tustvold commented on code in PR #2226: URL: https://github.com/apache/arrow-datafusion/pull/2226#discussion_r856290226
########## datafusion/scheduler/src/lib.rs: ########## @@ -0,0 +1,381 @@ +// Licensed to the Apache Software Foundation (ASF) under one +// or more contributor license agreements. See the NOTICE file +// distributed with this work for additional information +// regarding copyright ownership. The ASF licenses this file +// to you under the Apache License, Version 2.0 (the +// "License"); you may not use this file except in compliance +// with the License. You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, +// software distributed under the License is distributed on an +// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +// KIND, either express or implied. See the License for the +// specific language governing permissions and limitations +// under the License. + +use std::sync::Arc; + +use futures::stream::BoxStream; +use log::{debug, error}; + +use datafusion::error::Result; +use datafusion::execution::context::TaskContext; +use datafusion::physical_plan::ExecutionPlan; + +use crate::query::{Query, QueryBuilder, RoutablePipeline}; +use crate::task::{spawn_query, Task}; + +use rayon::{ThreadPool, ThreadPoolBuilder}; + +pub use task::QueryResults; + +mod pipeline; +mod query; +mod task; + +/// Builder for a [`Scheduler`] +#[derive(Debug)] +pub struct SchedulerBuilder { + inner: ThreadPoolBuilder, +} + +impl SchedulerBuilder { + /// Create a new [`SchedulerConfig`] with the provided number of threads + pub fn new(num_threads: usize) -> Self { + let builder = ThreadPoolBuilder::new() + .num_threads(num_threads) + .panic_handler(|p| error!("{}", format_worker_panic(p))) + .thread_name(|idx| format!("df-worker-{}", idx)); + + Self { inner: builder } + } + + /// Registers a custom panic handler + /// + /// Used by tests + #[allow(dead_code)] + fn panic_handler<H>(self, panic_handler: H) -> Self + where + H: Fn(Box<dyn std::any::Any + Send>) + Send + Sync + 'static, + { + Self { + inner: self.inner.panic_handler(panic_handler), + } + } + + /// Build a new [`Scheduler`] + fn build(self) -> Scheduler { + Scheduler { + pool: Arc::new(self.inner.build().unwrap()), + } + } +} + +/// A [`Scheduler`] maintains a pool of dedicated worker threads on which +/// query execution can be scheduled. This is based on the idea of [Morsel-Driven Parallelism] +/// and is designed to decouple the execution parallelism from the parallelism expressed in +/// the physical plan as partitions. +/// +/// # Implementation +/// +/// When provided with an [`ExecutionPlan`] the [`Scheduler`] first breaks it up into smaller +/// chunks called pipelines. Each pipeline may consist of one or more nodes from the +/// [`ExecutionPlan`] tree. +/// +/// The scheduler then maintains a list of pending [`Task`], that identify a partition within +/// a particular pipeline that may be able to make progress on some "morsel" of data. These +/// [`Task`] are then scheduled on the worker pool, with a preference for scheduling work +/// on a given "morsel" on the same thread that produced it. +/// +/// # Rayon +/// +/// Under-the-hood these [`Task`] are scheduled by [rayon], which is a lightweight, work-stealing +/// scheduler optimised for CPU-bound workloads. Pipelines may exploit this fact, and use [rayon]'s +/// structured concurrency primitives to express additional parallelism that may be exploited +/// if there are idle threads available at runtime +/// +/// # Shutdown +/// +/// Queries scheduled on a [`Scheduler`] will run to completion even if the +/// [`Scheduler`] is dropped +/// +/// [Morsel-Driven Parallelism]: https://db.in.tum.de/~leis/papers/morsels.pdf +/// [rayon]: https://docs.rs/rayon/latest/rayon/ +/// +pub struct Scheduler { + pool: Arc<ThreadPool>, +} + +impl Scheduler { + /// Create a new [`Scheduler`] with `num_threads` new threads in a dedicated thread pool + pub fn new(num_threads: usize) -> Self { + SchedulerBuilder::new(num_threads).build() + } + + /// Schedule the provided [`ExecutionPlan`] on this [`Scheduler`]. + /// + /// Returns a [`BoxStream`] that can be used to receive results as they are produced + pub fn schedule( + &self, + plan: Arc<dyn ExecutionPlan>, + context: Arc<TaskContext>, + ) -> Result<QueryResults> { + let query = QueryBuilder::new(plan, context).build()?; + Ok(self.schedule_query(query)) + } + + /// Schedule the provided [`Query`] on this [`Scheduler`]. + pub(crate) fn schedule_query(&self, query: Query) -> QueryResults { + spawn_query(query, self.spawner()) + } + + fn spawner(&self) -> Spawner { + Spawner { + pool: self.pool.clone(), + } + } +} + +/// Formats a panic message for a worker +fn format_worker_panic(panic: Box<dyn std::any::Any + Send>) -> String { + let maybe_idx = rayon::current_thread_index(); + let worker: &dyn std::fmt::Display = match &maybe_idx { + Some(idx) => idx, + None => &"UNKNOWN", + }; + + let message = if let Some(msg) = panic.downcast_ref::<&str>() { Review Comment: I don't think downcast_ref lets you cast to different types from the original Any, so I don't think this would work unless someone actually panic'ed with a `&'static dyn Display` type (which would be hard to construct if even possible) -- This is an automated message from the Apache Git Service. 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