Hey Matei and Imran, I think may be we can solve the problem without downgrading to 2.1.0 may by capturing dissociation and then setting a timeout if it associates again we keep moving else we shutdown the executor. This timeout can ofcourse be configurable.
Thoughts ? On Sat, Nov 2, 2013 at 3:29 AM, Matei Zaharia <[email protected]>wrote: > Hey Imran, > > Good to know that Akka 2.1 handles this — that at least will give us a > start. > > In the old code, executors certainly did get flagged as “down” > occasionally, but that was due to a timeout we controlled (we keep sending > heartbeats back and forth to track them). The timeout used to be smaller > and usually the reason to exceed it was GC. However, if Akka 2.2 can > sometimes drop the connections itself, this is a problem and we either have > to use the reliable proxies for everything or see if we can configure it > otherwise. Anyway, we’ll definitely look into it. > > Matei > > On Nov 1, 2013, at 1:09 PM, Imran Rashid <[email protected]> wrote: > > I downgraded spark to akka 2.1.0, and everything seems to work now. I'm > going to run my tests a few more times , but I'd really have expected to > see a failure by now w/ the 2.2.3 version. > > I'll submit a patch shortly (need to fix some compile errors in streaming > still). > > Matei -- I think I realize now that when you were talking about the > expectation of a tcp connection staying alive, you were explaining why this > is *not* a bug in the current release. You wouldn't end up in a situation > where the executor thinks it finished the task, but the driver doesn't know > about it, b/c if the connection dies, the executor wil get restarted. That > makes sense. But, it seems like if we upgrade to akka 2.2.x, a lot of > things change. I was probably wrong about seeing that problem in previous > releases -- it was just a vague recollection, which fit my current > theories, so I jumped to conclusions. > > thanks everyone > > > > On Fri, Nov 1, 2013 at 9:27 AM, Imran Rashid <[email protected]> wrote: > >> thanks everyone for all of the input. >> >> Matei: makes a lot more sense with your explanation of spark's expected >> behavior of tcp, I can see why this makes sense now. But, to show my total >> ignorance here, I'm wondering that when the connection does break, are you >> sure all of your messages that you thought you sent before the break were >> received? I'm guessing that you don't. Which is fine, if the response to >> that is to have the executor just die completely, and restart. that was >> the behavior I was initially observing with the code on the 2.10 branch, >> where the executor handles a DisassociatedEvent explicitly, and dies. >> >> But -- is that the behavior we want? do we want it to be robust to tcp >> connections breaking, without having to completely restart the executor? >> you might say that dying & restarting will lead to correct behavior, even >> if its inefficient. But sometimes, I've seen restarts so frequently that >> no progress is made. >> >> I don't see why this changed w/ the different versions of akka -- I don't >> see any relevant configuration settings that would change how "strongly" >> tcp tries to keep the connection alive, but I may be missing something. >> But it does seem like the netty configuration options have changed >> completely between the two versions: >> >> http://doc.akka.io/docs/akka/2.2.3/scala/remoting.html#Remote_Configuration >> vs >> http://doc.akka.io/docs/akka/2.0.5/scala/remoting.html >> >> btw, akka 2.1.0 also has been built for scala 2.10: >> >> http://search.maven.org/#artifactdetails|com.typesafe.akka|akka-remote_2.10|2.1.0|bundle >> and its netty configuration is closer to 2.0.5: >> http://doc.akka.io/docs/akka/2.1.0/scala/remoting.html >> >> perhaps someone more knowledge then me about netty & tcp can look through >> the changes and decide what the right changes are. >> >> Prashant said: >> >Before we conclude something about reliable messaging, I want you to for >> once consider other possibilities like >actual network reconnection and may >> be a GC pause ? Try connecting something like jconsole (or alike ) and >see >> what happens on the driver and executor. >> > >> >My doubt are since we are using standalone mode where even master and >> worker are also actors then if we see >a weird behaviour on the executor >> and driver then Why not on master and worker too ? They should also break >> >away from each other. For this reason, I am doubting our conclusions and >> may be if we narrow down the >problem first before we conclude something. >> It is a regression in akka 2.2.3 it uses more memory than it used to >be in >> 2.1.x. >> >See https://github.com/akka/akka/issues/1810 >> >> >> Well, there could easily be the same problem with dropped connections >> between master & worker -- they just communicate so little, it doesn't >> really matter. The odds that a message gets dropped between them is very >> low, only because there are barely any messages. >> >> I completely agree that the problem could be because of a contention, or >> gc pause, etc. In fact, I'm