In computer architecture, Amdahl's law (or Amdahl's argument) gives the theoretical speedup in latency of the execution of a task at fixed workload that can be expected of a system whose resources are improved. It is named after computer scientist Gene Amdahl, and was presented at the AFIPS Spring Joint Computer Conference in 1967.
Amdahl's law can be formulated the following way:
- Slatency is the theoretical speedup of the execution of the whole task;
- s is the speedup of the part of the task that benefits from improved system resources;
- p is the proportion of execution time that the part benefiting from improved resources originally occupied.
shows that the theoretical speedup of the execution of the whole task increases with the improvement of the resources of the system and that regardless of the magnitude of the improvement, the theoretical speedup is always limited by the part of the task that cannot benefit from the improvement.
Amdahl's law is often used in parallel computing to predict the theoretical speedup when using multiple processors. For example, if a program needs 20 hours using a single processor core, and a particular part of the program which takes one hour to execute cannot be parallelized, while the remaining 19 hours (p = 0.95) of execution time can be parallelized, then regardless of how many processors are devoted to a parallelized execution of this program, the minimum execution time cannot be less than that critical one hour. Hence, the theoretical speedup is limited to at most 20 times (1/(1 − p) = 20). For this reason parallel computing with many processors is useful only for very parallelizable programs.
A task executed by a system whose resources are improved compared to an initial similar system can be split up into two parts:
- a part that does not benefit from the improvement of the resources of the system;
- a part that benefits from the improvement of the resources of the system.
Example. — A computer program that processes files from disk. A part of that program may scan the directory of the disk and create a list of files internally in memory. After that, another part of the program passes each file to a separate thread for processing. The part that scans the directory and creates the file list cannot be sped up on a parallel computer, but the part that processes the files can.
The execution time of the whole task before the improvement of the resources of the system is denoted . It includes the execution time of the part that would not benefit from the improvement of the resources and the execution time of the one that would benefit from it. The fraction of the execution time of the task that would benefit from the improvement of the resources is denoted by . The one concerning the part that would not benefit from it is therefore . Then
It is the execution of the part that benefits from the improvement of the resources that is sped up by the factor after the improvement of the resources. Consequently, the execution time of the part that does not benefit from it remains the same, while the part that benefits from it becomes
The theoretical execution time of the whole task after the improvement of the resources is then
- Example 1
If 30% of the execution time may be the subject of a speedup, p will be 0.3; if the improvement makes the affected part twice as fast, s will be 2. Amdahl's law states that the overall speedup of applying the improvement will be
- Example 2
We are given a serial task which is split into four consecutive parts, whose percentages of execution time are p1 = 0.11, p2 = 0.18, p3 = 0.23, and p4 = 0.48 respectively. Then we are told that the 1st part is not sped up, so s1 = 1, while the 2nd part is sped up 5 times, so s2 = 5, the 3rd part is sped up 20 times, so s3 = 20, and the 4th part is sped up 1.6 times, so s4 = 1.6. By using Amdahl's law, the overall speedup is
Notice how the 20 times and 5 times speedup on the 2nd and 3rd parts respectively don't have much effect on the overall speedup when the 4th part (48% of the execution time) is sped up only 1.6 times.
Relation to law of diminishing returns
Amdahl's law is often conflated with the law of diminishing returns, whereas only a special case of applying Amdahl's law demonstrates law of diminishing returns. If one picks optimally (in terms of the achieved speedup) what to improve, then one will see monotonically decreasing improvements as one improves. If, however, one picks non-optimally, after improving a sub-optimal component and moving on to improve a more optimal component, one can see an increase in return. Note that it is often rational to improve a system in an order that is "non-optimal" in this sense, given that some improvements are more difficult or consuming of development time than others.
Amdahl's law does represent the law of diminishing returns if you are considering what sort of return you get by adding more processors to a machine, if you are running a fixed-size computation that will use all available processors to their capacity. Each new processor you add to the system will add less usable power than the previous one. Each time you double the number of processors the speedup ratio will diminish, as the total throughput heads toward the limit of 1/(1 − p).
This analysis neglects other potential bottlenecks such as memory bandwidth and I/O bandwidth, if they do not scale with the number of processors; however, taking into account such bottlenecks would tend to further demonstrate the diminishing returns of only adding processors.
Speedup in a serial program
For example, with a serial program in two parts A and B for which TA = 3 s and TB = 1 s,
- if part B is made to run 5 times faster, that is s = 5 and p = TB/(TA + TB) = 0.25, then
- if part A is made to run 2 times faster, that is s = 2 and p = TA/(TA + TB) = 0.75, then
Therefore, making part A to run 2 times faster is better than making part B to run 5 times faster. The percentage improvement in speed can be calculated as
- Improving part A by a factor of 2 will increase overall program speed by a factor of 1.60, which makes it 37.5% faster than the original computation.
- However, improving part B by a factor of 5, which presumably requires more effort, will only achieve an overall speedup factor of 1.25, which makes it 20% faster.
Amdahl's law only applies to cases where the problem size is fixed. In practice, as more computing resources become available, they tend to get used on larger problems (larger datasets), and the time spent in the parallelizable part often grows much faster than the inherently serial work. In this case, Gustafson's law gives a less pessimistic and more realistic assessment of parallel performance.
- (Rodgers 1985, p. 226)
- Michael McCool; James Reinders; Arch Robison (2013). Structured Parallel Programming: Patterns for Efficient Computation. Elsevier. p. 61.
- Rodgers, David P. (June 1985). "Improvements in multiprocessor system design". ACM SIGARCH Computer Architecture News archive. New York, NY, USA: ACM. 13 (3): 225–231. doi:10.1145/327070.327215. ISBN 0-8186-0634-7. ISSN 0163-5964.
- Amdahl, Gene M. (1967). "Validity of the Single Processor Approach to Achieving Large-Scale Computing Capabilities" (PDF). AFIPS Conference Proceedings (30): 483–485. doi:10.1145/1465482.1465560.
|Wikimedia Commons has media related to Amdahl's law.|
- Cases where Amdahl's law is inapplicable
- Oral history interview with Gene M. Amdahl Charles Babbage Institute, University of Minnesota. Amdahl discusses his graduate work at the University of Wisconsin and his design of WISC. Discusses his role in the design of several computers for IBM including the STRETCH, IBM 701, and IBM 704. He discusses his work with Nathaniel Rochester and IBM's management of the design process. Mentions work with Ramo-Wooldridge, Aeronutronic, and Computer Sciences Corporation
- A simple interactive Amdahl's Law calculator
- "Amdahl's Law" by Joel F. Klein, Wolfram Demonstrations Project, 2007.
- Amdahl's Law in the Multicore Era
- Blog Post: "What the $#@! is Parallelism, Anyhow?"
- Amdahl's Law applied to OS system calls on multicore CPU
- Evaluation of the Intel Core i7 Turbo Boost feature, by James Charles, Preet Jassi, Ananth Narayan S, Abbas Sadat and Alexandra Fedorova
- Calculation of the acceleration of parallel programs as a function of the number of threads, by George Popov, Valeri Mladenov and Nikos Mastorakis