Intel's Core i7 870 & i5 750, Lynnfield: Harder, Better, Faster Stronger
by Anand Lal Shimpi on September 8, 2009 12:00 AM EST- Posted in
- CPUs
Homework: How Turbo Mode Works
AMD and Intel both figured out the practical maximum power consumption of a desktop CPU. Intel actually discovered it first, through trial and error, in the Prescott days. At the high end that's around 130W, for the upper mainstream market that's 95W. That's why all high end CPUs ship with 120 - 140W TDPs.
Regardless of whether you have one, two, four, six or eight cores - the entire chip has to fit within that power envelope. A single core 95W chip gets to have a one core eating up all of that power budget. This is where we get very high clock speed single core CPUs from. A 95W dual core processor means that individually the cores have to use less than the single 95W processor, so tradeoffs are made: each core runs at a lower clock speed. A 95W quad core processor requires that each core uses less power than both a single or dual core 95W processor, resulting in more tradeoffs. Each core runs at a lower clock speed than the 95W dual core processor.
The diagram below helps illustrate this:
| Single Core | Dual Core | Quad Core | Hex Core | |
| TDP |  |  |  |  |
| Tradeoff |  |  |  |  |
The TDP is constant, you can't ramp power indefinitely - you eventually run into cooling and thermal density issues. The variables are core count and clock speed (at least today), if you increase one, you have to decrease the other.
Here's the problem: what happens if you're not using all four cores of the 95W quad core processor? You're only consuming a fraction of the 95W TDP because parts of the chip are idle, but your chip ends up being slower than a 95W dual core processor since its clocked lower. The consumer has to thus choose if they should buy a faster dual core or a slower quad core processor.
A smart processor would realize that its cores aren't frequency limited, just TDP limited. Furthermore, if half the chip is idle then the active cores could theoretically run faster.
That smart processor is Lynnfield.
Intel made a very important announcement when Nehalem launched last year. Everyone focused on cache sizes, performance or memory latency, but the most important part of Nehalem was far more subtle: the Power Gate Transistor.
Transistors are supposed to act as light switches - allowing current to flow when they're on, and stopping the flow when they're off. One side effect of constantly reducing transistor feature size and increasing performance is that current continues to flow even when the transistor is switched off. It's called leakage current, and when you've got a few hundred million transistors that are supposed to be off but are still using current, power efficiency suffers. You can reduce leakage current, but you also impact performance when doing so; the processes with the lowest leakage, can't scale as high in clock speed.
Using some clever materials engineering Intel developed a very low resistance, low leakage, transistor that can effectively drop any circuits behind it to near-zero power consumption; a true off switch. This is the Power Gate Transistor.
On a quad-core Phenom II, if two cores are idle, blocks of transistors are placed in the off-state but they still consume power thanks to leakage current. On any Nehalem processor, if two cores are idle, the Power Gate transistors that feed the cores their supply current are turned off and thus the two cores are almost completely turned off - with extremely low leakage current. This is why nothing can touch Nehalem's idle power:
Since Nehalem can effectively turn off idle cores, it can free up some of that precious TDP we were talking about above. The next step then makes perfect sense. After turning off idle cores, let's boost the speed of active cores until we hit our TDP limit.
On every single Nehalem (Lynnfield included) lies around 1 million transistors (about the complexity of a 486) whose sole task is managing power. It turns cores off, underclocks them and is generally charged with the task of making sure that power usage is kept to a minimum. Lynnfield's PCU (Power Control Unit) is largely the same as what was in Bloomfield. The architecture remains the same, although it has a higher sampling rate for monitoring the state of all of the cores and demands on them.
The PCU is responsible for turbo mode.
Facebook
Twitter
RSS
343 Comments
View All Comments
TA152H - Wednesday, September 9, 2009 - link
Your assumption would be wrong, I use computers for the normal stuff people do, and compiling.Go back to picking your nose, lowlife.
Skiprudder - Tuesday, September 8, 2009 - link
I'm starting to feel like some folks have Bloomfields and now they're trying to justify spending the money they did. At no point did Anand say Bloomfields were 'bad', he's just pointing out that due to current price/performance ratios the new chips are fantastic for the vast amount of folks here. They really area big step up for a lot of people, and frankly a lot of us feel we have better things to spend out money on than $300 X58 mobos. We can now get in on that sort of performance (or better) for a reasonable price, and how isn't this great?TA152H - Tuesday, September 8, 2009 - link
Well, you might be right, but within this context I'm going to assume you are talking about me, in which case you would be wrong.I don't have anything new, and I'm actually not going to get anything for a few months. Even if I did, I wouldn't let it cloud my perspective. Normally people who make these judgments are looking through their own personality flaws. I have many, but this is one area I do not.
Maybe I am annoyed at the processor because I was expecting more. I was expecting it to be a really nice processor, and it turned out disappointing. I get annoyed with stupid things sometimes, like the 4 clock cycle L1 cache irks me big time with the Nehalem. With the P55, the platform is just weak and I don't really like it when sites do everything they can to obfuscate the compromises in it.
I'll say this, though, about it, the power use is REALLY nice. I'm really impressed with that. Everything else though, just leaves me cold.
There are some questions that need to be asked, as well. Why is the P55 so damn expensive? It's the same cost as the P45, but without most of the logic.
Why is the 870 so expensive? Is there really any point to this processor at this cost? Maybe some, but not really for a broad segment of the buying population. I'd much rather have an LGA1366 if I were going into that type of expense, instead of the brain-damaged derivative.
