Empyrean Evaluation
summary
Skyframe is Bazel's incremental parallel evaluator, implemented as a framework of computations with a tiny Python toy and a real implementation of 15,000 lines of Java. Key behavior is the nonblocking restart model where a dependency lookup can return null to restart and the callee must yield, which caused quadratic refetching in naive code and repeated external repo downloads. Solutions include per-function persisted state via getState caching, explicit state machines, and heuristics to limit cache drops versus memory pressure; execution is now more parallelized across phases.
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chapters (13)
- 0:00Welcome
- 2:31Build systems neglected
- 6:09Theory versus practice
- 8:39Skyframe in Bazel
- 10:49Python Skyframe
- 13:17Bazel internals
- 16:11Bazel phases fiction
- 18:39Skyframe computations
- 20:03Skyframe restarts
- 22:31Caching state restarts
- 25:03State machines functions
- 27:11Nested machines complexity
- 28:14Q&A
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