Pipelining: 101

It seems like every time Intel releases a new processor we have to revisit the topic of pipelining to help explain why a 3GHz P4 performs like a 2GHz Athlon 64. With a 55% longer pipeline than Northwood, Prescott forces us to revisit this age old topic once again.

You've heard it countless times before: pipelining is to a CPU as the assembly line is to a car plant. A CPU's pipeline is not a physical pipe that data goes into and appears at the end of, instead it is a collection of "things to do" in order to execute instructions. Every instruction must go through the same steps, and we call these steps stages.

The stages of a pipeline do things like find out what instruction to execute next, find out what two numbers are going to be added together, find out where to store the result, perform the add, etc...

The most basic CPU pipeline can be divided into 5 stages:

1. Instruction Fetch
2. Decode Instructions
3. Fetch Operands
4. Execute
5. Store to Cache

You'll notice that those five stages are very general in their description, at the same time you could make a longer pipeline with more specific stages:

1. Instruction Fetch 1
2. Instruction Fetch 2
3. Decode 1
4. Decode 2
5. Fetch Operands
6. Dispatch
7. Schedule
8. Execute
9. Store to Cache 1
10. Store to Cache 2

Both pipelines have to accomplish the same task: instructions come in, results go out. The difference is that each of the five stages of the first pipeline must do more work than each of the ten stages of the second pipeline.

If all else were the same, you'd want a 5-stage pipeline like the first case, simply because it's easier to fill 5 stages with data than it is to fill 10. And if your pipeline is not constantly full of data, you're losing precious execution power - meaning your CPU isn't running as efficiently as it could.

The only reason you would want the second pipeline is if, by making each stage simpler, you can get the time it takes to complete each stage to be significantly quicker than in the previous design. Your slowest (most complicated) stage determines how quickly you can get data through each stage - keep that in mind.

Let's say that the first pipeline results in each stage taking 1ns to complete and if each stage takes 1 clock cycle to execute, we can build a 1GHz processor (1/1ns = 1GHz) using this pipeline. Now in order to make up for the fact that we have more stages (and thus have more of a difficult time keeping the pipeline full), the second design must have a significantly shorter clock period (the amount of time each stage takes to complete) in order to offer equal/greater performance to the first design. Thankfully, since we're doing less work per clock - we can reduce the clock period significantly. Assuming that we've done our design homework well, let's say we get the clock period down to 0.5ns for the second design.

Design 2 can now scale to 2GHz, twice the clock speed of the original CPU and we will get twice the performance - assuming we can keep the pipeline filled at all times. Reality sets in and it becomes clear that without some fancy footwork, we can't keep that pipeline full all the time - and all of the sudden our 2GHz CPU isn't performing twice as fast as our 1GHz part.

Make sense? Now let's relate this to the topic at hand.

Index 31 Stages: What’s this, Baskin Robbins?
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  • Jeff7181 - Thursday, March 11, 2004 - link

    #98

    Yes, increasing the drive current means increasing the current that's flowing through the transistors, which does explain the heat increase.

    watts = current x voltage

    If we do the math, we can figure out how many amps the current Prescott runs on...

    The 3.2 and 3.4 Ghz models have a spec of 103 watts, and the voltage is 1.385, so divide 103 by 1.385 and you get about 74.3 amps.

    The 3.4 Ghz Northwood has a spec of 89 watts, and the voltage is 1.550, that's 57.4 amps.

    That's a 30% increase in current, with only a 20% reduction in voltage. There's your extra heat. 103 watts vs. 89 watts... about a 16% increase in heat. We can take this a little further and say...

    The Prescott at 3.4 Ghz produces 103 watts of heat, max. The Prescott at 3.0 Ghz produces 89 watts of heat, max. That means a 3.4 GHz Prescott runs on 74.3 amps, and the 3.0 GHz Prescott runs on 64.3 amps. So increasing the speed by 400 Mhz requires 10 more amps.
    So a 3.6 GHz Prescott would run on 79.3 amps, which would create 109.8 watts...
    and a 3.8 GHz Prescott would run on 84.3 amps, which would create 116.8 watts...
    and a 4.0 GHz Prescott would run on 79.3 amps, which would create 123.7 watts...
    and a 5.0 GHz Prescott would run on 104.3 amps, which would create 144.5 watts.

