zhuzhurk commented on code in PR #20507: URL: https://github.com/apache/flink/pull/20507#discussion_r946673401
########## docs/content/docs/deployment/speculative_execution.md: ########## @@ -0,0 +1,112 @@ +--- +title: Speculative Execution +weight: 5 +type: docs + +--- +<!-- +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. +--> + +# Speculative Execution +Apache Flink supports speculative execution of batch jobs. + +This page describes the background of speculative execution, how to use it, and how to check the effectiveness of it. + +## Background +Speculative execution is a mechanism to mitigate job slowness which is caused by problematic nodes. +A problematic node may have hardware problems, accident I/O busy, or high CPU load. These problems may +make the hosted tasks run much slower than tasks on other nodes, and affect the overall execution time +of a batch job. + +In such cases, speculative execution will start a new attempt of the slow task on nodes that are not +detected as problematic. The new attempt processes the same input data and produces the same data as the +old one. The old attempt will not be affected and will keep running. The first finished attempt will be +admitted, its output will be seen and consumed by the downstream tasks, and the remaining attempts will be +canceled. + +To achieve this, Flink uses the slow task detector to detect slow tasks. The nodes that the slow tasks +locate in will be identified as problematic nodes and get blocked via the blocklist mechanism. The scheduler +will create new attempts for the slow tasks and deploy them on nodes that are not blocked. + +## Usage +This section describes how to use speculative execution, including how to enable it, how to tune it, and +how to develop/improve custom sources to work with speculative execution. + +{{< hint warning >}} +Note: Flink does not support speculative execution of sinks yet and will support it in follow-up releases. +{{< /hint >}} + +{{< hint warning >}} +Note: Flink does not support speculative execution of DataSet jobs because DataSet will be deprecated +in near future. DataStream API is now the recommended low level API to develop Flink batch jobs. +{{< /hint >}} + +### Enable Speculative Execution +To enable speculative execution, you need to set the following configuration options: +- `jobmanager.scheduler: AdaptiveBatch` + - Because only [Adaptive Batch Scheduler]({{< ref "docs/deployment/elastic_scaling" >}}#adaptive-batch-scheduler) supports speculative execution. +- `jobmanager.adaptive-batch-scheduler.speculative.enabled: true` + +### Tuning Configuration +To make speculative execution work better for different jobs, you can tune below configuration options of the scheduler: +- [`jobmanager.adaptive-batch-scheduler.speculative.max-concurrent-executions`]({{< ref "docs/deployment/config" >}}#jobmanager-adaptive-batch-scheduler-speculative-max-concurrent-e): +Controls the maximum number of execution attempts of each operator that can execute concurrently, including the original one and speculative ones. +- [`jobmanager.adaptive-batch-scheduler.speculative.block-slow-node-duration`]({{< ref "docs/deployment/config" >}}#jobmanager-adaptive-batch-scheduler-speculative-block-slow-node): +Controls how long an detected slow node should be blocked for. + +You can also tune below configuration options to control the slow task detector: +- [`slow-task-detector.check-interval`]({{< ref "docs/deployment/config" >}}#slow-task-detector-check-interval): +The interval to check slow tasks. +- [`slow-task-detector.execution-time.baseline-lower-bound`]({{< ref "docs/deployment/config" >}}#slow-task-detector-execution-time-baseline-lower-bound): +The lower bound of slow task detection baseline. +- [`slow-task-detector.execution-time.baseline-multiplier`]({{< ref "docs/deployment/config" >}}#slow-task-detector-execution-time-baseline-multiplier): +The multiplier to calculate the slow tasks detection baseline. Given that the parallelism is N and the ratio is R, define T as the median of the first `N*R` finished tasks' execution time. The baseline will be `T*M`, where M is the multiplier of the baseline. +- [`slow-task-detector.execution-time.baseline-ratio`]({{< ref "docs/deployment/config" >}}#slow-task-detector-execution-time-baseline-ratio): +The finished execution ratio threshold to calculate the slow tasks detection baseline. Given that the parallelism is N and the ratio is R, define T as the median of the first `N*R` finished tasks' execution time. The baseline will be `T*M`, where M is the multiplier of the baseline. + +### Enable Sources for Speculative Execution +If your job uses a custom {{< gh_link file="/flink-core/src/main/java/org/apache/flink/api/connector/source/Source.java" name="Source" >}}, +and the source uses custom {{< gh_link file="/flink-core/src/main/java/org/apache/flink/api/connector/source/SourceEvent.java" name="SourceEvent" >}}, +you need to change the {{< gh_link file="/flink-core/src/main/java/org/apache/flink/api/connector/source/SplitEnumerator.java" name="SplitEnumerator" >}} +of that source to implement {{< gh_link file="/flink-core/src/main/java/org/apache/flink/api/connector/source/SupportsHandleExecutionAttemptSourceEvent.java" name="SupportsHandleExecutionAttemptSourceEvent" >}} +interface. +```java +public interface SupportsHandleExecutionAttemptSourceEvent { + void handleSourceEvent(int subtaskId, int attemptNumber, SourceEvent sourceEvent); +} +``` +This means the SplitEnumerator should be aware of the attempt which sends the event. Otherwise, exceptions +will happen when the job manager receives a source event from the tasks and lead to job failures. + +No extra change is required for other sources to work with speculative execution, including +{{< gh_link file="/flink-streaming-java/src/main/java/org/apache/flink/streaming/api/functions/source/SourceFunction.java" name="SourceFunction sources" >}}, +{{< gh_link file="/flink-core/src/main/java/org/apache/flink/api/common/io/InputFormat.java" name="InputFormat sources" >}}, +and {{< gh_link file="/flink-core/src/main/java/org/apache/flink/api/connector/source/Source.java" name="new sources" >}}. +All the source connectors offered by Apache Flink can work with speculative execution. + +## Checking the Effectiveness of Speculative Execution +After enabling speculative execution, when there are slow tasks that trigger speculative execution, +the web UI will show the speculative attempts on the `SubTasks` tab of vertices on the job page. The web UI +also shows the blocked taskmanagers on the Flink cluster `Overview` and `Task Managers` pages. + +You can also check below metrics to see the effectiveness of speculative execution: +- **numSlowExecutionVertices**: Number of slow execution vertices at the moment. Review Comment: Yes, we should do that. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: issues-unsubscr...@flink.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org