LGA1366 motherboards are around $200 now, at least many are, so $300 is kind of an outdated number. For this, you get better i/o, better memory flexibility (you can use two or three dimms; you can't use three with a P55), better performance, etc...
Now, one thing Anand brought up, and I didn't respond to, was something about not needing more bandwidth unless you were tapping out all four processors. Needless to say, this is obviously false, and I wonder why he'd repeat it. A little thought would tell us that even with two cores running, you could gain advantages by having better bandwidth. The reason is simple. Let's say Core A and Core B are both plugging away, and Core A needs a memory read. OK, so it takes the bus, and starts the long process of getting a cache line read. Now, let's say a clock cycle, or two, or twenty, later, Core B needs memory outside of cache. So now it needs the memory bus. Hmmmm, but Core A has the bus. So, Core B has to wait. If you have three memory channels, you transfer data faster, satisfying the cache line faster, and freeing the bus sooner. So, now Core B can get it, and start working sooner. So, you could see better performance with lower processor use requirements than they indicate.
Now, the only possibility this premise is wrong is if the core only grabs 64-bit memory bank at a time, which seems very unlikely to me. In this event, there would still be one memory channel open to the second processor, thus no penalty would occur until three processors needed memory. I don't think this is what they did, as it would lower memory performance on the Bloomfield unless three processors were being used, and also, based on the shared L3 cache, it seems the entire memory bus is always used. Still, it is possible. Maybe Anand can answer this.
mesiah - Wednesday, September 9, 2009 - link
Dude, you are so full of shit they can smell it 3 states away. Don't you dare try to rationalize by saying "I was expecting more." Since lynnfield news first broke you have done nothing but trash the part, Call it brain damaged, and skew posted facts to fit your warped agenda. Then, when the retail part is finally unveiled, instead of eating crow (something you suggest Anand do) you act like every crazy half baked conspiracy theorist that ever existed and start ranting about biased reporting, facts being intentionally hidden, and flawed logic.While I will admit that this isn't the be all end all lynnfield review, this is day one of the launch. I believe more information was displayed in this review than can be reasonably expected for the initial review. In depth overclocking comparisons and the like will certainly come later, but that isn't to say overclocking was ignored.
You read the article and then go off on a tirade about biased reporting and the skewing of facts to meet an agenda when it is you that have been skewing facts all along, and you continue to do so. You make overclocking comparisons between lynnfield and bloomfield calling them similar, then ask why anyone would give up "all the other advantages" of the bloomfield if you are overclocking, but you fail to mention the big advantage for most, which is price comparison. Then later you try to rationalize price by quoting the lowest posted 920 sale price next to the 870 msrp. The parts hit the shelves today, give them a month for prices to normalize. You aren't going to get a smokin deal on the first one to roll off the production line.
Now, go ahead and call me a fool like you do everyone else that is smart enough to see you for what you are. Attempting to verbally abuse another person in order to make yourself feel smarter is a pretty common tactic for feeble minded people who are themselves afraid of looking stupid. Its the equivalent to bringing a gun to a fist fight just in case you start to get your ass beat. So, how about you stop being a pussy, eat a little crow, and admit that all of your talk about how terrible this part was going to be was wrong. Or better yet, just stop posting here because your pissy "I'm better than you" attitude does nothing but bring the site down.
You can reply and call me all the dirty names you want, you wont get a response. I only feed the trolls once a week.
TA152H - Wednesday, September 9, 2009 - link
Actually, you're an idiot, and you're changing my words to create an argument.I never said it was terrible, I said the pre-release performance was terrible, and I expected it to be better for the released version. It was, but I still don't think so highly of the processor, and I wouldn't consider it. I'd rather get the i7 920.
If you're going to argue, at least have the decency/intelligence to not misrepresent someone's position.
I didn't really read the rest of your crap. I could only skim through your sub-literate drivel. I doubt you would have said anything useful in it.
Skiprudder - Thursday, September 10, 2009 - link
Can you please stop referring to people as idiots and morons? I'm not sure where you acquired your rhetorical skills, but calling folks names is the last thing one should do if you're actually trying to convince people of your viewpoint. Call someone here a moron, and it makes you look like just a troll. Calm down, stay rational, and people will be much more likely to hear what you have to say (and if they aren't, why should you care anyway?) There's no excuse for rudeness.Skiprudder - Tuesday, September 8, 2009 - link
I agree, the 870 isn't priced at all sensibly (but I would argue the top Bloomfields are pretty darn unreasonable too). Prices are rather high right now, and it will be interesting to see what they do over the next few weeks as supplies and demands start to balance out.chrnochime - Tuesday, September 8, 2009 - link
The sad thing is, right now(at least), in order to get i7 CPU and decent MB to OC with, a P55 set up would run:~300 for the i7 860
plus
~200 for an UD4P or an Asus P55 Pro.
Whereas for a x58 setup,
I can get an i 7 920 for 200 pre-tax, and should spend about 230-250 for a decent motherboard.
So for me, it's actually cheaper to go with the x58 setup, even though P55 MB are supposed to be cheaper...
TA152H - Tuesday, September 8, 2009 - link
Yes, and you'd end up with better performance too. And better i/o, and better flexibility.Once you factor in overclocking, the P55 isn't really much of an option unless you go to it's really low end, where you simply can't build an x58. Then it's competing with the Core 2, and has a chance.
ClownPuncher - Tuesday, September 8, 2009 - link
Good article, thanks for the clarifications too.