计算机系统结构2

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Principles - Computer Design Or Speedup = (Execution time for entire task without using the enhancement)/ (Execution time for the entire task using the enhancement when possible). Speedup tells us how much faster a task will run using the machine with the enhancement as opposed to the original machine.
1.
Then WAM =
2.
Weight Tim e
i 1 i i
n
n Wi 1 i 1
Weighted Harmonic Mean (WHM) 1 WHM =
Weighti i 1 Rate i
Both WAM and WHM will give a clearer picture (in relative terms) of performance when different workloads (i.e. portions of P1, P2..) are involved with different machines.
Average normalized execution time can be expressed as either arithmetic or geometric means (GM) where :
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Principles - Benchmark Suites
GM n
execution_ tim e_ ratio
n
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Principles - Benchmark Suites
A further approach has been tried when unequal mixtures of programs are involved in each machine’s workload. This involves normalizing execution times to a reference machine, and then take the average of the normalized execution times.
In making design tradeoff, favor the frequent case over the infrequent case. This will help performance. The frequent case is often simpler and can be done faster than the infrequent case. e.g. overflow in CPU’s In applying this principle, we have to decide what the frequent case is and how much performance can be improved by making that case faster.
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However, these two approaches only remain valid if each of the (benchmark) programs (say P1 and P2) were run an equal number of times on each machine. If different workloads are required (i.e. P1 and P2 are not running equally often on each machine) then two further approaches using weighted means may be then used to represent (summarize) relative l important guidelines, principles and equations which are very useful in the analysis and evaluation of a computers design, performance and its cost/performance ratio. 1. Make the Common Case Fast:
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Principles - Benchmark Suites
Several approaches are used: 1. An average of the time is the Arithmetic Mean n
1 AM Tim ei n i 1
Where Timei is the execution time for the ith program of n programs in the workload.
In other words, it defines the speedup that can be gained by using a particular feature. Speedup factor = (Performance for entire task using the enhancement when possible) / (Performance for entire task without using the enhancement).
2.
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If the performance is expressed as a rate, then the average that tracks total execution time is the Harmonic mean n HM n 1 i 1 ratei
Principles - Benchmark Suites Where ratei = 1/timei , the reciprocal of the execution time for the ith of n programs in the workload.
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Principles - Computer Design Amdahl’s law can be used to quantify this principle.
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Amdahl’s Law: The performance improvement to be gained from using a faster mode of execution is limited by the fraction of the time the faster mode can be used.
Principles - Benchmark Suites
Weighted arithmetic mean (WAM) Assign a weighting factor (Wi) as follows: If P1 is run 20% of the time : WP1 = 0.2, If P2 is run 80% of the time : WP2 = 0.8,
Department of Computer Science and Computer Engineering
Latrobe University CSE2CSA Computer Architecture
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Principles - Benchmark Suites Lecture 2
A benchmark is a program – normally a collection of programs into 1 program (the benchmark) and are typically used to measure the performance of the machines. (A typical benchmark is SPEC2006 (p.31) used to characterize performance in the workstation and server markets) Often when comparing and summarizing performance of several benchmarks (programs) over several machines we need a consistent summary measure which accurately reflects performance with respect to execution times.
Amdahl’s Law gives us a quick way to find the speedup from some enhancements by only knowing two factors
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Principles - Computer Design The fraction of the computation time in the original machine that can be converted to take advantage of the enhancement = fraction enhanced (this is often called percentage of vectorization (POV)) The improvement gained by the enhanced execution mode: that is, how much faster the task would run if the enhanced mode were used for the entire program (i.e. 100% of the time) = speedupenhanced. Then : Execution_timenew