#message-passing #queue


A high-performance async-task scheduler

65 releases

0.4.2 Nov 24, 2023
0.4.0 Oct 16, 2023
0.3.52 Dec 5, 2022
0.3.49 Nov 18, 2022
0.1.0 Oct 26, 2020

#422 in Asynchronous

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Used in 27 crates (21 directly)

ISC license

503 lines


A global, auto-scaling scheduler for [async-task] using work-balancing.

What? Another executor?

smolscale is a work-balancing executor based on [async-task], designed to be a drop-in replacement to smol and async-global-executor. It is designed based on the idea that work-stealing, the usual approach in async executors like async-executor and tokio, is not the right algorithm for scheduling huge amounts of tiny, interdependent work units, which are what message-passing futures end up being. Instead, smolscale uses work-balancing, an approach also found in Erlang, where a global "balancer" thread periodically balances work between workers, but workers do not attempt to steal tasks from each other. This avoids the extremely frequent stealing attempts that work-stealing schedulers generate when applied to async tasks.

smolscale's approach especially excels in two circumstances:

  • When the CPU cores are not fully loaded: Traditional work stealing optimizes for the case where most workers have work to do, which is only the case in fully-loaded scenarios. When workers often wake up and go back to sleep, however, a lot of CPU time is wasted stealing work. smolscale will instead drastically reduce CPU usage in these circumstances --- a async-executor app that takes 80% of CPU time may now take only 20%. Although this does not improve fully-loaded throughput, it significantly reduces power consumption and does increase throughput in circumstances where multiple thread pools compete for CPU time.
  • When a lot of message-passing is happening: Message-passing workloads often involve tasks quickly waking up and going back to sleep. In a work-stealing scheduler, this again floods the scheduler with stealing requests. smolscale can significantly improve throughput, especially compared to executors like async-executor that do not special-case message passing.

License: ISC


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