Fix starvation when using LIFO slot without consuming budget #5686
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As an optimization to improve locality, the multi-threaded scheduler maintains a single slot (LIFO slot). When a task is scheduled, it goes into the LIFO slot. The scheduler will run tasks in the LIFO slot first, before checking the local queue. In ping-ping style workloads where task A notifies task B, which notifies task A again, this can cause starvation as these two tasks will repeatedly schedule the other in the LIFO slot. #5686, a first attempt at solving this problem, consumes a unit of budget each time a task is scheduled from the LIFO slot. However, at the time of this commit, the scheduler allocates 128 units of budget for each chunk of work. This is quite high in situation where tasks do not perform many async operations, yet have meaningful poll times (even 5-10 microsecond poll times can have outsized impact on the scheduler). In an ideal world, the scheduler would adapt to the workload it is executing. However, as a stopgap, this commit limits the number of times the LIFO slot is prioritized per scheduler tick.
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As an optimization to improve locality, the multi-threaded scheduler maintains a single slot (LIFO slot). When a task is scheduled, it goes into the LIFO slot. The scheduler will run tasks in the LIFO slot first before checking the local queue. Ping-ping style workloads where task A notifies task B, which notifies task A again, can cause starvation as these two tasks repeatedly schedule the other in the LIFO slot. #5686, a first attempt at solving this problem, consumes a unit of budget each time a task is scheduled from the LIFO slot. However, at the time of this commit, the scheduler allocates 128 units of budget for each chunk of work. This is relatively high in situations where tasks do not perform many async operations yet have meaningful poll times (even 5-10 microsecond poll times can have an outsized impact on the scheduler). In an ideal world, the scheduler would adapt to the workload it is executing. However, as a stopgap, this commit limits the times the LIFO slot is prioritized per scheduler tick.
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This PR contains the following updates: | Package | Type | Update | Change | |---|---|---|---| | [tokio](https://tokio.rs) ([source](https://github.com/tokio-rs/tokio)) | dependencies | minor | `1.28.2` -> `1.29.1` | | [tokio](https://tokio.rs) ([source](https://github.com/tokio-rs/tokio)) | dev-dependencies | minor | `1.28.2` -> `1.29.1` | --- ### Release Notes <details> <summary>tokio-rs/tokio (tokio)</summary> ### [`v1.29.1`](https://github.com/tokio-rs/tokio/releases/tag/tokio-1.29.1): Tokio v1.29.1 [Compare Source](tokio-rs/tokio@tokio-1.29.0...tokio-1.29.1) ##### Fixed - rt: fix nesting two `block_in_place` with a `block_on` between (#​5837]) #​5837]: tokio-rs/tokio#5837 ### [`v1.29.0`](https://github.com/tokio-rs/tokio/releases/tag/tokio-1.29.0): Tokio v1.29.0 [Compare Source](tokio-rs/tokio@tokio-1.28.2...tokio-1.29.0) Technically a breaking change, the `Send` implementation is removed from `runtime::EnterGuard`. This change fixes a bug and should not impact most users. ##### Breaking - rt: `EnterGuard` should not be `Send` (#​5766]) ##### Fixed - fs: reduce blocking ops in `fs::read_dir` (#​5653]) - rt: fix possible starvation (#​5686], #​5712]) - rt: fix stacked borrows issue in `JoinSet` (#​5693]) - rt: panic if `EnterGuard` dropped incorrect order (#​5772]) - time: do not overflow to signal value (#​5710]) - fs: wait for in-flight ops before cloning `File` (#​5803]) ##### Changed - rt: reduce time to poll tasks scheduled from outside the runtime (#​5705], #​5720]) ##### Added - net: add uds doc alias for unix sockets (#​5659]) - rt: add metric for number of tasks (#​5628]) - sync: implement more traits for channel errors (#​5666]) - net: add nodelay methods on TcpSocket (#​5672]) - sync: add `broadcast::Receiver::blocking_recv` (#​5690]) - process: add `raw_arg` method to `Command` (#​5704]) - io: support PRIORITY epoll events (#​5566]) - task: add `JoinSet::poll_join_next` (#​5721]) - net: add support for Redox OS (#​5790]) ##### Unstable - rt: add the ability to dump task backtraces (#​5608], #​5676], #​5708], #​5717]) - rt: instrument task poll times with a histogram (#​5685]) #​5766]: tokio-rs/tokio#5766 #​5653]: tokio-rs/tokio#5653 #​5686]: tokio-rs/tokio#5686 #​5712]: tokio-rs/tokio#5712 #​5693]: tokio-rs/tokio#5693 #​5772]: tokio-rs/tokio#5772 #​5710]: tokio-rs/tokio#5710 #​5803]: tokio-rs/tokio#5803 #​5705]: tokio-rs/tokio#5705 #​5720]: tokio-rs/tokio#5720 #​5659]: tokio-rs/tokio#5659 #​5628]: tokio-rs/tokio#5628 #​5666]: tokio-rs/tokio#5666 #​5672]: tokio-rs/tokio#5672 #​5690]: tokio-rs/tokio#5690 #​5704]: tokio-rs/tokio#5704 #​5566]: tokio-rs/tokio#5566 #​5721]: tokio-rs/tokio#5721 #​5790]: tokio-rs/tokio#5790 #​5608]: tokio-rs/tokio#5608 #​5676]: tokio-rs/tokio#5676 #​5708]: tokio-rs/tokio#5708 #​5717]: tokio-rs/tokio#5717 #​5685]: tokio-rs/tokio#5685 </details> --- ### Configuration 📅 **Schedule**: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined). 🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied. ♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox. 🔕 **Ignore**: Close this PR and you won't be reminded about these updates again. --- - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box --- This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate). <!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiIzNi4wLjAiLCJ1cGRhdGVkSW5WZXIiOiIzNi4wLjAiLCJ0YXJnZXRCcmFuY2giOiJkZXZlbG9wIn0=--> Co-authored-by: cabr2-bot <cabr2.help@gmail.com> Reviewed-on: https://codeberg.org/Calciumdibromid/CaBr2/pulls/1958 Reviewed-by: crapStone <crapstone01@gmail.com> Co-authored-by: Calciumdibromid Bot <cabr2_bot@noreply.codeberg.org> Co-committed-by: Calciumdibromid Bot <cabr2_bot@noreply.codeberg.org>
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This fixes a problem where using the LIFO slot without consuming budget can result in starvation.