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Pipelining

Introduction to pipelining in computer architecture, overlapping tasks to enhance performance, handling hazards and stalls in the pipeline, solutions for pipeline hazards, and redesigning the datapath for pipelined execution.

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Pipelining

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  1. Pipelining Chapter 6

  2. Introduction to Pipelining • Pipelining is overlapping of tasks to realize improvement in overall performance • Consider 4 sub-tasks making up a major task. Lets consider the example given in your text: wash, dry, iron and fold clothes (W D I F) • Now consider n-students want to do this WDIF operation this weekend. • WDIFWDIFWDIFWDIF • WDIF • WDIF • WDIF • WDIF

  3. Instruction Cycle • Fetch: Fetch instruction from memory • Read: Read registers while decoding the instructions • Execute: Execute the operation or calculate an address • Access Memory: Read memory • Write: Write result to register • Assume each of the above operation takes clock cycle. • Assume read and write to register happen in different halves of the cycle. Now we can overlap register read and write.

  4. Pipelining • Time between instructions in pipelined = time between instructions in non-pipelined / # pipelined stages • We want a balanced set of instructions to realized best performance by pipelining • Lets examine the MIPS instruction pipelining page: 373 • How do we design instruction set for pipelining? • MIPS: • instructions of same length • Only few instruction formats • Memory operands only in load and store • Operands must be aligned in the memory

  5. Life is not simple • It is full of hazards • There are situations in pipelining where the next instruction cannot execute in the following cycle. • These are called hazards and there are three different types. • Structural hazards: instruction fetch and data access of memory • Data hazards: • add $s0,$t0,$t1 • sub $t2,$s0,$t3 • Solution: data forwarding • Control hazards: branch…delayed branch, rearranging instructions • Lets look at some examples

  6. How to address pipeline hazards? • Stalls in the pipeline occur when instructions due to • structural hazards (two instructions needing memory at the same time), • control hazards (branch instruction), and • data hazards (results from an instruction needed as data in another instruction). • Solution 1: Forwarding… need to be made during the design of the datapath • Solution 2: introducing a delay or bubble in the pipeline; this is usually done after load and store; delayed load; • Example:

  7. Rendering Code to Avoid Pipeline Stalls Original code Rearranged code • A = B + E • C = B + F lw $t1,0(t0) lw $t2,4(t0) add $t3, $t1, $t2 sw $t3, 12($t0) lw $t4, 8($t0) add $t5, $t1, $t4 sw $t5,16($t0) • A = B + E • C = B + F lw $t1,0(t0) lw $t2,4(t0) lw $t4, 8($t0) add $t3, $t1, $t2 sw $t3, 12($t0) add $t5, $t1, $t4 sw $t5,16($t0)

  8. Control Hazards • There are benchmark program that are used for evaluating the performance of the hardware called SPEC benchmarks • SPECint2000 is one of them. According to this benchmark 13% of the instructions executed are branch. • After a branch we a nop to stall; 13% of the time one extra cycle is added to the time. • Also the instructions loaded into the pipeline need to flushed if the branch is taken. • Branch prediction is another solution: based on the prediction you may want to stall or prefetch.

  9. Revisit and redesign Datapath • Lets redesign our datapath to allow pipelined execution: • See. Figs., 6.9, 6.10, 6.11…

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