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https://issues.apache.org/jira/browse/SPARK-17911?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15572289#comment-15572289
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Imran Rashid commented on SPARK-17911:
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Copying the earlier discussion on the PR here
from squito:
bq.I find myself frequently wondering about the purpose of this. Its commented
very tersely on RESUBMIT_TIMEOUT, but I think it might be nice to add a longer
comment here. I guess something like
bq. "If we get one fetch-failure, we often get more fetch failures across
multiple executors. We will get better parallelism when we resubmit the
mapStage if we can resubmit when we know about as many of those failures as
possible. So this is a heuristic to add a small delay to see if we gather a few
more failures before we resubmit."
bq.We do not need the delay to figure out exactly which shuffle-map outputs are
gone on the executor -- we always mark the executor as lost on a fetch failure,
which means we mark all its map output as gone. (This is really confusing -- it
looks like we only remove the one shuffle-map output that was involved in the
fetch failure, but then the entire removal is buried inside another method a
few lines further.)
bq.I did some browsing through history, and there used to be this comment
{noformat}
// Periodically resubmit failed stages if some map output fetches have
failed and we have
// waited at least RESUBMIT_TIMEOUT. We wait for this short time because
when a node fails,
// tasks on many other nodes are bound to get a fetch failure, and they
won't all get it at
// the same time, so we want to make sure we've identified all the reduce
tasks that depend
// on the failed node.
{noformat}
bq. at least in the current version, this also sounds like a bad reason to have
the delay. failedStage won't be resubmitted till mapStage completes anyway, and
then it'll look to see what tasks it is missing. Adding a tiny delay on top of
the natural delay for mapStage seems pretty pointless.
bq. I don't even think that the reason I gave in my suggested comment is a good
one -- do you really expect failures in multiple executors? But it is at least
logically consistent.
from markhamstra
bq. I don't like "Periodically" in your suggested comment, since this is a
one-shot action after a delay of RESUBMIT_TIMEOUT milliseconds.
bq.I agree that this delay-before-resubmit logic is suspect. If we are both
thinking correctly that a 200 ms delay on top of the time to re-run the
mapStage is all but inconsequential, then removing it in this PR would be fine.
If there are unanticipated consequences, though, I'd prefer to have that change
in a separate PR.
from squito
bq. yeah probably a separate PR, sorry this was just an opportunity for me to
rant :)
bq.And sorry if I worded it poorly, but I was not suggesting the one w/
"Periodically" as a better comment -- in fact I think its a bad comment, just
wanted to mention it was another description which used to be there long ago.
bq. This was my suggestion:
bq. If we get one fetch-failure, we often get more fetch failures across
multiple executors. We will get better parallelism when we resubmit the
mapStage if we can resubmit when we know about as many of those failures as
possible. So this is a heuristic to add a small delay to see if we gather a few
more failures before we resubmit.
from markhamstra
bq. Ah, sorry for ascribing the prior comment to your preferences. That comment
actually did make sense a long time ago when the resubmitting of stages really
was done periodically by an Akka scheduled event that fired every something
seconds. I'm pretty sure the RESUBMIT_TIMEOUT stuff is also legacy code that
doesn't make sense and isn't necessary any more.
bq. So, do you want to do the follow-up PR to get rid of it, or shall I?
bq. BTW, nothing wrong with your wording -- but my poor reading can create
misunderstanding of even the clearest text.
from squito
bq. if you are willing, could you please file the follow up? I am bouncing
between various things in my backlog -- though that change is small, I have a
feeling it will be merit extra discussion as a risky change, would be great if
you drive it
bq. Ok, I can get started on that. I believe that leaves this PR ready to merge.
bq. I think that I am going to backtrack on creating a new PR, because I think
that the RESUBMIT_TIMEOUT actually does still make sense.
bq. If we go way back in DAGScheduler history
(https://github.com/apache/spark/blob/branch-0.8/core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala)
we'll find that we had an event queue that was polled every 10 millis
(POLL_TIMEOUT) and that fetch failures didn't produce separate resubmit tasks
events, but rather we called resubmitFailedStages within the event
polling/handling loop if 50 millis (RESUBMIT_TIMEOUT) had passed since the last
time a FetchFailed was received in a CompletionEvent. The handling of a
FetchFailure didn't have any check of whether a failed stage was already in
failedStages or any other kind of heuristic as to whether a resubmit was
already scheduled for a failed stage, so the RESUBMIT_TIMEOUT was effectively
the only thing preventing multiple resubmits from occurring as quickly as every
10 ms.
bq. RESUBMIT_TIMEOUT has persisted through the multiple iterations of the
DAGScheduler since then, and now it is effectively making sure that the time
between ResubmitFailedStages events is at least 200 ms. The time to actually
complete any of the stages in failedStages when the ResubmitFailedStages event
is handled doesn't really come into play, since failedStages is cleared within
resubmitFailedStages and that method returns as soon as the stages that were
queued up are resubmitted, not actually done re-calculating. In other words,
handling a ResubmitFailedStages event should be quick, and causes failedStages
to be cleared, allowing the next ResubmitFailedStages event to be posted from
the handling of another FetchFailed. If there are the expected lot of fetch
failures for a single stage, and there is no RESUBMIT_TIMEOUT, then it is quite
likely that there will be a burst of resubmit events (and corresponding log
messages) and submitStage calls made in rapid succession.
bq. I think I'm inclined to keep the RESUBMIT_TIMEOUT throttle.
> Scheduler does not need messageScheduler for ResubmitFailedStages
> -----------------------------------------------------------------
>
> Key: SPARK-17911
> URL: https://issues.apache.org/jira/browse/SPARK-17911
> Project: Spark
> Issue Type: Improvement
> Components: Scheduler
> Affects Versions: 2.0.0
> Reporter: Imran Rashid
>
> Its not totally clear what the purpose of the {{messageScheduler}} is in
> {{DAGScheduler}}. It can perhaps be eliminated completely; or perhaps we
> should just clearly document its purpose.
> This comes from a long discussion w/ [~markhamstra] on an unrelated PR here:
> https://github.com/apache/spark/pull/15335/files/c80ad22a242255cac91cce2c7c537f9b21100f70#diff-6a9ff7fb74fd490a50462d45db2d5e11
> But its tricky so breaking it out here for archiving the discussion.
> Note: this issue requires a decision on what to do before a code change, so
> lets just discuss it on jira first.
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