[GitHub] [arrow-datafusion] xudong963 commented on a diff in pull request #2226: Introduce new optional scheduler, using Morsel-driven Parallelism + rayon (#2199)

[GitBox <gi...@apache.org>]

xudong963 commented on code in PR #2226:
URL: https://github.com/apache/arrow-datafusion/pull/2226#discussion_r856941791


##########
datafusion/scheduler/src/plan.rs:
##########
@@ -0,0 +1,276 @@
+// 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 datafusion::error::Result;
+use datafusion::execution::context::TaskContext;
+use datafusion::physical_plan::coalesce_partitions::CoalescePartitionsExec;
+use datafusion::physical_plan::repartition::RepartitionExec;
+use datafusion::physical_plan::{ExecutionPlan, Partitioning};
+
+use crate::pipeline::{
+    execution::ExecutionPipeline, repartition::RepartitionPipeline, Pipeline,
+};
+
+/// Identifies the [`Pipeline`] within the [`PipelinePlan`] to route output to
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct OutputLink {
+    /// The index of the [`Pipeline`] in [`PipelinePlan`] to route output to
+    pub pipeline: usize,
+
+    /// The child of the [`Pipeline`] to route output to
+    pub child: usize,
+}
+
+/// Combines a [`Pipeline`] with an [`OutputLink`] identifying where to send its output
+#[derive(Debug)]
+pub struct RoutablePipeline {
+    /// The pipeline that produces data
+    pub pipeline: Box<dyn Pipeline>,
+
+    /// Where to send output the output of `pipeline`
+    ///
+    /// If `None`, the output should be sent to the query output
+    pub output: Option<OutputLink>,
+}
+
+/// [`PipelinePlan`] is the scheduler's representation of the [`ExecutionPlan`] passed to
+/// [`super::Scheduler::schedule`]. It combines the list of [Pipeline`] with the information
+/// necessary to route output from one stage to the next
+#[derive(Debug)]
+pub struct PipelinePlan {
+    pub pipelines: Vec<RoutablePipeline>,
+}
+
+/// When converting [`ExecutionPlan`] to [`Pipeline`] we may wish to group
+/// together multiple operators, [`OperatorGroup`] stores this state
+struct OperatorGroup {
+    /// Where to route the output of the eventual [`Pipeline`]
+    output: Option<OutputLink>,
+
+    /// The [`ExecutionPlan`] from which to start recursing
+    root: Arc<dyn ExecutionPlan>,
+
+    /// The number of times to recurse into the [`ExecutionPlan`]'s children
+    depth: usize,
+}
+
+/// A utility struct to assist converting from [`ExecutionPlan`] to [`PipelinePlan`]
+///
+/// The [`ExecutionPlan`] is visited in a depth-first fashion, gradually building
+/// up the [`RoutablePipeline`] for the [`PipelinePlan`]. As nodes are visited depth-first,
+/// a node is visited only after its parent has been.
+pub struct PipelinePlanner {
+    task_context: Arc<TaskContext>,
+
+    /// The current list of completed pipelines
+    in_progress: Vec<RoutablePipeline>,

Review Comment:
   `in_progress` with the annotation `completed` seems a little confusing 🤔



##########
datafusion/scheduler/src/lib.rs:
##########
@@ -0,0 +1,411 @@
+// 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.
+
+//! 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/
+//!
+//! # Example
+//!
+//! ```rust
+//! # use futures::TryStreamExt;
+//! # use datafusion::prelude::{CsvReadOptions, SessionConfig, SessionContext};
+//! # use datafusion_scheduler::Scheduler;
+//!
+//! # #[tokio::main]
+//! # async fn main() {
+//! let scheduler = Scheduler::new(4);
+//! let config = SessionConfig::new().with_target_partitions(4);
+//! let context = SessionContext::with_config(config);
+//!
+//! context.register_csv("example", "../core/tests/example.csv", CsvReadOptions::new()).await.unwrap();
+//! let plan = context.sql("SELECT MIN(b) FROM example")
+//!     .await
+//!    .unwrap()
+//!    .create_physical_plan()
+//!    .await
+//!    .unwrap();
+//!
+//! let task = context.task_ctx();
+//! let stream = scheduler.schedule(plan, task).unwrap();
+//! let scheduled: Vec<_> = stream.try_collect().await.unwrap();
+//! # }
+//! ```
+//!
+
+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::plan::{PipelinePlan, PipelinePlanner, RoutablePipeline};
+use crate::task::{spawn_plan, Task};
+
+use rayon::{ThreadPool, ThreadPoolBuilder};
+
+pub use task::ExecutionResults;
+
+mod pipeline;
+mod plan;
+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)]

Review Comment:
   If the method only used by tests, how about repace `#[allow(dead_code)]` with `#[cfg(test)]`?



##########
datafusion/scheduler/src/task.rs:
##########
@@ -0,0 +1,437 @@
+// 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 crate::plan::PipelinePlan;
+use crate::{is_worker, spawn_local, spawn_local_fifo, RoutablePipeline, Spawner};
+use arrow::record_batch::RecordBatch;
+use datafusion::error::{DataFusionError, Result};
+use futures::channel::mpsc;
+use futures::task::ArcWake;
+use futures::{Stream, StreamExt};
+use log::{debug, trace};
+use std::pin::Pin;
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::{Arc, Weak};
+use std::task::{Context, Poll};
+
+/// Spawns a [`PipelinePlan`] using the provided [`Spawner`]
+pub fn spawn_plan(plan: PipelinePlan, spawner: Spawner) -> ExecutionResults {
+    debug!("Spawning pipeline plan: {:#?}", plan);
+
+    let (sender, receiver) = mpsc::unbounded();
+    let context = Arc::new(ExecutionContext {
+        spawner,
+        pipelines: plan.pipelines,
+        output: sender,
+    });
+
+    for (pipeline_idx, query_pipeline) in context.pipelines.iter().enumerate() {
+        for partition in 0..query_pipeline.pipeline.output_partitions() {
+            context.spawner.spawn(Task {
+                context: context.clone(),
+                waker: Arc::new(TaskWaker {
+                    context: Arc::downgrade(&context),
+                    wake_count: AtomicUsize::new(1),
+                    pipeline: pipeline_idx,
+                    partition,
+                }),
+            });
+        }
+    }
+
+    ExecutionResults {
+        context: context,

Review Comment:
   ```suggestion
           context,
   ```



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