Enhancing Performance with Pipelining

1. Enhancing Performance with Pipelining Slides developed by Rami Abielmona and modified by Miodrag Bolic High-Level Computer Systems Design

2. Presentation Outline (1) • What is pipelining ? • Pipeline Taxonomies • Instruction Pipelines • MIPS Instruction Pipeline • Pipeline Hazards • MIPS Pipelined Datapath • Load Word Instruction Example • Pipeline Datapath Example • Pipeline Control • Pipeline Instruction Example

3. Presentation Outline (2) • Pipeline Hazards • Control Hazards • Data Hazards • Detecting Data Hazards • Resolving Data Hazards • Forwarding Example • Stalling Example • Branch Hazards • Branching Example • Key terms

4. What is Pipelining ? (1) • There are two main ways to increase the performance of a processor through high-level system architecture • Increasing the memory access speed • Increasing the number of supported concurrent operations • Pipelining ! • Parallelism ? • Pipelining is the process by which instructions are parallelized over several overlapping stages of execution, in order to maximize datapath efficiency

5. What is Pipelining ? (2) • Pipelining is analogous to many everyday scenarios • Car manufacturing process • Batch laundry jobs • Basically, any assembly-line operation applies • Two important concepts: • New inputs are accepted at one end before previously accepted inputs appear as outputs at the other end; • The number of operations performed per second is increased, even though the elapsed time needed to perform any one operation remains the same

6. What is Pipelining ? (3) Looking at the textbook’s example, we have a 4-stage pipeline of laundry tasks: • Place one dirty load of clothes into washer • Place the washed clothes into a dryer • Place a dry load on a table and fold • Put the clothes away Graphically speaking: • Sequential (top) vs. • Pipelined (bottom) execution

7. Pipeline Taxonomies • There are two types of pipelines used in computer systems • Arithmetic pipelines • Used to pipeline data intensive functionalities • Instruction pipelines • Used to pipeline the basic instruction fetch and execute sequence • Other classifications include • Linear vs. nonlinear pipelines • Presence (or lack) of feedforward and feedback paths between stages • Static vs. dynamic pipelines • Dynamic pipelines are multifunctional, taking on a different form depending on the function being executed • Scalar vs. vector pipelines • Vector pipelines specifically target computations using vector data

8. MIPS Instruction Pipeline (1) • Let us now introduce the pipeline we’re working with • It’s a 5-stage instruction, linear, static and scalar pipeline, consisting of the following steps: • Fetch instruction from Memory (IF) • Read registers while decoding the instruction (ID) • Execute the operation or calculate an address (EX) • Access an operand in data memory (MEM) • Write the result into a register (WB) • Again, theoretically, pipeline speedup = number of stages in pipeline

9. MIPS Instruction Pipeline (2) Inst. Fetch (2ns), Reg. read/write (1ns), ALU op. (2ns), Data access (2ns)

10. Ifetch Reg Exec Mem Wr Ifetch Reg Exec Mem Ifetch Ifetch Reg Exec Mem Wr Ifetch Reg Exec Mem Wr Ifetch Reg Exec Mem Wr Single Cycle, Multiple Cycle, vs. Pipeline [1] Cycle 1 Cycle 2 Clk Single Cycle Implementation: Load Store Waste Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6 Cycle 7 Cycle 8 Cycle 9 Cycle 10 Clk Multiple Cycle Implementation: Load Store R-type Pipeline Implementation: Load Store R-type

11. Why Pipeline? • Suppose • 100 instructions are executed • The single cycle machine has a cycle time of 45 ns • The multicycle and pipeline machines have cycle times of 10 ns • The multicycle machine has a CPI of 4.6 • Single Cycle Machine • 45 ns/cycle x 1 CPI x 100 inst = 4500 ns • Multicycle Machine • 10 ns/cycle x 4.6 CPI x 100 inst = 4600 ns • Ideal pipelined machine • 10 ns/cycle x (1 CPI x 100 inst + 4 cycle drain) = 1040 ns • Ideal pipelined vs. single cycle speedup • 4500 ns / 1040 ns = 4.33 • What has not yet been considered?

12. MIPS Instruction Pipeline (3) [2] • What makes it easy • all instructions are the same length • just a few instruction formats • memory operands appear only in loads and stores • What makes it hard? • structural hazards: suppose we had only one memory • control hazards: need to worry about branch instructions • data hazards: an instruction depends on a previous instruction • We’ll build a simple pipeline and look at these issues

13. Pipeline Hazards [1] • structural hazards: attempt to use the same resource two different ways at the same time • E.g., two instructions try to read the same memory at the same time • data hazards: attempt to use item before it is ready • instruction depends on result of prior instruction still in the pipeline add r1, r2, r3 sub r4, r2, r1 • control hazards: attempt to make a decision before condition is evaulated • branch instructions beq r1, loop add r1, r2, r3 • Can always resolve hazards by waiting • pipeline control must detect the hazard • take action (or delay action) to resolve hazards

