shell: fix delay in completion of jobs with a single shell rank #4159
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Problem: The rank 0 job shell normally waits for an "eof" message to the shell output.write service in order to know when it can flush pending events in eventlogger and release the shell completion reference for the output plugin. However, this reference counting is skipped when there is only one shell involved in the job. This causes a delay after all tasks output is closed of up to output.batch-timeout seconds (.5s by default). Instead of ignoring the contribution of the rank 0 shell to the output completion reference count, have each local task contribute to the reference count on this shell, and decrement the reference count for these tasks in the task.exit callback. This allows the output plugin to flush output immediately when the last task is complete. Since tasks are not considered complete until they have exited *and* all streams have EOF, this should be perfectly safe.
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Rebased. Thanks! I'll set MWP. |
Codecov Report
@@ Coverage Diff @@
## master #4159 +/- ##
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+ Coverage 83.43% 83.46% +0.02%
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Files 379 378 -1
Lines 63420 63427 +7
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+ Hits 52915 52937 +22
+ Misses 10505 10490 -15
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Commit 1d08bf7 modified how the shell output plugin detects completion, but it introduced a performance issue for single rank jobs, which now wait at least
output.batch-timeoutseconds before the output plugin flushes pending events and exits the reactor.This PR fixes that issue by having the rank 0 shell add a reference count per task - so that once all tasks are complete the output eventlogger can be flushed and exit immediately.
Without this fix single rank jobs have a minimum runtime of ~500 ms, which affects single node job throughput.