Added local queue scheduling and "next_task" optimization#22
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nullchinchilla wants to merge 41 commits intosmol-rs:masterfrom
Closed
Added local queue scheduling and "next_task" optimization#22nullchinchilla wants to merge 41 commits intosmol-rs:masterfrom
nullchinchilla wants to merge 41 commits intosmol-rs:masterfrom
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…om being put there
…t_task field cannot be stolen. this can lead to deadlocks because run() is a future that may not be constantly polled, and thus there's no guarantee that local queues will make progress.
Contributor
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@nullchinchilla Would you be open to rebasing/cutting down on this PR? These optimizations are important and I would be open to reviewing it. |
Author
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I've actually decided on a different course, since I've realized that local scheduling and an unstealable next_task cell can cause issues (such as deadlocks if we nest smol::block_on). You can check my latest executor work in the "smolscale" crate, which uses smol::Task as well and is fully compatible with the smol-rs ecosystem, but to be easier to optimize forces a global executor. |
Contributor
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Thanks for letting me know! I think that this crate should act more as a "reference" executor, that aims to implement features rather than be as optimal as possible. I'll close this for now. |
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Two major changes significantly improve performance:
Executor::run()is called, a handle to the local queue and ticker are cached into TLS. This lets tasks schedule to a thread-local queue rather than always to the global queue.next_taskoptimization (see https://tokio.rs/blog/2019-10-scheduler) to greatly reduce context-switch costs in message-passing patterns. We avoid putting the same task intonext_tasktwice to avoid starvation.Through both unit testing and production deployment in https://github.com/geph-official/geph4, whose QUIC-like
sosistabprotocol is structured in an actor-like fashion that greatly stresses the scheduler, I see significant improvements in real-world throughput (up to 30%, and this is in a server dominated by cryptography CPU usage) and massive improvements in microbenchmarks (up to 10x faster in theyield_nowbenchmark and similar context-switch benchmarks). I see no downsides --- the code should gracefully fall back to pushing to the global queeu in case e.g. nestingExecutors invalidates the TLS cache.I also added
criterionbenchmarks.