[jira] [Created] (FLINK-26590) Triggered checkpoints can be delayed by discarding shared state

["Roman Khachatryan (Jira)" <ji...@apache.org>]

Roman Khachatryan created FLINK-26590:
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             Summary: Triggered checkpoints can be delayed by discarding shared state
                 Key: FLINK-26590
                 URL: https://issues.apache.org/jira/browse/FLINK-26590
             Project: Flink
          Issue Type: Improvement
          Components: Runtime / Checkpointing
    Affects Versions: 1.15.0, 1.14.3
            Reporter: Roman Khachatryan
            Assignee: Roman Khachatryan
             Fix For: 1.16.0


Quick note: CheckpointCleaner is not involved here.

When a checkpoint is subsumed, SharedStateRegistry schedules its unused shared state for async deletion. It uses common IO pool for this and adds a Runnable per state handle. ( see SharedStateRegistryImpl.scheduleAsyncDelete)

When a checkpoint is started, CheckpointCoordinator uses the same thread pool to initialize the location for it. (see CheckpointCoordinator.initializeCheckpoint)

The thread pool is of fixed size [jobmanager.io-pool.size|https://nightlies.apache.org/flink/flink-docs-master/docs/deployment/config/#jobmanager-io-pool-size]; by default it's the number of CPU cores) and uses FIFO queue for tasks.

When there is a spike in state deletion, the next checkpoint is delayed waiting for an available IO thread.

Back-pressure seems reasonable here (similar to CheckpointCleaner); however, this shared state deletion could be spread across multiple subsequent checkpoints, not neccesarily the next one.

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I believe the issue is an pre-existing one; but it particularly affects changelog state backend, because 1) such spikes are likely there; 2) workloads are latency sensitive.

In the tests, checkpoint duration grows from seconds to minutes immediately after the materialization.



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