3:04:06 PM

WR2 said:
Who are those "some people" ??



oh, not me if thats what you mean lol. i wish i owned a dual core cpu in still using a 1.7ghz pent4
a c 203 à CPUs
a b K Overclocking
May 4, 2008 3:11:19 PM

It might be OK to say a dual core CPU that runs @ 2Ghz can do about the same amount of work a single 4Ghz could do - everything else being equal which it almost never is.
I suppose someone might be concerned that a customer might think an older Pentium 4 570 3.8Ghz might be more powerful than a C2Q E4300 @ 1.8Ghz.

May 4, 2008 3:13:43 PM

It depends on how efficiently a program utilizes cores. Most encoding applications scale almost linearly, meaning a 3.0ghz quad is effectively 12ghz. Typical programs do not scale nearly as well. On average, typical multithreaded applications scale anywhere between 40 to 80%, or 7-10ghz. Old single threaded prgrams that only use one core and let the rest idle do not scale at all.
May 4, 2008 3:14:40 PM

well for instance this article on toms hardware provoked the debate some: http://www.tomshardware.com/news/Skulltrail-6GHz-Overcl... one of the guys posted the last comment because i told him if he was certain he was right then he should tell the article writer hes wrong. i don't know so i didn't post. the article writer claimed 48ghz in the article. these "pros" that were telling me i was wrong said for instance a 2.5ghz dual core would be 1.25ghz per core when both cores are used.
a c 203 à CPUs
a b K Overclocking
May 4, 2008 3:24:49 PM

Most people just want to know how fast it will run a favorite game.


If they're tossing Skulltrail at you - you can can toss this back at 'em.
64-Processor Quad-Core Computer (256 cpu cores)
But becareful - someone is sure to what to know how well it would play Crysis.
May 4, 2008 3:51:14 PM

Answering your one question:

A dual core 2.5ghz cpu, IS 2 cpu cores, each operating at 2.5ghz. Dont let someone try to tell you otherwise.

A good example of how you can prove this: Take a small but demanding (single threaded) program, (I use a little program that I wrote in vb6, that simply eats up processor cycles, telling you how many times it can loop per second, ill post a link here if anyone asks for it.) and then find 2 cpus of the same architecture (ex. 2 athlon 64's) but get one that is single core, and one that is dual core, and run them at the same frequency (2ghz or whatever you like). Now, open 1 copy of the program on the single core. lets say it loops 100 million times a second. Now open a second copy. Both (programs) should drop substantially, to approximately 50 million a second.

Now, go over to your dual core machine of the same arch. and at the same clock speed, and open a copy of the program. 100 million you see, alright, now open a second copy. If windows works its magic correctly and puts the second copy on the second cpu core, both programs will continue to run at 100 million loops per second, because each program has its own 2ghz cpu to hog.


The other question, can you call a dual core 2ghz chip a 4ghz chip? I would say no, because there are far too many single threaded apps still in use to make this an effective way of rating chips. A single core app, on a 2ghz dual core, is still only going to get 2ghz of processing power, no matter how you look at it. And along with what dagger said, even lots of multithreaded apps dont scale perfectly, so you still wouldnt be using all of your available processing power. Thus, it is untrue to say "a dual core 2ghz is the same as 4ghz" because 90% of the time, you cant use all the power effectively at the same time.
May 12, 2008 8:56:44 AM

This was on Trademe, and I was one of the ones in the tread. Now seriouslycgi hasn't quite said this correctly. The Point alot of us was pointing out is, a 2.5 Quad core is still 2.5 max speed (So Intel says) the 4 cpu's can run to a max of 2.5 each, but really only one is full speed as the load is then sent over the other 3. This doesn't make it a 10Ghz CPU (The tech at Intel NZ had a laugh at that). As he said "If it was 10Ghz then we would sell them at that, But they arn't). This all came to head as Members are advertising systems as 10Ghz CPU systems or 12Ghz etc (you get the Idea).
May 19, 2008 10:59:26 AM

so can a 2.5ghz quad core cpu ever hit 10ghz yes or no?
May 19, 2008 11:22:10 AM

seriouslycgi said:
Ok so i have had much debate recently with some pro's, who tell me the multicore cpu's ghz rating stays constant over multiple cores, they tell me that, say a 2.5ghz dual core is 2.5ghz no matter how many cores are used or how hard they are used ( not counting overclocking) i for some reason thought a 2.5ghz dual core had two 2.5ghz cores. i would really like to know if i have this wrong so i can wipe the whole debate from my head and get on with some real work. lol. the debate started because some people are selling cpu's (eg 2.5ghz) at twice or four times the ghz rating (eg 5ghz or 10ghz max) i wondered if they were technically right to sell them like that?


