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L' Appunto
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Case Studies with Exercises by Diana Franklin
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In this set of exercises, you are to make sense of Figure 1.26, which presents the
performance of selected processors and a fictional one (Processor X), as reported
by www.tomshardware.com. For each system, two benchmarks were run. One
benchmark exercised the memory hierarchy, giving an indication of the speed of
the memory for that system. The other benchmark, Dhrystone, is a CPU-intensive
benchmark that does not exercise the memory system. Both benchmarks are displayed in order to distill the effects that different design decisions have on memory and CPU performance.
[10/10/Discussion/10/20/Discussion] <1.7> Make the following calculations on
the raw data in order to explore how different measures color the conclusions one
can make. (Doing these exercises will be much easier using a spreadsheet.)
a. [10] <1.8> Create a table similar to that shown in Figure 1.26, except express
the results as normalized to the Pentium D for each benchmark.
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b. [10] <1.9> Calculate the arithmetic mean of the performance of each processor. Use both the original performance and your normalized performance calculated in part (a).
e.
c. [Discussion] <1.9> Given your answer from part (b), can you draw any conflicting conclusions about the relative performance of the different processors?
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d. [10] <1.9> Calculate the geometric mean of the normalized performance of
the dual processors and the geometric mean of the normalized performance
of the single processors for the Dhrystone benchmark.
e. [20] <1.9> Plot a 2D scatter plot with the x-axis being Dhrystone and the yaxis being the memory benchmark.
f.
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[Discussion] <1.9> Given your plot in part (e), in what area does a dualprocessor gain in performance? Explain, given your knowledge of parallel
processing and architecture, why these results are as they are.
Chip
# of cores
Clock frequency
(MHz)
Memory
performance
Dhrystone
performance
Athlon 64 X2 4800+
2
2,400
3,423
20,718
Pentium EE 840
2
2,200
3,228
18,893
Pentium D 820
2
3,000
3,000
15,220
Athlon 64 X2 3800+
2
3,200
2,941
17,129
Pentium 4
1
2,800
2,731
7,621
Athlon 64 3000+
1
1,800
2,953
7,628
Pentium 4 570
1
2,800
3,501
11,210
Processor X
1
3,000
7,000
5,000
Figure 1.26 Performance of several processors on two benchmarks.
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Chapter One Fundamentals of Computer Design
1.13
[10/10/20] <1.9> Imagine that your company is trying to decide between a
single-processor system and a dual-processor system. Figure 1.26 gives the performance on two sets of benchmarks—a memory benchmark and a processor
benchmark. You know that your application will spend 40% of its time on
memory-centric computations, and 60% of its time on processor-centric computations.
a. [10] <1.9> Calculate the weighted execution time of the benchmarks.
b. [10] <1.9> How much speedup do you anticipate getting if you move from
using a Pentium 4 570 to an Athlon 64 X2 4800+ on a CPU-intensive application suite?
c. [20] <1.9> At what ratio of memory to processor computation would the performance of the Pentium 4 570 be equal to the Pentium D 820?
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[10/10/20/20] <1.10> Your company has just bought a new dual Pentium processor, and you have been tasked with optimizing your software for this processor.
You will run two applications on this dual Pentium, but the resource requirements
are not equal. The first application needs 80% of the resources, and the other only
20% of the resources.
e.
a. [10] <1.10> Given that 40% of the first application is parallelizable, how
much speedup would you achieve with that application if run in isolation?
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b. [10] <1.10> Given that 99% of the second application is parallelizable, how
much speedup would this application observe if run in isolation?
c. [20] <1.10> Given that 40% of the first application is parallelizable, how
much overall system speedup would you observe if you parallelized it?
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d. [20] <1.10> Given that 99% of the second application is parallelizable, how
much overall system speedup would you get?
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2.9
2.10
2.11
2.12
2.13
Instruction-Level Parallelism: Concepts and Challenges
Basic Compiler Techniques for Exposing ILP
Reducing Branch Costs with Prediction
Overcoming Data Hazards with Dynamic Scheduling
Dynamic Scheduling: Examples and the Algorithm
Hardware-Based Speculation
Exploiting ILP Using Multiple Issue and Static Scheduling
Exploiting ILP Using Dynamic Scheduling,
Multiple Issue, and Speculation
Advanced Techniques for Instruction Delivery and Speculation
Putting It All Together: The Intel Pentium 4
Fallacies and Pitfalls
Concluding Remarks
Historical Perspective and References
Case Studies with Exercises by Robert P. Colwell
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2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
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80
89
97
104
114
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121
131
138
140
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“Who’s first?”
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Instruction-Level
Parallelism and Its
Exploitation
“America.”
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“Who’s second?”
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“Sir, there is no second.”
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Dialog between two observers
of the sailing race later named
“The America’s Cup” and run
every few years—the inspiration for John Cocke’s naming of
the IBM research processor as
“America.” This processor was
the precursor to the RS/6000
series and the first superscalar
microprocessor.
Chapter Two Instruction-Level Parallelism and Its Exploitation
Instruction-Level Parallelism: Concepts and Challenges
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All processors since about 1985 use pipelining to overlap the execution of
instructions and improve performance. This potential overlap among instructions
is called instruction-level parallelism (ILP), since the instructions can be evaluated in parallel. In this chapter and Appendix G, we look at a wide range of techniques for extending the basic pipelining concepts by increasing the amount of
parallelism exploited among instructions.
This chapter is at a considerably more advanced level than the material on
basic pipelining in Appendix A. If you are not familiar with the ideas in Appendix
A, you should review that appendix before venturing into this chapter.
We start this chapter by looking at the limitation imposed by data and control
hazards and then turn to the topic of increasing the ability of the compiler and the
processor to exploit parallelism. These sections introduce a large number of concepts, which we build on throughout this chapter and the next. While some of the
more basic material in this chapter could be understood without all of the ideas in
the first two sections, this basic material is important to later sections of this
chapter as well as to Chapter 3.
There are two largely separable approaches to exploiting ILP: an approach
that relies on hardware to help discover and exploit the parallelism dynamically,
and an approach that relies on software technology to find parallelism, statically
at compile time. Processors using the dynamic, hardware-based approach,
including the Intel Pentium series, dominate in the market; those using the static
approach, including the Intel Itanium, have more limited uses in scientific or
application-specific environments.
In the past few years, many of the techniques developed for one approach
have been exploited within a design relying primarily on the other. This chapter
introduces the basic concepts and both approaches. The next chapter focuses on
the critical issue of limitations on exploiting ILP.
In this section, we discuss features of both programs and processors that limit
the amount of parallelism that can be exploited among instructions, as well as the
critical mapping between program structure and hardware structure, which is key
to understanding whether a program property will actually limit performance and
under what circumstances.
The value of the CPI (cycles per instruction) for a pipelined processor is the
sum of the base CPI and all contributions from stalls:
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Pipeline CPI = Ideal pipeline CPI + Structural stalls + Data hazard stalls + Control stalls
The ideal pipeline CPI is a measure of the maximum performance attainable by
the implementation. By reducing each of the terms of the right-hand side, we
minimize the overall pipeline CPI or, alternatively, increase the IPC (instructions
per clock). The equation above allows us to characterize various techniques by
what component of the overall CPI a technique reduces. Figure 2.1 shows the
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