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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  • terrywongintra - Monday, February 02, 2004 - link

    anybody benchmark prescott over northwood in entry-server environment? i'm installing 3 servers later by using intel 875p (s875wp1-e) entry server board n p4 2.8, need to decide prescott or northwood to use. Reply
  • sipc660 - Monday, February 02, 2004 - link

    i don't understand why some people are bashing such a good inovation that was long overdue from intel.

    a pc that doubles as a heater and at only 100-200W power consumption.

    Let me remind you that a conventional fan heater eats up a kilowatt/hour of power.

    Think positive

    * space reduction
    * enormous power savings (pc + fan heater)
    * extremly sophisticated looking fan haeter
    * extremly safe casing. reduces burn injuries
    to pets and children.
    * finely tunable temperature settings (only need
    to overclock by small increments)
    * coupled with an lcd it features the best
    looking temperature adjustment one has ever
    witnessed on a heater
    * child proof as it features thermal shutdown
    * anyone having a laugh thus far
    * will soon feature on american idol
    the worst singers will receive one p4 E based
    unit each. That should make people
    think twice about auditioning thus making
    sure only true talent shows up.
    * gives dell new marketing potential and a crack
    at a long desired consumer heating electronic
    * amd is nowhere near this advancement in thermal
    thechnology leaving intel way ahead


    hope you enjoyed some of my thoughts

    Other than that good article and some good comments.

    on another note i don't understand why people run and fill intels pockets so intel can hide their engineering mistakes with unseen propaganda, while there is an obvious choice.

    choice is Advanced Micro Devices all until intel gets their act together.

    go amd...
    Reply
  • Stlr22 - Monday, February 02, 2004 - link

    INTC - "Intel roadmap says Prescott will hit 4.2 GHz by Q1 '05. My guess is that it is already running at 4 GHz but just needs to be fine tuned to reduce the heat."


    Maybe they are trying to keep it under the 200watt mark? ;-)
    Reply
  • INTC - Monday, February 02, 2004 - link

    I think CRAMITPAL must have sat on a hot Prescott and got it stuck where the sun doesn't shine - that would explain all of the yelling and screaming and friggin this and friggin that going on. "Approved mobo, approved PC case cooling system, approved heatsink & fan - and you better not use Artic Silver or else it will void your warranty..." gee - didn't we just hear that when Athlon XPs came out? It brings to mind when TechTV put their dual Athlon MP rig together and it started smoking and catching on fire when they fired it up the first time on live television during their show.

    Intel roadmap says Prescott will hit 4.2 GHz by Q1 '05. My guess is that it is already running at 4 GHz but just needs to be fine tuned to reduce the heat. I bet the experts (or self proclaimed experts such as CRAM) were betting that Northwood could not hit 3 GHz and look where it is at today. Video card GPUs today are hitting 70 degrees C plus at full load but they do fine with cooling in the same PC cases.
    Reply
  • CRAMITPAL - Monday, February 02, 2004 - link

    Dealing with the FLAME THROWER's heat issues is only one aspect of Prescott's problems. The chip is a DOG and it requires an "approved Mobo" and an "approved PC case cooling system", a premo PSU cause the friggin thing draws 100+ Watts and this crap all costs money you don't need to spend on an A64 system that is faster, runs cooler, and does both 32/64 bit processing faster. How difficult is THIS to comprehend???

    Ain't no way Intel is gonna be able to Spin this one despite the obvious "press material" they supplied to all the reviewers to PIMP that Prescott was designed to reach 5 Gigs. Pigs will fly lightyears before Prescott runs at 5 Gigs.

    Time to GET REAL folks. Prescott sucks and every hardware review site politely stated so in "political speak".
    Reply
  • Stlr22 - Monday, February 02, 2004 - link

    ((((((((((((CRAMITPAL)))))))))))))))




    It's ok man. It's ok. Everything will be alright.


    ;-)
    Reply
  • scosta - Monday, February 02, 2004 - link

    #38 - About your "Did anyone catch the error in Pipelining: 101?".

    There is no error. The time it takes to travel the pipelane is just a kind of process delay. What matters is the rate at witch finished/processed results come out of the pipeline. In the case of the 0.5ns/10 stage pipelane you will get one finished result every 0.5ns, twice as many as in the case of the 1ns/5 stage pipeline.

    If the pipelines were building motorcycles, you woud get, respectively, 1 and 2 motorcycles every ns. And that is the point.
    Reply
  • LordSnailz - Monday, February 02, 2004 - link

    I'm sure the prescotts will get hotter as the speed increases but you can't forget there are companies out there that specializes in this area. There are 3 companies that I know of that are doing research on ways to reduce the heat, for instance, they're planning on placing a piece of silicon with etch lines on top of the CPU and run some type of coolant through it. Much like the radiator concept.

    My point is, Intel doesn't have to worry about the heat too much since there are companies out there fighting that battle. Intel will just concentrate on achieving those higher speeds and the temp control solution will come.
    Reply
  • scosta - Monday, February 02, 2004 - link

    You can find thermal power information in the also excelent "Aces Hardware" Prescot review here:
    [L=myurl]http://www.aceshardware.com/read.jsp?id=60000317[/l]

    In resume, we have the following Typical Thermal Power :
    P4 3.2 GHz (Northwood) - 82W
    P4E 3.2 GHz (Prescot) - 103W

    Note that, at the same clock speed and with the same or lesser performance, the Prescot dissipates 25% more power than Northwood. This means that with a similar cooling system, the Prescot has to run substancially hoter.

    As AcesHardware says,
    [Q]After running a 3DSMax rendering and restarting the PC, the BIOS reported that the 3.2 GHz Northwood was at about 45-47°C, while Prescott was flirting with 64-66°C. Mind you, this is measured on a motherboard completely exposed to the cool air (18°C) of our lab.[/Q]

    So, what will the ~5GHx Prescot dissipate? 200W ?
    Will we all be forced to run PCs with bulky, expensive, etc, criogenic cooling systems?. I for one wont. This power consumption escalation has to stop. Intel and AMD have to improve the performace of their CPUs by improving the CPU archytecture and manufacturing processes, not by trowing more and more electrical power at the problem.

    And those are my 2 cents.
    Reply
  • CRAMITPAL - Monday, February 02, 2004 - link

    Prescott will never go above 3.8 Gig. even with the 3rd revision of the 90 nano process. Tejas will make it to just over 4.0 Gig. with a little luck but it won't be anything to write home about either based on current knowledge.

    Intel has fallen and can't get it up!
    Reply

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