Hey Prashant, do messages still get lost while we’re dissociated? Or can you set the timeouts high enough to proven that?
Matei On Nov 13, 2013, at 12:39 AM, Prashant Sharma <[email protected]> wrote: > We may no longer need to track disassociation and IMHO use the *improved* > feature in akka 2.2.x called remote death watch. Which lets us acknowledge a > remote death both in case of a natural demise and accidental deaths. This was > not the case with remote death watch in previous akka releases. Please take a > closer look at the patch at > https://github.com/apache/incubator-spark/pull/163 and let us know. > > This patch does not make disassociation disappear, they are added to akka as > such but gives us sufficient knobs to tune things as to when they occur. > Don't forget to tune those extra properties apart from other timeouts. > > > > > > On Sat, Nov 2, 2013 at 11:08 AM, Matei Zaharia <[email protected]> > wrote: > Prashant, the problem seems to be that messages sent while we’re > disassociated are lost. I think we’d have to just prevent disassociation > altogether, or replace all remote actor refs with the reliable proxies (which > sounds painful). > > Matei > > On Nov 1, 2013, at 7:53 PM, Prashant Sharma <[email protected]> wrote: > >> 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 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 > > > > > -- > s
