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Amdahl’s Law – Understanding Hold-ups and Speedup Limits

Amdahl’s Law – Understanding Hold-ups and Speedup Limits


Amdahl’s Law is a fundamental principle in parallel computing that quantifies the potential speedup of a program when part of it is parallelized. It was formulated by computer scientist Gene Amdahl in 1967. The following formula states the Law:




  • Speedup is the improvement in performance,
  • F is the fraction of the program executed serially (not parallelized),
  • P is the number of processors.

Key Points of Amdahl’s Law:

  1. Limitations of Parallelization: Amdahl’s Law highlights that the sequential portion of the code limits the speedup of a program. Even with significant parallel processing, a serial fraction imposes an upper bound on achievable speedup.
  2. Importance of Optimization: To maximize speedup, efforts should focus on optimizing the sequential portion of the program. Improving the parallelized portion alone may not yield substantial overall performance gains if the serial fraction is not addressed.
  3. Diminishing Returns: As the number of processors (P) increases, the potential speedup diminishes. It highlights the diminishing returns of parallelization, especially when the serial fraction is significant.
  4. Practical Implications: This Law emphasizes the balancing approach to parallel computing. However, parallelization can offer substantial performance improvements, it is also crucial to identify and optimize the critical serial sections for meaningful gains.
  5. Parallel Efficiency: The ratio of the speedup attained by the parallelized portion to the maximum speedup is known as parallel efficiency.
  6. Applications: This Law’s uses widely extend in designing and analyzing parallel algorithms, guiding decisions on resource allocation and system design.

Factors of Amdahl’s Law:

Amdahl’s Law pivots on two key factors:

Fraction of Sequential Code (F): This factor represents the portion of the task execution sequentially. Amdahl’s Law emphasizes that this sequential portion restricts the speedup of a program. As (F) increases, the potential speedup achievable through parallelization diminishes.

Number of Processors (P): The second vital factor in this Law is the number of processors used for parallel execution. The Law illustrates that as (P) increases, the potential speedup of the program improves, but with diminishing returns. Furthermore, this diminishing return occurs because the sequential fraction F continues to limit on the overall speedup, and adding more processors provides less benefit.

Benefits of Amdahl’s Law:

Amdahl’s Law delivers valuable insights into the limitations and potential benefits of parallel computing. While it emphasizes the limitations posed by the sequential fraction of a program, understanding and applying the Law offers several advantages:

  1. Performance Prediction: Amdahl’s Law allows for the estimation of the potential speedup in a parallel computing system based on the proportion of the code that can be parallelized.
  2. Resource Allocation: The Law assists in making informed decisions about resource allocation, such as determining the optimal number of processors for a given task.
  3. Optimization Focus: Amdahl’s Law highlights the importance of optimizing the sequential portion of a program. Developers can prioritize efforts to enhance the non-parallelizable components’ performance to maximize overall speedup.
  4. Parallel Efficiency Evaluation: The ratio of the speedup achieved to the maximum possible speedup can be analyzed to gauge how well a parallelized application utilizes available resources.
  5. Guidance for Parallelization Strategies: The Law leads developers in choosing effective parallelization strategies. It stresses the significance of identifying and addressing sequential hold-ups to achieve meaningful speedup rather than focusing merely on parallel components.
  6. Realistic Expectations: This Law boosts a realistic understanding of the potential gains from parallelization.

Disadvantages of Amdahl’s Law:

The Law has some disadvantages and assumptions impacting its applicability and accuracy in certain scenarios:

  • It assumes a fixed problem size, meaning the total workload remains constant. In real-world scenarios, the problem size may vary, and this assumption may not hold long. Therefore, it may affect the Law’s accuracy in dynamic computing environments.
  • The Law assumes uniform processor speeds, which may not reflect the reality of heterogeneous computing environments where processors have different speeds or capabilities.
  • The Law simplifies the view of parallelization by considering only one sequential fraction. The Law may provide an oversimplified representation in complex systems with multiple levels of parallelism or different types of parallel tasks.
  • It assumes ideal scalability; adding more processors will always lead to proportional speedup.


In conclusion, while Amdahl’s Law provides a foundational agenda for understanding the limitations of parallel computing, it comes with certain disadvantages and assumptions that may limit its accuracy in real-world scenarios.

Moreover, the Law’s fixed problem size assumption, uniform processor speed model, and disregard of communication overhead may not fully capture the complexities of modern computing environments.

Despite these limitations, the Law remains a valuable tool for valuing potential speedup and directing decisions on resource allocation and optimization strategies. However, its application should be complemented with a nuanced understanding of dynamic computing scenarios, varying problem sizes, and thoughts of communication overhead. Consequently, for a more comprehensive analysis of parallel system performance.

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