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2020-11-01tracing: replace future names with spawn locations in task spans (#3074)Eliza Weisman
## Motivation Currently, the per-task `tracing` spans generated by tokio's `tracing` feature flag include the `std::any::type_name` of the future that was spawned. When future combinators and/or libraries like Tower are in use, these future names can get _quite_ long. Furthermore, when formatting the `tracing` spans with their parent spans as context, any other task spans in the span context where the future was spawned from can _also_ include extremely long future names. In some cases, this can result in extremely high memory use just to store the future names. For example, in Linkerd, when we enable `tokio=trace` to enable the task spans, there's a spawned task whose future name is _232990 characters long_. A proxy with only 14 spawned tasks generates a task list that's over 690 KB. Enabling task spans under load results in the process getting OOM killed very quickly. ## Solution This branch removes future type names from the spans generated by `spawn`. As a replacement, to allow identifying which `spawn` call a span corresponds to, the task span now contains the source code location where `spawn` was called, when the compiler supports the `#[track_caller]` attribute. Since `track_caller` was stabilized in Rust 1.46.0, and our minimum supported Rust version is 1.45.0, we can't assume that `#[track_caller]` is always available. Instead, we have a RUSTFLAGS cfg, `tokio_track_caller`, that guards whether or not we use it. I've also added a `build.rs` that detects the compiler minor version, and sets the cfg flag automatically if the current compiler version is >= 1.46. This means users shouldn't have to enable `tokio_track_caller` manually. Here's the trace output from the `chat` example, before this change: ![Screenshot_20201030_110157](https://user-images.githubusercontent.com/2796466/97741071-6d408800-1a9f-11eb-9ed6-b25e72f58c7b.png) ...and after: ![Screenshot_20201030_110303](https://user-images.githubusercontent.com/2796466/97741112-7e899480-1a9f-11eb-9197-c5a3f9ea1c05.png) Closes #3073 Signed-off-by: Eliza Weisman <eliza@buoyant.io>
2020-10-12rt: simplify rt-* features (#2949)Taiki Endo
tokio: merge rt-core and rt-util as rt rename rt-threaded to rt-multi-thread tokio-util: rename rt-core to rt Closes #2942
2020-10-12rt: Remove `threaded_scheduler()` and `basic_scheduler()` (#2876)Lucio Franco
Co-authored-by: Alice Ryhl <alice@ryhl.io> Co-authored-by: Carl Lerche <me@carllerche.com>
2020-07-13task: add Tracing instrumentation to spawned tasks (#2655)Eliza Weisman
## Motivation When debugging asynchronous systems, it can be very valuable to inspect what tasks are currently active (see #2510). The [`tracing` crate] and related libraries provide an interface for Rust libraries and applications to emit and consume structured, contextual, and async-aware diagnostic information. Because this diagnostic information is structured and machine-readable, it is a better fit for the task-tracking use case than textual logging — `tracing` spans can be consumed to generate metrics ranging from a simple counter of active tasks to histograms of poll durations, idle durations, and total task lifetimes. This information is potentially valuable to both Tokio users *and* to maintainers. Additionally, `tracing` is maintained by the Tokio project and is becoming widely adopted by other libraries in the "Tokio stack", such as [`hyper`], [`h2`], and [`tonic`] and in [other] [parts] of the broader Rust ecosystem. Therefore, it is suitable for use in Tokio itself. [`tracing` crate]: https://github.com/tokio-rs/tracing [`hyper`]: https://github.com/hyperium/hyper/pull/2204 [`h2`]: https://github.com/hyperium/h2/pull/475 [`tonic`]: https://github.com/hyperium/tonic/blob/570c606397e47406ec148fe1763586e87a8f5298/tonic/Cargo.toml#L48 [other]: https://github.com/rust-lang/chalk/pull/525 [parts]: https://github.com/rust-lang/compiler-team/issues/331 ## Solution This PR is an MVP for instrumenting Tokio with `tracing` spans. When the "tracing" optional dependency is enabled, every spawned future will be instrumented with a `tracing` span. The generated spans are at the `TRACE` verbosity level, and have the target "tokio::task", which may be used by consumers to filter whether they should be recorded. They include fields for the type name of the spawned future and for what kind of task the span corresponds to (a standard `spawn`ed task, a local task spawned by `spawn_local`, or a `blocking` task spawned by `spawn_blocking`). Because `tracing` has separate concepts of "opening/closing" and "entering/exiting" a span, we enter these spans every time the spawned task is polled. This allows collecting data such as: - the total lifetime of the task from `spawn` to `drop` - the number of times the task was polled before it completed - the duration of each individual time that the span was polled (and therefore, aggregated metrics like histograms or averages of poll durations) - the total time a span was actively being polled, and the total time it was alive but **not** being polled - the time between when the task was `spawn`ed and the first poll As an example, here is the output of a version of the `chat` example instrumented with `tracing`: ![image](https://user-images.githubusercontent.com/2796466/87231927-e50f6900-c36f-11ea-8a90-6da9b93b9601.png) And, with multiple connections actually sending messages: ![trace_example_1](https://user-images.githubusercontent.com/2796466/87231876-8d70fd80-c36f-11ea-91f1-0ad1a5b3112f.png) I haven't added any `tracing` spans in the example, only converted the existing `println!`s to `tracing::info` and `tracing::error` for consistency. The span durations in the above output are generated by `tracing-subscriber`. Of course, a Tokio-specific subscriber could generate even more detailed statistics, but that's follow-up work once basic tracing support has been added. Note that the `Instrumented` type from `tracing-futures`, which attaches a `tracing` span to a future, was reimplemented inside of Tokio to avoid a dependency on that crate. `tracing-futures` has a feature flag that enables an optional dependency on Tokio, and I believe that if another crate in a dependency graph enables that feature while Tokio's `tracing` support is also enabled, it would create a circular dependency that Cargo wouldn't be able to handle. Also, it avoids a dependency for a very small amount of code that is unlikely to ever change. There is, of course, room for plenty of future work here. This might include: - instrumenting other parts of `tokio`, such as I/O resources and channels (possibly via waker instrumentation) - instrumenting the threadpool so that the state of worker threads can be inspected - writing `tracing-subscriber` `Layer`s to collect and display Tokio-specific data from these traces - using `track_caller` (when it's stable) to record _where_ a task was `spawn`ed from However, this is intended as an MVP to get us started on that path. Signed-off-by: Eliza Weisman <eliza@buoyant.io>