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From Chang Song <tru64...@me.com>
Subject Re: Serious problem processing hearbeat on login stampede
Date Thu, 14 Apr 2011 14:03:43 GMT

2011. 4. 14., 오후 1:53, Patrick Hunt 작성:

> two additional thoughts come to mind:
> 
> 1) try running the ensemble with a single zk server, does this help at
> all? (it might provide a short term workaround, it also might provide
> some insight into what's causing the issue)


We are going to try this to see if we identify a culprit.

Thanks.



> 2) can you hold off some of the clients from the stampede? Perhaps add
> a random holdoff to each of the clients before connecting,
> additionally a similar random holdoff from closing the session. this
> seems like a straightforward change from your client side (easy to
> implement/try) but hard to tell given we don't have much insight into
> what your use case is.
> 
> 
> Anyone else in the community have any ideas?
> 
> 
> Patrick
> 
> 2011/4/13 Patrick Hunt <phunt@apache.org>:
>> 2011/4/13 Chang Song <tru64ufs@me.com>:
>>> 
>>> Patrick.
>>> Thank you for the reply.
>>> 
>>> We are very aware of all the things you mentioned below.
>>> None of those.
>>> 
>>> Not GC (we monitor every possible resource in JVM and system)
>>> No IO. No Swapping.
>>> No VM guest OS. No logging.
>>> 
>> 
>> Hm. ok, a few more ideas then:
>> 
>> 1) what is the connectivity like btw the servers?
>> 
>> What is the ping time btw them?
>> 
>> Is the system perhaps loading down the network during this test,
>> causing network latency to increase? Are all the nic cards (server and
>> client) configured correctly? I've seen a number of cases where
>> clients and/or server had incorrectly configured nics (ethtool
>> reported 10 MB/sec half duplex for what should be 1gigeth)
>> 
>> 2) regarding IO, if you run 'iostat -x 2' on the zk servers while your
>> issue is happening, what's the %util of the disk? what's the iowait
>> look like?
>> 
>> 3) create a JIRA and upload your 3 server configuration files. Include
>> the log4j.properties file you are using and any other details you
>> think might be useful. If you can upload a log file from when you see
>> this issue that would be useful. Upload any log file if you can't get
>> it from the time when you see the issue.
>> 
>>> 
>>> Oh, one thing I should mention is that it is not 1000 clients,
>>> 1000 login/logout per second. All operations like closeSession,
>>> ping takes more than 8 seconds (peak).
>>> 
>> 
>> Are you continuously logging in and the logging out, 1000 times per
>> second? That's not a good use case for ZK sessions in general. Perhaps
>> if you describe your use case in more detail it would help.
>> 
>> Patrick
>> 
>>> It's about CommitProcessor thread queueing (in leader).
>>> QueuedRequests goes up to 800, so does commitedRequests and
>>> PendingRequestElapsedTime. PendingRequestElapsedTime
>>> goes up to 8.8 seconds during this flood.
>>> 
>>> To exactly reproduce this scenario, easiest way is to
>>> 
>>> - suspend All JVM client with debugger
>>> - Cause all client JVM OOME to create heap dump
>>> 
>>> in group B. All clients in group A will not be able to receive
>>> ping response in 5 seconds.
>>> 
>>> We need to fix this as soon as possible.
>>> What we do as a workaround is to raise sessionTimeout to 40 sec.
>>> At least clients in Group A survives. But this increases
>>> our cluster failover time significantly.
>>> 
>>> Thank you, Patrick.
>>> 
>>> 
>>> ps. We actually push ping request to FinalRequestProcessor as soon
>>>     as the packet identifies itself as ping. No dice.
>>> 
>>> 
>>> 
>>> 2011. 4. 14., 오전 12:21, Patrick Hunt 작성:
>>> 
>>>> Hi Chang, it sounds like you may have an issue with your cluster
>>>> environment/setup, or perhaps a resource (GC/mem) issue. Have you
>>>> looked through the troubleshooting guide?
>>>> https://cwiki.apache.org/confluence/display/ZOOKEEPER/Troubleshooting
>>>> 
>>>> In particular 1000 clients connecting should be fine, I've personally
>>>> seen clusters of 7-10 thousand clients. Keep in mind that each session
>>>> establishment is essentially a write (so the quorum in involved) and
>>>> what we typically see there is that the cluster configuration has
>>>> issues. 14 seconds for a ping response is huge and indicates one of
>>>> the following may be an underlying cause:
>>>> 
>>>> 1) are you running in a virtualized environment?
>>>> 2) are you co-locating other services on the same host(s) that make up
>>>> the ZK serving cluster?
>>>> 3) have you followed the admin guide's "things to avoid"?
>>>> http://zookeeper.apache.org/doc/r3.3.3/zookeeperAdmin.html#sc_commonProblems
>>>> In particular ensuring that you are not swapping or going into gc
>>>> pause (both on the server and the client)
>>>> a) try turning on GC logging and ensure that you are not going into GC
>>>> pause, see the troubleshooting guide, this is the most common cause of
>>>> high latency for the clients
>>>> b) ensure that you are not swapping
>>>> c) ensure that other processes are not causing log writing
>>>> (transactional logging) to be slow.
>>>> 
>>>> Patrick
>>>> 
>>>> On Wed, Apr 13, 2011 at 6:35 AM, Chang Song <tru64ufs@me.com> wrote:
>>>>> Hello, folks.
>>>>> 
>>>>> We have ran into a very serious issue with Zookeeper.
>>>>> Here's a brief scenario.
>>>>> 
>>>>> We have some Zookeeper clients with session timeout of 15 sec (thus 5
sec ping), let's called
>>>>> these clients, group A.
>>>>> 
>>>>> Now 1000 new clients (let's call these, group B) starts up at the same
time trying to
>>>>> connect to a three-node ZK ensemble, creating ZK createSession stampede.
>>>>> 
>>>>> Now almost all clients in group A is not able to exchange ping within
session expire time (15 sec).
>>>>> Thus clients in group A drops out of the cluster.
>>>>> 
>>>>> We have looked into this issue a bit, found mostly synchronous nature
of session queue processing.
>>>>> Latency between ping request and response ranges from 10ms up to 14 seconds
during this login stampede.
>>>>> 
>>>>> Since session timeout is serious matter for our cluster, thus ping should
be done in psuedo realtime fashion.
>>>>> 
>>>>> I don't know exactly how these ping timeout policy in clients and server,
but failure to receive ping
>>>>> response in clients due to zookeeper login session seem very nonsense
to me.
>>>>> 
>>>>> Shouldn't we have a separate ping/heartbeat queue and thread?
>>>>> Or even multiple ping queues/threads to keep realtime heartbeat?
>>>>> 
>>>>> THis is very serious issue with Zookeeper for our mission-critical system.
Could anyone
>>>>> look into this?
>>>>> 
>>>>> I will try to file a bug.
>>>>> 
>>>>> Thank you.
>>>>> 
>>>>> Chang
>>>>> 
>>>>> 
>>>>> 
>>> 
>>> 
>> 


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