[jira] [Commented] (SPARK-19326) Speculated task attempts do not get launched in few scenarios

["Tejas Patil (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-19326?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15833270#comment-15833270 ] 

Tejas Patil commented on SPARK-19326:
-------------------------------------

cc [~kayousterhout], [~irashid] [~vanzin] 

`ExecutorAllocationManager` currently determines "is there any need to allocate new executor ?" based on the `running` and `pending` tasks. This accounting is purely based on a listener which does not know anything speculated tasks. From the code I could see that this was [intentionally done|https://github.com/apache/spark/blob/6ee28423ad1b2e6089b82af64a31d77d3552bb38/core/src/main/scala/org/apache/spark/ExecutorAllocationManager.scala#L579] in the original patch which introduced this. Changing this doesn't look like a point fix to me so would like to get your opinion about this.

> 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: 2.0.2, 2.1.0
>            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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