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

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

tustvold commented on code in PR #2226:
URL: https://github.com/apache/arrow-datafusion/pull/2226#discussion_r850809762


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datafusion/scheduler/src/lib.rs:
##########
@@ -0,0 +1,275 @@
+// 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, StreamExt};
+use log::debug;
+
+use datafusion::arrow::error::Result as ArrowResult;
+use datafusion::arrow::record_batch::RecordBatch;
+use datafusion::error::Result;
+use datafusion::execution::context::TaskContext;
+use datafusion::physical_plan::ExecutionPlan;
+
+use crate::query::Query;
+use crate::task::{spawn_query, Task};
+
+use rayon::{ThreadPool, ThreadPoolBuilder};
+
+mod pipeline;
+mod query;
+mod task;
+
+/// 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]
+/// which decouples execution parallelism from the parallelism expressed in the physical plan
+///
+/// # 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
+///
+/// TBC
+///
+/// [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` threads in its thread pool
+    pub fn new(num_threads: usize) -> Self {
+        let pool = ThreadPoolBuilder::new()
+            .num_threads(num_threads)
+            .thread_name(|idx| format!("df-worker-{}", idx))
+            .build()
+            .unwrap();
+
+        Self {
+            pool: Arc::new(pool),
+        }
+    }
+
+    /// 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<BoxStream<'static, ArrowResult<RecordBatch>>> {
+        let (query, receiver) = Query::new(plan, context, self.spawner())?;
+        spawn_query(Arc::new(query));
+        Ok(receiver.boxed())
+    }
+
+    fn spawner(&self) -> Spawner {
+        Spawner {
+            pool: self.pool.clone(),
+        }
+    }
+}
+
+/// Returns `true` if the current thread is a worker thread
+fn is_worker() -> bool {
+    rayon::current_thread_index().is_some()

Review Comment:
   I'd rather keep the check cheap, as it will be called fairly frequently, it's mainly intended as a sanity check



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