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?
POST A COMMENT

103 Comments

View All Comments

  • sprockkets - Monday, February 02, 2004 - link

    Hmmm... on Intel's website on the new processor news: "Thermal Monitoring: Allows motherboards to be cost-effectively designed to expected application power usages rather than theoretical maximums."

    Not sure what it means. I'm thinking clock throttling so that if your particular chip is hotter than it should be it will run on under engineered motherboards/coolers.

    This chip dissipates around the same heat as Northwoods clock for clock! And of course, Intel style is wait 6-12, then the new stuff will actually be good. Still, is it really that important to increase performance so much that heat becomes an issue? I.E., will Dell be able to make the cooling whisper quiet? They can with the processor sitting at 80-90c, but now that with normal cooling it's almost there, now what will they do? Why can't we just have new processors that run so cool that we can just use heatsinks without fans? Oh well.
    Reply
  • Novaoblivion - Monday, February 02, 2004 - link

    Great article :) I found it very interesting I dont think I'll be buying a prescott till they hit about 4Ghz. My 2.4C is nice and fast for now. Reply
  • CRAMITPAL - Monday, February 02, 2004 - link


    http://www.theinquirer.net/?article=13927


    http://www.theinquirer.net/?article=13947
    Reply
  • johnsonx - Monday, February 02, 2004 - link

    To Vanners, #38:

    "if you halve the time for a stage in the pipeline and double the number of stages. Yes this means you can run at 2GHz instead of 1GHz but the reality is you're still taking 5ns to complete the pipe."

    Yes and no... In the example, you're right that a single instruction takes the same 5ns to complete. But you're not just executing a single instruction... rather, thousands to millions! The 10 stage pipe has twice as many instructions in flight as the 5 stage pipe. Therefore in the example, you get one result out of the 5-stage/1Ghz cpu every 1ns, but TWO results out of the 10-stage/2Ghz cpu in the same 1ns... twice as many.

    What I find interesting is that as pipelines get longer and longer, we might have to start talking about Instruction Latency: the number of clocks and ns between the time an instruction goes in and when the result comes out. It'll never be anything a human could notice directly, but it might come into play in high-performance realtime apps that deal with input from the outside world, and have to produce synchronized output. Any CPU calculates somewhat "back-in-time" as instructions fly down the pipe... right now, a Prescott calculates about twice as far behind 'reality' as an A64 does. I don't know if there is any realworld application where this really could make a difference, or if there ever will be, but it's interesting to ponder, particularly if the pipeline lengths of Intel vs. AMD continue to diverge.
    Reply
  • cliffa3 - Monday, February 02, 2004 - link

    i don't see how a 4+GHz prescott will match up with intel's new pico BTX form factor...with that much heat (using air cooling), you need to keep a safe zone around the proc unless you like your RAM DDR+BBQ.
    I'd have to say that a lot of enthusiasts are younger and live in limited space conditions...might work well for people up north who don't want to run the heater, but as for me in texas, i have all the cool air pumping in to my bedroom and it still takes a lot to keep it cool. Can you imagine a university or corporation having a room full of those?..if they think about that, then it's no bueno for DELL and others as well.
    I'd also have to agree with the others about the heat/power being a major part of the article that was left out...otherwise a tremendous read, thanks for all the effort that goes into these.
    Reply
  • tfranzese - Monday, February 02, 2004 - link

    But - I need to add - the correction was needed and is welcome. Not trying to pick a bone with the editors. Reply
  • tfranzese - Monday, February 02, 2004 - link

    #55, you read what I read. I'll vouch for you. Reply
  • Icewind - Monday, February 02, 2004 - link

    #55
    Better go back to sleep me thinks :)
    Reply
  • Spearhawk - Monday, February 02, 2004 - link

    Is it just me (who was extremely tired yesterday) or has the 101 on pipeline part changed since the article was put up?
    I seem to rememeber reading someting about how a 5 staged CPU at 1 Ghz should be exactly as fast as a 2 GHz CPU with 10 stages (all else being equal of course) and that the secret of geting any profit out of going to more stages was to make sure that it couldn't only scale to 2 Ghz but to 3 Ghz or more.
    Reply
  • Icewind - Monday, February 02, 2004 - link

    I think shuttle owners are SOL with prescott. Reply

Log in

Don't have an account? Sign up now