collegestudent11 :
I am buying a pc for school, and I was wondering why some of the intel core i7 processors get slower as they increase in price? (GHz)
That's because there are two different lines of Core i7 CPU. The most expensive ones are 6-core units, whereas the lower-priced ones are 4-core units. Processors today all have their clock speed limited because of heat production, and thus a CPU with more cores will produce more heat than a comparable CPU at the same clock speed with fewer cores. Thus, CPUs with more cores have to be clocked lower than CPUs with fewer cores to fit in the acceptable heat production limit (roughly 130-150 watts.) The reasons the higher-core-count CPUs are more expensive are because 1) some people will pay that premium for those chips, and 2) the 6-core units use more silicon and have more transistors, so they are simply more expensive to make as they use more silicon and have a higher production error rate (resulting in a lower percentage of salable CPUs) due to their larger size than the smaller quad-core CPUs. CPUs also have a variance in how much voltage is required to hit a certain clock speed, and a CPU with more cores has less of a voltage range to play with than a lower-cored CPU does because more voltage = a lot more heat and a high-core-count CPU already is putting out a lot more heat than a lower-core-count CPU. So, you need the best dies to make the high-core CPUs whereas you can use lesser dies to make quad-core CPUs and not have them overheat.
The old adage of "good, fast, and cheap- pick any two" certainly applies here:
- You can have a CPU with a lot of cores with a low price. It would have multiple CPU dies, since as die size increases beyond a certain point (roughly 250 mm^2, or a current 4-6 core CPU), yields drop off sharply and the price of a fully-functional CPU rises accordingly. It would be clocked at a low speed so that relatively poor dies that require a lot of voltage can be used without exceeding the desired thermal limit. This is the methodology that AMD uses with CPUs like the Opteron 6128s I have in my workstation. It's a CPU that uses two quad-core dies clocked at a mediocre 2.0 GHz with an allowable voltage of a pretty high 1.30 volts, but it sells for only $266 and is usable in quad-processor motherboards. Intel's cheapest 8-core CPU uses one massive die and sells for $1334 and clocks in at 2.00 GHz.
- You can have a CPU with a lot of cores and a high clock speed, but it will cost a ton of money. A 6-core i7 running at 3.46 GHz costs $1000, an 8-core 2.67 GHz Xeon runs $2280, a 2.40 GHz 10-core Xeon costs over $4000, and a 12-core Opteron that runs at 2.5 GHz costs about $1500. Those are the cream of the crop of the big-die, low-yielding CPUs and you will pay dearly for them.
- You can have a CPU with a lot of cores and a (relatively) low price, but it will be very low-clocked. You will be using the absolute bottom of the barrel full-core-count dies with regards to voltage, and clock speeds will be kept very low to keep thermals within limits. CPUs like the Opteron 6168 are good examples as the 6168 provides 12 cores that work in quad-processor arrangements for about $700 (Intel won't sell you a 10-core unit for less than $2558) but it runs at a paltry 1.90 GHz. Ramping up the speed to 2.50 GHz will cost you roughly twice as much.
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