2740p - i5 versus i7 processor

Discussion in 'HP Elitebooks' started by taztech, May 18, 2011.

  1. Hattori Hanzo

    Hattori Hanzo Scribbler - Standard Member

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    @ pmatulew

    Answering this precisely is hard and requires two discrete CPUs and their exact operating power states. If we take the i5 and i7 from above, it might be that the i7 is more efficient in the lower power states than the i5 is - or vice versa.
    Practically the question is less important.

    If you barely use the CPU at all, it will be sleeping most of the time and consume barely nothing. The idle power draw of the first gen Core-i CPUs should be next to nothing, even if it differs notably between NV and LV chips. They all have power gating and even the Core2 chips barely used any power when in deep sleep.

    If you have chips of similar efficiency, you will practically use the same amount of energy for a given set of operations. While a 3Ghz chip will work 1 second and sleep 9, a 1GHz chip will work 3 and sleep 7. If both have the same efficiency, the 1GHz chip will use 10W under load and the 3GHz one will use 30W.
    There's of course a lot of other minor effects to take into consideration, like the power usage of the chipset during CPU operation etc

    Practically you often have to compare platforms. The Core2 ULV CPUs came with a different chipset, which also was slower and less power consuming. Hence the platform idle draw was a good bit lower and you could get notably longer battery life if you barely poked the machine. And they could be built smaller and lighter.

    In this case I guess it's just about the CPUs and they should be roughly the same efficiency, so paying more for barely anything is, erm, questionable ;)

    If you don't run anything CPU intensive anyways, the question is even less important. Let's look at a 10h work period and assume you have an average load of 10% with the i5. I know this is far from precise or even correct, but let's assume this could be translated into 10% of the TDP, so roughly 3.5W. Add another 0.35W as the chipset's TDP is 3.5W (HM55/57).
    If you take a look into the history of Intel's mobile platforms, you'll notice that usual 12" units consume about 10W while being idle, with the screen dimmed strongly.
    The 945 Express in the T4210/15 could go down to about 8W, the 965 Express in the T4220 was more around 10, the 45 Express about 9 and I'm assuming the whatshisname for the first Core-i to be about 9, too. This estimations are derived from various manufacturer datasheets and represent my own experience quite well. Not reflected are other changes, like the changes in backlight technology and such.

    But you can safely assume a normal system will not drop below 8W in the long term, with background tasks like indexing and such that are usually switched off for the manufacturer benchmarks. If you turn on your wireless card, the story differs again!

    So if you now have a base usage of 10W, 3.5W for the load, and 0.35 for the chipset, how much more efficient does the CPU need to be to make a real difference?
    Bringing the CPU down to use just 2W would a be huge (!!!) efficiency increase and still net you just 1.5W. With a 50Wh battery that is 3.61 hours or 4.04 hours.

    That is not even half an hour given an enormous difference between the CPUs. In this case of i5 and i7 I'd assume the practical difference to be below half a Watt in average.

    Bottom line:
    The peak power consumption of the CPU is basically only interesting when designing the (cooling) system. The importance of the efficiency of the CPU rises with it's average load.
    Practically, LV CPUs are not that much more efficient. Hence they only make sense to buy when getting a unit optimized for the small thermal footprint.

    @ AndreR
    Don't worry. Your i7 is practically as fast if you don't have it under heavy load and whoever buys your machine second hand will likely also pay more because it has an i7 label ;)
  2. pmatulew

    pmatulew Scribbler - Standard Member

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    Wonderful explanation! Thank you.
  3. KaiYop

    KaiYop Pen Pal - Newbie

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    Hey, i currently have to decide between the elitebook 2760p with the i5-2540M and the i7-2620M.
    Is it still the same issue mit the i5 being better, vor is it worth spending more (20$) for the i7 in this case.
    (Batterylife doesnt really matter to me)
    Thanks in advance!
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