a dual core at 2.0GHZ DOSE NOT run at 4.0GHZ, only means there 2 cores running at 2.0ghz meaning it can handle as much info per clock cycle as fast as signal core 4.0ghz could. But intel realized it not the speed of the CPU it how much info the CPU can process per clock cycle that is most impotent when it comes to permanence not how fast. Thats why we got dual cores at 2.0ghz and not signal cores at 4GHZ.
May 19, 2008 11:43:28 AM

The point many people seems to miss is that Hz are a measure of FREQUENCY not PROCESSING POWER. And you can't add frequency like that. If I play a note on the piano, say, A4 at 440 Hz; then get a second piano to play the same note, I don't suddenly have A5 at 880Hz - I just have A4 twice as loud. Hz = cycles per second, that's all. To add frequency in that fashion is simply a sign of poor knowledge of basic physics. I can make a 30GHz processor really easy - I just need to tell it to execute an instruction every 10th clock pulse. BUT, this CPU will not be the equivalent of 10X 3GHz CPUs executing at each pulse. Keep your units straight ppl.
May 19, 2008 12:11:57 PM

i know hz has a different meaning depending on the subject matter, but if your piano frequency was counting the vibrations per second and you generated two frequencies of equal value from two different pianos wouldn't the room be filled with twice the vibrations per second? isnt that how tone frequency is measured?

can a 2.5ghz quad core cpu hit a total of 10ghz collectively yes or no?
May 19, 2008 2:16:20 PM

No, hertz has the same meaning in all subjects. When talking cpus we are talking about the internal clock frequency. It's ONLY a timing signal that allows the CPU to sync it's operations. It does not measure the processing power of the CPU, for example a 2GHz C2D will outperform a 3GHz P4. So Hz, means exactly the same thing whether you talk pianos or computers.

Two answer your question, you will have the same number of frequencies per second, just two sources. Will a given volume of space contain a greater number of pressure changes, yes - your brain can carry out a "fourier" analysis to separate them out, but they are still two sources at original frequency. Otherwise we would hear the sound go up an octave.

So, can you overclock a 2.5GHz cpu to 10GHz - possibly, with some very exotic cooling - and a custom circuit board. I'm sure there are prototypes in Intel's lab that have gone past that speed. Could you deploy one in practice - probably not. But, that CPU would have 4 cores running at 10GHz; NOT a 40GHz CPU.
May 19, 2008 3:44:59 PM

That's exactly right, hertz is only a timing measurement and that is one cycle per second. Basically 1000 000 000 000 single instructions per second per Ghz. This is why SMT and Nehalem will be interesting.This is why the extreme CPU's cost so much more and system performance is not strictly the CPU's domain but alot about how much and how fast that CPU can be loaded up. The QX9650 has 820 million transistors vs 410 million for the Q9450 which everyone thinks is a bargain. The whole debate about dual vs quad revolves around it too, the e8400 also has 410 million transistors, hence we see similar benches between that and the Q6600. I have a strange suspicion Intel simply split the 410m transistors into 4 rather than 2 and came up with a new product called a quad core, then went on about its "multitasking" capability, which is technically true. Thats why I think the QX9650 and above are the true quad cores. Thats also why I suggest people try and find a cheap QX6*** series cpu if they want to upgrade, because the QX6700 for examply has 584 million transistors and significantly outperforms the q6600 stock.

If you look at the skulltrail video benches it generally underperformed against the QX9650 despite both the 771 cpu's having 820 million transistors each, why, because the system couldn't deliver instructions quick enough.
That is why we bench, otherwise we'd plug the stats into a formula and produce a chart.

I don't think the 4 core version of Nehalem is going to do much for most people and the 8 core will be great for servers.
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