    This is of course assuming they don't make core changes that require less current, and that they don't make core changes that require less voltage. It will be VERY interesting to see how they deal with this increased thermal output... considering it looks like the 2.8 Ghz Prescotts are maxing out at 50 degrees C with the retail heatsinks... and the thermal output of a 5 Ghz Prescott is about twice that, so, with the same heatsink as the 2.8... a 5 Ghz Prescott should run at about 100 degrees C, lol. 5 Ghz is a ways away though, 4 is much closer, but still, that's about a 75% increase in heat over the 2.8... so you're gonna be looking at full load temps around 80 degrees C unless Intel pulls something out of their hat.

    On a side note...

    Strained Silicon is supposed to reduce current leakage, and it does. But what I think Intel maybe didn't foresee is the 30% increase in current… or maybe they thought they could run on 1.0 – 1.2 volts.

    See, voltage is electrical pressure, current is electrical volume. If you increase the volume of electricity moving through, but decrease the pressure, not as much current will leak. Think of it like a water hose. If you need a certain amount of water in a certain amount of time, you can increase the water pressure, and it will move faster so you'll have more water, but you might spring a leak in the hose... or you can just get a bigger hose and use less water pressure, which is basically what Intel did with Strained Silicon.

    AMD’s approach with using SOI has been, dare I say, more successful. When you look at the specifications, the 3400+, 2.2 GHz has a maximum of 89 watts at 1.5 volts. When the PowerNow feature is used, it drops down to 2.0 GHz, and 1.4 volts, the wattage drops down to a cool 69 watts. When it drops down again to 1.8 GHz and 1.3 volts, the wattage drops to 50 watts. And finally when it drops down to 1.0 GHz and 1.1 volts, the wattage is a frigid 22 watts. Normally you would think that means for every 200 MHz increase, your wattage increases by 10 watts. However… the FX-53 runs at 2.4 GHz and it’s maximum wattage is also 89 watts. So it seems as though AMD may be estimating very high with these early processors if a 2.4 GHz chip has the same maximum heat dissipation of a 2.0 GHz chip. The only explanation I can come up with is that as they get more experience at manufacturing these chips, current leakage just gets better and better. We can only hope to see the same from Intel with the Prescott as they refine their Strained Silicon and 90nm process.
    Reply
  • slashbinslashbash - Sunday, February 29, 2004 - link

    Oh, and I think that the conspicuous silence by AT and everybody else on this subject only confirms that Intel indeed has something up its sleeve. They all say "Prescott has higher energy consumption" and "a larger transistor count" without even speculating as to what could create the wild disparity that we see with the transistor math. Reply
  • slashbinslashbash - Sunday, February 29, 2004 - link

    #96: I'm no CPU designer, but it seems to me that the "add transistors to dissapate heat more evenly" argument doesn't make sense. Why not just have empty silicon if you need to spread things out? Adding actual transistors will also increase the amount of heat output, so the density of heat would stay the same.

    Lots of good speculation on Prescott/Yamhill here: http://www.chip-architect.com/
    Reply
  • Regs - Tuesday, February 17, 2004 - link

    Ah god, I'm sorry. This is suppose to be about the Prescott, and I just completely made a "fan boy" remark. Reply
  • Regs - Tuesday, February 17, 2004 - link

    So it's pricey, runs hot, shows little improvement over the earlier northwoods, and did I mention pricey? The 3.4c is 415 dollars at newegg let alone what a 3.2 o 3.4E would cost.

    To us tech-gurus it comes down to common sense, but everybody knows marketing will always get the better of AMD. Intel well shovel "you're paying for the best performer", which is sadly true by a small margin if that for a huge price difference. And how people ignore the A64 completely just because 64-bit is not needed as of right now is just frustrating.

    AMD made a remarkable achievement for making affordable technology while satisfying the need for higher performance.
    Reply
  • TrogdorJW - Wednesday, February 04, 2004 - link

    What does it mean to increase the transistor drive current by 10-20%? Does that mean that they need to run, say, 1.1 to 1.2 Amps instead of 1.0 Amps? (I know that's not what the processors use; I'm just using those numbers because they're easy to work with.) If that's correct, then it would certainly account for some of the heat increase.