only giving spark 24 out of 32 cores available >> on each box, and 90g out of 125g memory. I've looked at gc a little with >> jstat, and I did see some gc pauses but nothing ridiculous. >> >> But, I think the question remains. Suppose it is gc pauses, etc. that >> cause the disassociation events; what do we do to fix it? How can we >> diagnose the problem, and figure out which of the configuration variables >> to tune? clearly, there *will be* long gc pauses, and the networking layer >> needs to be able to deal with them. >> >> still I understand your desire to see if that might be the cause of the >> problem in this particular case, so I will dig a little more. >> >> >> (btw, should I move this thread to the dev list now? it is getting into >> the nitty-gritty of implementation ...) >> >> On Fri, Nov 1, 2013 at 1:15 AM, Matei Zaharia <[email protected]>wrote: >> >>> Yes, so far they’ve been built on that assumption — not that Akka would >>> *guarantee* delivery in that as soon as the send() call returns you know >>> it’s delivered, but that Akka would act the same way as a TCP socket, >>> allowing you to send a stream of messages in order and hear when the >>> connection breaks. Maybe that isn’t what they want to provide, but I'd find >>> it weird, because it’s very easy to write a server with this property. >>> >>> Matei >>> >>> On Oct 31, 2013, at 9:58 PM, Sriram Ramachandrasekaran < >>> [email protected]> wrote: >>> >>> Sorry if I my understanding is wrong. May be, for this particular case >>> it might be something to do with the load/network, but, in general, are you >>> saying that, we build these communication channels(block manager >>> communication, task events communication, etc) assuming akka would take >>> care of it? I somehow feel that, it's being overly optimistic. Correct me >>> if I am wrong. >>> >>> >>> >>> On Fri, Nov 1, 2013 at 10:08 AM, Matei Zaharia >>> <[email protected]>wrote: >>> >>>> It’s true that Akka’s delivery guarantees are in general at-most-once, >>>> but if you look at the text there it says that they differ by transport. In >>>> the previous version, I’m quite sure that except maybe in very rare >>>> circumstances or cases where we had a bug, Akka’s remote layer always kept >>>> connections up between each pair of hosts. So the guarantee was that as >>>> long as you haven’t received a “disconnected” event, your messages are >>>> being delivered, though of course when you do receive that event you don’t >>>> know which messages have really made it through unless you acked them. But >>>> that didn’t matter for our use case — from our point of view an executor >>>> was either up or down. >>>> >>>> For this reason I still think it should be possible to configure Akka >>>> to do the same on 2.2. Most likely some timeouts just got lower. With large >>>> heaps you can easily get a GC pause of 60 seconds, so these timeouts should >>>> be in the minutes. >>>> >>>> If for some reason this isn’t the case, then we have a bigger problem — >>>> there are *lots* of messages beyond task-finished that need to be sent >>>> reliably, including things like block manager events (a block was added / >>>> removed on this node) and commands to tell the block manager to drop data. >>>> It would be silly to implement acks at the application level for all these. >>>> But I doubt this is the case. Prashant’s observation that the standalone >>>> cluster manager stayed up is a further sign that this might be due to GC. >>>> >>>> Matei >>>> >>>> On Oct 31, 2013, at 9:11 PM, Sriram Ramachandrasekaran < >>>> [email protected]> wrote: >>>> >>>> Hi Imran, >>>> Just to add, we've noticed dis-associations in a couple projects that >>>> we built(using akka 2.2.x not spark). We went to some details to find out >>>> what was happening. As Matei, suggested, Akka keeps the TCP connection open >>>> and uses that to talk to peers. We noticed that in our case, initially, we >>>> were seeing dis-associations generally at the end of keep-alive duration. >>>> So, when the keep-alive duration ends, at the TCP layer, a keep-alive probe >>>> gets sent to inform the peer on the other side that the connection is still >>>> alive/valid. For some reason, the probe dint renew the keep-alive >>>> connection and we saw a lot of dis-associations during that time. Later, we >>>> realized this was not a pattern either. This >>>> thread<https://groups.google.com/forum/#!msg/akka-user/RYxaPl_nby4/1USHDFIRgOkJ>contains >>>> the full history of our discussions with the Akka team. It's still >>>> open and unclear as to what was causing it for our case. >>>> We tried tweaking various settings of akka(wrt heartbeats, failure >>>> detector, even plugged-in our own failure detector with no effect). >>>> >>>> Imran - Just to clarify your point on message delivery - akka's message >>>> delivery policy is at-most-once. However, there's no guarantee for a >>>> message to be delivered to a peer. The documentation clearly explains that. >>>> http://doc.akka.io/docs/akka/2.0.2/general/message-send-semantics.html. >>>> It's >>>> the responsibility of the application