14. MIPS Pipelined Datapath (1) What do we need to split the datapath into stages ?

15. MIPS Pipelined Datapath (2) Pipeline registers (buffers) are similar to multicycle processor design

16. Load Word Instruction (1) Instruction fetch stage

17. Load Word Instruction (2) Instruction decode and register file read stage

18. Load Word Instruction (3) Execute or address calculation stage

19. Load Word Instruction (4) Memory access stage

20. Load Word Instruction (5) Write back stage

21. Load Word Corrected Datapath Write register number comes from the MEM/WB pipeline register along with the data

22. Graphical Representations Multiple-clock cycle (vs. single-clock cycle) pipelined diagrams

23. Pipeline Datapath Example (1) Single-cycle pipeline diagram with one instruction on the pipeline

24. Pipeline Datapath Example (2) Single-cycle pipeline diagram with two instructions on the pipeline

25. Pipeline Control (1) • What control signals are required ? • First, notice that the pipeline registers are written every clock cycle, hence do not require explicit control signals, otherwise: • Instruction fetch and PC increment • Again, asserted at every clock cycle • Instruction decode and register file read • Again, asserted at every clock cycle • Execution and address calculation • Need to select the result register, the ALU operation, and either Read data 2 or the sign-extended immediate for the ALU • Memory access • Need to read from memory, write to memory or complete branch • Write back • Need to send back either ALU result or memory value to the register file

26. Pipeline Control (2)

27. Pipeline Control (3)

28. Pipeline Datapath with Control

29. Pipeline Instruction Example (1)

30. Pipeline Instruction Example (2)

31. Pipeline Instruction Example (3)

32. Pipeline Instruction Example (4)

33. Pipeline Instruction Example (5)

34. Pipeline Instruction Example (6)

35. Pipeline Instruction Example (7)

36. Pipeline Instruction Example (8)

37. Pipeline Instruction Example (9)

38. Pipeline Hazards • Structural hazard • Occurs when a combination of instructions is not supported by the datapath • For example, a unified memory unit would need to be accessed in stages 1 (IF) and 4 (MEM), which would cause a contention • Pipeline outright fails in the presence of structural hazards • Control hazard • Occurs when a decision is made based on the results of one instructions, while others are executing • For example, a branch instruction is either taken or not • Solutions that exist are stalling and predicting • Data hazard • Occurs when an instruction depends on the results of an instruction resident on the pipeline • For example, adding two register contents and storing their result into a third register, then using that register’s contents for another operation • Solutions that exist are based on forwarding

39. Control Hazards - Stalling • Three major solutions • Stall • Predict • Delayed branch slot • Stalling involves always waiting for the PC to be updated with the correct address before moving on • A pipeline stall (or bubble) allows us to perform this wait • Quite costly, as we have to stall even if the branch fails

40. Control Hazards - Predicting • Predicting involves guessing whether the branch is taken or not, and acting on that guess • If correct, then proceed with normal pipeline execution • If incorrect, then stall pipeline execution

41. Control Hazards – Delayed branch • Delayed branch involves executing the next sequential instruction with the branch taking place after that delayed branch slot • The assembler automatically adjusts the instructions to make it transparent from the programmer • The instruction has to be safe, as in it shouldn’t affect the branch • Longer pipelines requires the use of more branch delay slots • Actual MIPS architecture solution

42. Data Hazards – Forwarding (1) • Forwarding involves providing the inputs to a stage of one instruction before the completion of another instruction • Valid if destination stage is later in time than the source stage • Left diagram shows typical forwarding scenario (add then sub) • Right diagram shows that we still need a stall in the case of a load-use data hazard (load then R-type)

43. Data Hazards – Forwarding (2) sub $2, $1, $3 and $12, $2, $5 or $13, $6, $2 add $14, $2, $2 sw $14, 100($2)

44. Data Hazards – Crude Solution • We could insert “no operation” (nop) instructions to delay the pipeline execution until the correct result is in the register file sub $2, $1, $3 nop nop and $12, $2, $5 or $13, $6, $2 add $14, $2, $2 sw $14, 100($2) • Too slow as it adds extra useless clock cycles • In reality, we try to find useful instructions to execute between data-dependent instructions, but this happens too often to be efficient

45. Data Hazards – Detection (1) • Let us try to formalize detecting a data hazard • EX/MEM.RegisterRd = ID/EX.RegisterRs • EX/MEM.RegisterRd = ID/EX.RegisterRt • MEM/WB.RegisterRd = ID/EX.RegisterRs • MEM/WB.RegisterRd = ID/EX.RegisterRt sub $2, $1, $3 and $12, $2, $5 Data hazard of type #1 or $13, $6, $2 Data hazard of type #4 add $14, $2, $2 No data hazard – register file sw $14, 100($2) No data hazard – correct operation

46. Data Hazards – Detection (2) • Two modifications are in order • Firstly, we don’t have to forward all the time! • Some instructions don’t write registers (e.g. beq) • Use RegWrite signal in WB control block to determine condition • Secondly, the $0 register must always return 0 • Can’t limit programmer of using it as a destination register • Use RegisterRd to determine if $0 is being used • If (EX/MEM.RegWrite & (EX/MEM.RegisterRd ≠ 0) & (EX/MEM.RegisterRd=ID/EX.RegisterRs)) ForwardA= 10 • If (EX/MEM.RegWrite & (EX/MEM.RegisterRd ≠ 0) & (EX/MEM.RegisterRd=ID/EX.RegisterRt)) ForwardB= 10 • If (MEM/WB.RegWrite & (MEM/WB.RegisterRd ≠ 0) & (MEM/WB.RegisterRd=ID/EX.RegisterRs)) ForwardA= 01 • If (MEM/WB.RegWrite & (MEM/WB.RegisterRd ≠ 0) & (MEM/WB.RegisterRd=ID/EX.RegisterRt)) ForwardB= 01 • Let us examine the hardware changes to our datapath

47. Data Hazards – Forwarding Unit (1)

48. Data Hazards – Forwarding Unit (2) • Remember that there is no hazard in the WB stage, because the register file is able to be written and read in the same stage

49. Data Hazards – Forwarding Unit (3)

50. Data Hazards – Forwarding Unit (4)