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Unformatted text preview: L08 - Pipelining 1 6.004 – Fal 2010 10/5/10 Pipelining what Seymour Cray taught the laundry industry I’ve got 3 months Worth of laundry To do tonight… Funny, considering that he’s only got one outfit… L08 - Pipelining 2 6.004 – Fal 2010 10/5/10 Forget circuits… lets solve a “Real Problem” Device: Washer Function: Fill, Agitate, Spin Washer PD = 30 mins Device: Dryer Function: Heat, Spin Dryer PD = 60 mins INPUT: dirty laundry OUTPUT: 6 more weeks L08 - Pipelining 3 6.004 – Fal 2010 10/5/10 Step 1: Step 2: Total = Washer PD + Dryer PD = _________ mins 90 One load at a time Everyone knows that the real reason that MIT students put o ff doing laundry so long is not because they procrastinate, are lazy, or even have be er things to do. The fact is, doing one load at a time is not smart. L08 - Pipelining 4 6.004 – Fal 2010 10/5/10 Doing N loads of laundry Here’s how they do laundry at Harvard, the “combinational” way. Step 1: Step 2: Step 3: Step 4: Total = N*(Washer PD + Dryer PD ) = ____________ mins N*90 … (Of course, this is just an urban legend. No one at Harvard actually does laundry. The butlers all arrive on Wednesday morning, pick up the dirty laundry and return it all pressed and starched in time for afternoon tea) L08 - Pipelining 5 6.004 – Fal 2010 10/5/10 Doing N Loads… the MIT way MIT students “pipeline” the laundry process. That’s why we wait! Step 1: Step 2: Step 3: Total = N * Max(Washer PD , Dryer PD ) = ____________ mins N*60 … Actually, it’s more like N*60 + 30 if we account for the startup transient correctly. When doing pipeline analysis, we’re mostly interested in the “steady state” where we assume we have an infinite supply of inputs. L08 - Pipelining 6 6.004 – Fal 2010 10/5/10 Performance Measures Latency: The delay from when an input is established until the output associated with that input becomes valid. (Harvard Laundry = _________ mins) ( MIT Laundry = _________ mins) Throughput: The rate of which inputs or outputs are processed. (Harvard Laundry = _________ outputs/min) ( MIT Laundry = _________ outputs/min) 90 120 1/90 1/60 Assuming that the wash is started as soon as possible and waits (wet) in the washer until dryer is available. L08 - Pipelining 7 6.004 – Fal 2010 10/5/10 Okay, back to circuits… F G H X P(X) For combinational logic: latency = t PD , throughput = 1/t PD. We can’t get the answer faster, but are we making e ff ective use of our hardware at all times? G(X) F(X) P(X) X F & G are “idle”, just holding their outputs stable while H performs its computation L08 - Pipelining 8 6.004 – Fal 2010 10/5/10 Pipelined Circuits use registers to hold H’s input stable!...
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