« Previous 1 2
Planning Performance Without Running Binaries
Blade Runner
Roy Batty's Tears
The power of Amdahl's law is found in its analytical insight. Code is measured in time, not in lines, so some minimal performance testing is still required. If you determine that only 50 percent of an algorithm's critical section can be parallelized, its theoretical speedup can't exceed 2x, as you see in Figure 2. Furthermore, it's not practical to use more than 12 cores to run this code, because it can attain more than 90 percent of the maximum theoretical speedup with 12 cores (a 1.84x speedup). You know this before attempting any optimization, saving you effort if the best possible result is inadequate to achieving your aims.
In an alternative scenario with only five percent serial code in the bottleneck (Figure 3), the asymptote is at 20x speedup. In other words, if you can successfully parallelize 95 percent of the problem, under ideal circumstances the maximum speedup for that problem is 20x. This handy analysis tool can quickly determine what can be accomplished by accelerating code for a problem of fixed size.
Without invoking the C-beams speech or even going near the Tannhäuser Gate [7], one must point out parallelism's massive overhead, already obvious from the examples here. Execution time can be significantly reduced, yet you accomplish this by throwing resources at the problem – perhaps suboptimally. On the other hand, one could argue that idle CPU cores would not be doing anything productive. All those cycles would be lost … like tears in rain.
Infos
- MPI: https://www.mpi-forum.org/docs/
- MPI pi implementation: https://github.com/PacktPublishing/Build-Supercomputers-with-Raspberry-Pi-3/blob/master/Chapter02/03_MPI_08_b.c
- Morrison, Carlos. Build Supercomputers with Raspberry Pi 3 . Pakt Publishing, 2016
- Amdahl's law: https://webhome.phy.duke.edu/~rgb/Beowulf/beowulf_book/beowulf_book/node21.html
- Embarrassingly parallel algorithms: https://en.wikipedia.org/wiki/Embarrassingly_parallel
- Gustafson's law: https://en.wikipedia.org/wiki/Gustafson%27s_law
- Tears in rain monologue from Blade Runner : https://en.wikipedia.org/wiki/Tears_in_rain_monologue
« Previous 1 2
Buy this article as PDF
(incl. VAT)