Pipelining Principles Prof. Sirer CS 316 Cornell University

Meet the Cast Alice

Meet the Cast Alice Bob

Meet the Cast Alice Bob They don’t like each other!

The Laundry Four sequential tasks

Laundry Room Design #1 A large room with a one entry-door and one exit-door

Laundry Room Design #1 First Alice owns the room

Laundry Room Design #1 First Alice owns the room Bob can enter as soon as she is done No possibility for Alice and Bob to fight

Laundry Room Design #1 Elapsed Time for Alice: 4 Elapsed Time for Bob: 4 Elapsed Time for both: 8 A better laundry room can improve utilization and speed up task completion Time

Laundry Room Design #2 Elapsed Time for Alice: 4 Elapsed Time for Bob: 4 Elapsed Time for both: 5!!! Time

Laundry Room Design #2 The room is partitioned into stages One person owns a stage at a time, the room can hold up to four people simultaneously

Laundry Room Design #2 Alice

Laundry Room Design #2 BobAlice

Laundry Room Design #2 BobAlice Charlie

Laundry Room Design #2 BobAlice CharlieDave

Implications Principle: Latencies can be masked by running isolated operations in parallel Need mechanisms for isolation Need mechanisms for handling dependencies between tasks Let’s apply this principle to processor design…

MIPS Processor Functional decomposition of our processor

A MIPS Pipeline Pipeline stages for a MIPS processor

Pipeline Isolation Latches between pipeline stages hold the values for the stage to operate on Latches and register file are triggered on opposite edges of the clock

Pipeline Logic Additional latches required to store the instruction for each pipeline stage Copy the instruction forward, generate local control signals separately in each pipeline stage

Pipelining for Other Circuits Pipelining can be applied to any combinational logic circuit What’s the circuit latency at 2ns. per gate? What’s the throughput? What is the stage breakdown? Where would you place latches? What’s the throughput after pipelining? Pipelining vs. superpipelining