    Initially, I read about strained silicon and thought that the idea was that it would take less power to run the chips at the same speed. The atoms are further apart, electrons flow more easily... doesn't that mean that strained silicon should make things run cooler? (I'll be honest - the electromagnetic physics course I had to take in college was *NOT* my favorite course. Talk about a HARD class....)
    Reply
  • PrinceGaz - Wednesday, February 04, 2004 - link

    I think Intel's heat problems are in part down to the Strained-Silicon technology they've introduced with the 90nm process as much as anything else. If as it says it increases the transistor drive-current by 10-20% then thats 10-20% more power and therefore heat being generated by each transistor for a given voltage.

    AMD however has opted to go for SOI now and that reduces leakage-current (waste) from the transistors, which means less heat is generated by them.

    Intel is expected to introduce SOI with their 65nm process in 2005 and that should help reduce their heat problem a bit, and AMD will no doubt adopt Strained-Silicon around about the same time which will raise the amount of heat in their chips making them both about even again.

    The difference now is that Intel implemented the heat-increasing performance improvement first, while AMD implemented the heat-decreasing one first.
    Reply
  • TrogdorJW - Wednesday, February 04, 2004 - link

    Aceshardware has some information on the transistors as well, on the bottom of page one of their review: http://www.aceshardware.com/read.jsp?id=60000315

    Of course, they also end up concluding the same things as me: the changes that Intel has really told us about don't seem like they should really be using up the 45 million added transistors. (A Northwood with 1 MB of L2 would be an 80 million transistor CPU.)

    Intel did make numerous small changes to the processor, so I guess that it is possible that they could have used up all of the extra transistors. Who knows?

    One other thing that isn't really being talked about anywhere is transistor density. In the past, shrinking the transistor size always ended up making chips run cooler. It appears that this may not be the case with 90nm processes and beyond. If Intel had stuck with a straight Northwood core and simply moved to 90nm, then the CPU die size would be something like half of what it currently is. So instead of 112 mm2, it would be 60 mm2 or something.

    With all of the heat being generated in such a small area, maybe they had to add transistors and size just to spread out the heat dissipation? It's a weird argument, but it *could* be true. When AMD releases 90nm chips and we see how hot they get, we'll probably gain more insight into this. If AMD's chips run slightly hotter, then 90nm will have marked a transition to a new set of problems in processor die shrinks.
    Reply
  • Pumpkinierre - Wednesday, February 04, 2004 - link

    The only other explanation is that prescott is dual core. Really if the stages get smaller as the pipeline gets deeper then the transistor count should stay the same. So a dual core with double the cache should be 2xNorthwood= 110 million transistors- still 15 million unaccounted for and available for other things. Other people are saying that the 31 stage pipeline cant be right as the processor's power would be much weaker than the observed performance cf. equivalent (20 stage pipe) Northwood, despite the tweaks. It seems to perform well on the hyperthreaded enabled software and dual cpu may explain the slowness of the cache like duallies where one cpu has to keep tabs on the other. It also explains the heat for which a size reduction on a single core should augur less heat in contrast to Prescott's > 100 Watts. Reply
  • Pumpkinierre - Tuesday, February 03, 2004 - link

    That's it. Prescott is already 64bit enabled. They have'nt bothered to switch them off as no intel mobo BIOS detects the 64bit extensions anyway. That's where the extra heat is coming from. I mean Northwood is ~130MM2 (55million transistors) and Prescott is close in size 112mm2 but 125million transistors - so approximately the same size but far greater transistor density so more heat. Even with the extra cache it should have been around 80 million and thus heat would have been at Northwood levels. The extra transistors still seem excessive for x86-64. So it might even be IA-64. Sckt 478 might not be pinned enough but 775 should do it. Here's my prediction then: ** 64bit WILL be available when Sckt LBGA 775 Prescott cpus come out in April with the new Grantsdale and Alderwood mobos **. And thats what is going on display in coupla of weeks time. How to check it, maybe write some assembler using X86-64 or IA-64 commands and see if they work. Reply

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