MRMCD 2018

»Pitfalls for performance: Typical latencies to keep in mind«
2018-09-08, 13:00–13:50, Lichtspielhaus

"Less computation implies faster code": Sounds reasonable at a first glance, but a closer inspection considering memory or disk latency numbers of modern computer architectures quickly reveals this to be a fallacy. In this talk I will discuss some ballpark latency numbers every programmer should know and hint at implications for designing algorithms in the context of high-performance computing as well as scientific simulations.

In recent years a number of algorithmic approaches have appeared in high-performance computing, which are no longer based on storing intermediate results on disk or in memory, but which explicitly avoid any kind of storage operations as much as possible. Often these methods willingly pay the price of recomputing parts of the intermediate results over and over. As counter-intuitive as it sounds at first, this may reduce runtime provided that recomputation is faster than typical latencies required for operating on the storage medium.

A key aspect to keep in mind in this development is typical latency numbers for access into storage. For example modern hardware typically allows to perform on the order of 1000 floating point operations per load from main memory. Recent trends in memory band width and CPU design suggest this number to increase in the future. In this sense having a feel about latencies helps to judge, whether storing data is worth it and not actually more expensive than recomputing it.

In this talk I will discuss typical latency numbers and in the light of this especially hint at approaches to solving linear algebra problems, which avoid access to main memory. Most notably I will discuss so-called matrix-free or contraction-based methods. No knowledge of numerical linear algebra is required to follow the talk, but naturally there will be maths and some formulae, so beware!