Comment by nucleardog | Hacker News Reader

Comment by nucleardog | Hacker News Reader

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> Unfortunately, here's what I actually measured with my trusty Kill-a-Watt for each server build as I performed my standard stability testing, with completely identical parts except for the CPU:

The two CPUs were in identical test rigs. There is a 80W difference changing only the CPU. While you can expect some of this is lost in the PSU, simple fact is more power is ending up inside the computer and it has nowhere to go but out as heat. The most reasonable explanation given otherwise identical builds would be to expect this difference to be due to the changed component.

So if we assume that Intel's 4-core is actually 140W TDP, then there's no way this 6-core can also be 140W.

Yes, this isn't an exact, scientific test, but it's certainly reliable enough to say "this 6-core processor is emitting more heat than the 4-core, although they are rated identically" which it seems to me was the only point he was trying to make given the context which was "two more cores, slightly lower clock speed, that might be an okay tradeoff - OH WAIT, MORE HEAT".

> So if we assume that Intel's 4-core is actually 140W TDP, then there's no way this 6-core can also be 140W.

Why? How can you say that under typical usage the thermal dissipation for both chips isn't the same? Atwood's numbers measure overall power consumption while idle and under heavy load with mprime, neither of which is what TDP seeks to measure.

TDP is like fuel economy for cars. You don't claim it's a lie when you go one hundred and fifty miles an hour for 20 miles and burn two gallons of gas. You simply realize those highway numbers are meant for more of a sixty mile per hour journey over the same distance.

Okay:

Yes, TDP is an inexactly defined and meaningless term and while to the layman it would generally be understood to have some relation to the heat emitted by the CPU during normal operation, it's possible that both chips in fact only generate 1W of heat and were given a 140W TDP because Intel had a pre-existing cooling solution and a warehouse full of spare parts. Yes, you are correct on the semantics. Power consumption has no relation to TDP because heat generated has no defined relation to TDP.

However, given any meaningfully bounded definition of TDP, the case is still made that the TDP of these chips should be dissimilar. The later numbers show that under full load the power draw at the wall increases by 20W/core used. Unless your "typical load" used for determining TDP does not actually make use of the full number of cores (which I would think could be fine for a desktop processor, certainly not a server) then it's clear that the heat generated by these two processors should be dissimilar under any load.

If the 4 core actually needs to dissipate 140W under a typical use case, then the 6 core should absolutely need to dissipate more unless the "typical use case" is uselessly applied.

If we want to talk cars... Let's say I sell a base model with a top speed of 80mph, and a sport model with a top speed of 120mph. But I tell you you only need to put 80mph rated tires on the sport model because that's as fast as a typical person drives. Would you really claim that the sport model's tires are correctly rated? Would you not be surprised when you drove 100mph in the sport model and the tires exploded? Why on Earth would anyone even buy the sport model if it's crippled to nearly the same performance as the base model?

Let's get to the crux, here:

> Unless your "typical load" used for determining TDP does not actually make use of the full number of cores

> If the 4 core actually needs to dissipate 140W under a typical use case, then the 6 core should absolutely need to dissipate more unless the "typical use case" is uselessly applied.

Taken from Intel's own specs for the E5-1650:

"Thermal Design Power (TDP) represents the average power, in watts, the processor dissipates when operating at Base Frequency with all cores active under an Intel-defined, high-complexity workload. Refer to Datasheet for thermal solution requirements."

Immediately notice "Base Frequency", not Max Turbo, "all cores active", and "Intel-defined, high-complexity workload." Until you can perform the same test on both chips, you cannot assume that "these two processors should be dissimilar under any load."

In regard to your car analogy, if the sport version had a management interface to limit speed due to the rating on the tires, then it would be like an Intel CPU. Read this:

http://www.intel.com/content/dam/www/public/us/en/documents/...

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