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https://issues.apache.org/jira/browse/FLINK-12576?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16938406#comment-16938406
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Yingjie Cao commented on FLINK-12576:
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[~alpinegizmo] Because only one of the two upstream task will emit records to
the downstream task. One of downstream task will always get its input from
local input channel and no buffer in the bufferPool for input will be used. So
inPoolUsage is always zero for one of the channels should be what's expected.
For one two-slot TM case, both input channels for the Backpressure vertex are
local input channel. So the inPoolUsage should be also zero.
> inputQueueLength metric does not work for LocalInputChannels
> ------------------------------------------------------------
>
> Key: FLINK-12576
> URL: https://issues.apache.org/jira/browse/FLINK-12576
> Project: Flink
> Issue Type: Bug
> Components: Runtime / Metrics, Runtime / Network
> Affects Versions: 1.6.4, 1.7.2, 1.8.0, 1.9.0
> Reporter: Piotr Nowojski
> Priority: Major
> Labels: pull-request-available
> Fix For: 1.9.0
>
> Attachments: Screen Shot 2019-09-24 at 3.11.15 PM.png, Screen Shot
> 2019-09-24 at 3.13.05 PM.png, Screen Shot 2019-09-24 at 3.22.36 PM.png,
> Screen Shot 2019-09-24 at 3.22.53 PM.png,
> flink-1.8-2-single-slot-TMs-input.png,
> flink-1.8-2-single-slot-TMs-output.png, flink-1.8-input-subtasks.png,
> flink-1.8-output-subtasks.png, image-2019-09-26-11-34-24-878.png,
> image-2019-09-26-11-36-06-027.png
>
> Time Spent: 20m
> Remaining Estimate: 0h
>
> Currently {{inputQueueLength}} ignores LocalInputChannels
> ({{SingleInputGate#getNumberOfQueuedBuffers}}). This can can cause mistakes
> when looking for causes of back pressure (If task is back pressuring whole
> Flink job, but there is a data skew and only local input channels are being
> used).
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