developer to handle cases where >>>> message is suspected to be not have been delivered. >>>> >>>> I hope this helps. >>>> >>>> >>>> >>>> >>>> On Fri, Nov 1, 2013 at 8:35 AM, Imran Rashid <[email protected]>wrote: >>>> >>>>> >>>>> unfortunately that change wasn't the silver bullet I was hoping for. >>>>> Even with >>>>> 1) ignoring DisassociatedEvent >>>>> 2) executor uses ReliableProxy to send messages back to driver >>>>> 3) turn up akka.remote.watch-failure-detector.threshold=12 >>>>> >>>>> >>>>> there is a lot of weird behavior. First, there are a few >>>>> DisassociatedEvents, but some that are followed by AssociatedEvents, so >>>>> that seems ok. But sometimes the re-associations are immediately followed >>>>> by this: >>>>> >>>>> 13/10/31 18:51:10 INFO executor.StandaloneExecutorBackend: got >>>>> lifecycleevent: AssociationError >>>>> [akka.tcp://sparkExecutor@<executor>:41441] >>>>> -> [akka.tcp://spark@<driver>:41321]: Error [Invalid address: >>>>> akka.tcp://spark@<driver>:41321] [ >>>>> akka.remote.InvalidAssociation: Invalid address: >>>>> akka.tcp://spark@<driver>:41321 >>>>> Caused by: >>>>> akka.remote.transport.Transport$InvalidAssociationException: The remote >>>>> system has quarantined this system. No further associations to the remote >>>>> system are possible until this system is restarted. >>>>> ] >>>>> >>>>> On the driver, there are messages like: >>>>> >>>>> [INFO] [10/31/2013 18:51:07.838] >>>>> [spark-akka.actor.default-dispatcher-3] [Remoting] Address [ >>>>> akka.tcp://sparkExecutor@<executor>:46123] is now quarantined, all >>>>> messages to this address will be delivered to dead letters. >>>>> [WARN] [10/31/2013 18:51:10.845] >>>>> [spark-akka.actor.default-dispatcher-20] >>>>> [akka://spark/system/remote-watcher] >>>>> Detected unreachable: [akka.tcp://sparkExecutor@<executor>:41441] >>>>> >>>>> >>>>> and when the driver does decide that the executor has been terminated, >>>>> it removes the executor, but doesn't start another one. >>>>> >>>>> there are a ton of messages also about messages to the block manager >>>>> master ... I'm wondering if there are other parts of the system that need >>>>> to use a reliable proxy (or some sort of acknowledgement). >>>>> >>>>> I really don't think this was working properly even w/ previous >>>>> versions of spark / akka. I'm still learning about akka, but I think you >>>>> always need an ack to be confident w/ remote communicate. Perhaps the old >>>>> version of akka just had more robust defaults or something, but I bet it >>>>> could still have the same problems. Even before, I have seen the driver >>>>> thinking there were running tasks, but nothing happening on any executor >>>>> -- >>>>> it was just rare enough (and hard to reproduce) that I never bothered >>>>> looking into it more. >>>>> >>>>> I will keep digging ... >>>>> >>>>> On Thu, Oct 31, 2013 at 4:36 PM, Matei Zaharia < >>>>> [email protected]> wrote: >>>>> >>>>>> BTW the problem might be the Akka failure detector settings that seem >>>>>> new in 2.2: http://doc.akka.io/docs/akka/2.2.3/scala/remoting.html >>>>>> >>>>>> Their timeouts seem pretty aggressive by default — around 10 seconds. >>>>>> This can easily be too little if you have large garbage collections. We >>>>>> should make sure they are higher than our own node failure detection >>>>>> timeouts. >>>>>> >>>>>> Matei >>>>>> >>>>>> On Oct 31, 2013, at 1:33 PM, Imran Rashid <[email protected]> >>>>>> wrote: >>>>>> >>>>>> pretty sure I found the problem -- two problems actually. And I >>>>>> think one of them has been a general lurking problem w/ spark for a >>>>>> while. >>>>>> >>>>>> 1) we should ignore disassociation events, as you suggested >>>>>> earlier. They seem to just indicate a temporary problem, and can >>>>>> generally >>>>>> be ignored. I've found that they're regularly followed by >>>>>> AssociatedEvents, and it seems communication really works fine at that >>>>>> point. >>>>>> >>>>>> 2) Task finished messages get lost. When this message gets sent, we >>>>>> dont' know it actually gets there: >>>>>> >>>>>> >>>>>> https://github.com/apache/incubator-spark/blob/scala-2.10/core/src/main/scala/org/apache/spark/executor/StandaloneExecutorBackend.scala#L90 >>>>>> >>>>>> (this is so incredible, I feel I must be overlooking something -- but >>>>>> there is no ack somewhere else that I'm overlooking, is there??) So, >>>>>> after >>>>>> the patch, spark wasn't hanging b/c of the unhandled DisassociatedEvent. >>>>>> It hangs b/c the executor has sent some taskFinished messages that never >>>>>> get received by the driver. So the driver is waiting for some tasks to >>>>>> finish, but the executors think they are all done. >>>>>> >>>>>> I'm gonna add the reliable proxy pattern for this particular >>>>>> interaction and see if its fixes the problem >>>>>> >>>>>> http://doc.akka.io/docs/akka/2.2.3/contrib/reliable-proxy.html#introducing-the-reliable-proxy >>>>>> >>>>>> imran >>>>>> >>>>>> >>>> >>>> >>>> -- >>>> It's just about how deep your longing is! >>>> >>>> >>>> >>> >>> >>> -- >>> It's just about how deep your longing is! >>> >>> > > -- s
