Lazy and Speculative Execution
Recorded 16 June 2010 in Lausanne, Vaud, Switzerland
Event: SuRI - I&C - Summer Research Institute
The distinction between lazy and eager (or strict) evaluation has been studied in programming languages since Algol 60s call by name, as a way to avoid unnecessary work and to deal gracefully with infinite structures such as streams. It is deeply integrated in some languages, notably Haskell, and can be simulated in many languages by wrapping a lazy expression in a lambda.
Less well studied is the role of laziness, and its opposite, speculation, in computer systems, both hardware and software. A wide range of techniques can be understood as applications of these two ideas.
Laziness is the idea behind:
- Redo logging for maintaining persistent state and replicated state machines: the log represents the current state, but it is evaluated only after a failure or to bring a replica online
- Copy-on-write schemes
- Clipping regions and expose events in graphics and window systems
- Infinity and Not a number results of floating point operations
- Futures (in programming) and out of order execution (in CPUs), which launch a computation but are lazy about consuming the result
- Formatting operators in text editors, which apply properties such as italic to large regions of text by attaching a sequence of functions that compute the properties; the functions are not evaluated until the text needs to be displayed
Speculation is the idea behind:
- Optimistic concurrency control in databases, and more recently in transactional memory
- Prefetching in memory and file systems
- Branch prediction, and speculative execution in general in modern CPUs
- Exponential backoff schemes for scheduling a resource, most notably in LANs such as WiFi or classical Ethernet
- All forms of caching, which speculate that its worth filling up some memory with data in the hope that it will be used again
In both cases it is usual to insist that the laziness or speculation is strictly a matter of scheduling that doesn't affect the result of a computation but only improves the performance. Sometimes, however, the spec is weakened, for example in eventual consistency. I will discuss many of these examples in detail and examine what they have in common, how they differ, and what factors govern the effectiveness of laziness and speculation in computer systems.
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