Tejas Patil created SPARK-19326:
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Summary: Speculated task attempts do not get launched in few
scenarios
Key: SPARK-19326
URL: https://issues.apache.org/jira/browse/SPARK-19326
Project: Spark
Issue Type: Bug
Components: Scheduler
Affects Versions: 2.1.0, 2.0.2
Reporter: Tejas Patil
Speculated copies of tasks do not get launched in some cases.
Examples:
- All the running executors have no CPU slots left to accommodate a speculated
copy of the task(s). If the all running executors reside over a set of slow /
bad hosts, they will keep the job running for long time
- `spark.task.cpus` > 1 and the running executor has not filled up all its CPU
slots. Since the [speculated copies of tasks should run on different
host|https://github.com/apache/spark/blob/2e139eed3194c7b8814ff6cf007d4e8a874c1e4d/core/src/main/scala/org/apache/spark/scheduler/TaskSetManager.scala#L283]
and not the host where the first copy was launched.
In both these cases, `ExecutorAllocationManager` does not know about pending
speculation task attempts and thinks that all the resource demands are well
taken care of. ([relevant
code|https://github.com/apache/spark/blob/6ee28423ad1b2e6089b82af64a31d77d3552bb38/core/src/main/scala/org/apache/spark/ExecutorAllocationManager.scala#L265])
This adds variation in the job completion times and more importantly SLA misses
:( In prod, with a large number of jobs, I see this happening more often than
one would think. Chasing the bad hosts or reason for slowness doesn't scale.
Here is a tiny repro. Note that you need to launch this with (Mesos or YARN or
standalone deploy mode) along with `spark.speculation=true`
{code}
val someRDD = sc.parallelize(1 to 8, 8)
someRDD.mapPartitionsWithIndex( (index: Int, it: Iterator[Int]) => {
if (index == 8) {
Thread.sleep(Long.MaxValue) // fake long running task(s)
}
it.toList.map(x => index + ", " + x).iterator
}).collect
{code}
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