Zhilong Hong created FLINK-22643:
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Summary: Too many TCP connections among TaskManagers for large
scale jobs
Key: FLINK-22643
URL: https://issues.apache.org/jira/browse/FLINK-22643
Project: Flink
Issue Type: Improvement
Components: Runtime / Network
Affects Versions: 1.13.0
Reporter: Zhilong Hong
Fix For: 1.14.0
For the large scale jobs, there will be too many TCP connections among
TaskManagers. Let's take an example.
For a streaming job with 20 JobVertices, each JobVertex has 500 parallelism. We
divide the vertices into 5 slot sharing groups. Each TaskManager has 5 slots.
Thus there will be 400 taskmanagers in this job. Let's assume that job runs on
a cluster with 20 machines.
If all the job edges are all-to-all edges, there will be 19 * 20 * 399 * 2 =
303,240 TCP connections for each machine. If we run several jobs on this
cluster, the TCP connections may exceed the maximum limit of linux, which is
1,048,576. This will stop the TaskManagers from creating new TCP connections
and cause task failovers.
As we run our production jobs on a K8S cluster, the job always failover due to
exceptions related to network, such as {{Sending the partition request to
'null' failed}}, and etc.
We think that we can decrease the number of connections by letting tasks reuse
the same connection. We implemented a POC that makes all tasks on the same
TaskManager reuse one TCP connection. For the example job we mentioned above,
the number of connections will decrease from 303,240 to 15960. With the POC,
the frequency of meeting exceptions related to network in our production jobs
drops significantly.
The POC is illustrated in:
https://github.com/wsry/flink/commit/bf1c09e80450f40d018a1d1d4fe3dfd2de777fdc
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