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https://issues.apache.org/jira/browse/FLINK-26306?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17496736#comment-17496736
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Piotr Nowojski commented on FLINK-26306:
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Thanks for the explanation, I get it now.
> 1. Batch deletions and leave one thread idle (e.g. group 1K handles into 10
> big batches handled by 11 IO threads)
Is this the right level to provide back pressure functionality? Would it even
work if you hardcoded in the {{CheckpointCoordinator}} assumptions about pool
size and the number of used threads? We don't know how else this thread pool is
being used.
Apart of that. Don't we already have a backpressure mechanism on a higher
level? {{CheckpointRequestDecider#numberOfCleaningCheckpointsSupplier}}? It
looks like simple fair io thread pool as I described above, without any
priorities + addjusting/relaxing
{{numberOfCleaningCheckpointsSupplier.getAsInt() >
maxConcurrentCheckpointAttempts}} check to something like
{{numberOfCleaningCheckpointsSupplier.getAsInt() >
maxConcurrentCheckpointAttempts + CONSTANT}} would do the trick, wouldn't it?
> Triggered checkpoints can be delayed by discarding shared state
> ---------------------------------------------------------------
>
> Key: FLINK-26306
> URL: https://issues.apache.org/jira/browse/FLINK-26306
> Project: Flink
> Issue Type: Improvement
> Components: Runtime / Checkpointing
> Affects Versions: 1.15.0, 1.14.3
> Reporter: Roman Khachatryan
> Assignee: Roman Khachatryan
> Priority: Major
> Fix For: 1.15.0
>
>
> 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.
> ----
> 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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