Details
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Bug
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Status: Resolved
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P1
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Resolution: Fixed
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2.20.0
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None
Description
Nexmark tests show that the throughput of FlinkRunner with Rocksdb state backend dropped by 50%~80% in Query 4/5/6/9/11. Some other queries also dropped but not as much as these queries. Affected queries contain Keyed State.
Nexmark tests results. Tests have been run on the same machine and can be reproduced.
- before regression:
Performance:
Conf Runtime(sec) (Baseline) Events(/sec) (Baseline) Results (Baseline)
0000 3.8 26171.2 100000
0001 3.9 25967.3 92000
0002 1.9 53447.4 351
0003 2.8 35791.0 580
0004 2.5 4045.3 40
0005 9.6 10448.2 12
0006 1.2 8532.4 401
0007 4.0 25018.8 1
0008 2.9 34928.4 6000
0009 1.1 9066.2 298
0010 9.5 10564.1 2
0011 11.1 9005.0 1919
0012 4.5 22075.1 1919
0013 4.4 22547.9 92000
0014 9.7 10261.7 92000
- after regression:
Performance:
Conf Runtime(sec) (Baseline) Events(/sec) (Baseline) Results (Baseline)
0000 4.5 22036.1 100000
0001 3.9 25839.8 92000
0002 2.3 43763.7 351
0003 3.5 28669.7 580
0004 3.6 2801.1 40
0005 22.6 4429.1 12
0006 2.5 3993.6 401
0007 7.5 13320.9 1
0008 2.7 36737.7 6000
0009 2.5 3930.8 298
0010 16.2 6178.6 2
0011 82.9 1206.3 1919
0012 5.9 16874.8 1919
0013 4.2 23889.2 92000
The regression comes from the "updateWatermarkHold()" function recently added in Flink DoFnOperator in [PR#10534|hhttps://github.com/apache/beam/pull/10534]
It's to allow FinkRunner to set watermark holds. However, the implementation was not performance efficient:
- "pendingTimersById" is an interface of the RocksDB key/value map state. Iterate all values via "pendingTimersById.values()" is a big cost. According to the bellow CPU cycles profiling result of Query 4 by [async-profiler|https://github.com/jvm-profiling-tools/async-profiler], we also can see that most of the CPU time was spent on "RocksIterator.next()" brought by "pendingTimersById.values()".
- Another overhead is that this function will be called multiple times in each setTimer and deleteTimer;
CPU time profiling result (FlameGraph) of Query 4 in Nexmark:
https://drive.google.com/open?id=1muVQipv-JidxVceQkOze5PZozPgPB_bh
