chang, if the problem is on client startup, then it isn't the heartbeat stamped, it is session establishment. the heartbeats are very light weight, so i can't imagine them causing any issues.
the two key issues we need to know are: 1) the version of the server you are running, and 2) if you are using a dedicated device for the transaction log. ben 2011/4/14 Patrick Hunt <ph...@apache.org>: > 2011/4/14 Chang Song <tru64...@me.com>: >>> 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? >>> >> >> Again, no I/O at all. 0% >> > > This is simply not possible. > > Sessions are persistent. Each time a session is created, and each time > it is closed, a transaction is written by the zk server to the data > directory. Additionally log4j based logs are also being streamed to > the disk. Each of these activities will cause disk IO that will show > up on iostat. > >> Patrick. They are not continuously login/logout. >> Maybe a couple of times a week. and before they push new feature. >> When this happens, clients in group A drops out of clusters, which causes >> problem to other unrelated services. >> > > Ok, good to know. > >> >> It is not about use case, because ZK clients simply tried to connect to >> ZK ensemble. No use case applies. Just many clients login at the >> same time or expires at the same time or close session at the same time. >> > > As I mentioned, I've seen cluster sizes of 10,000 clients (10x what > you report) that didn't have this issue. While bugs might be lurking, > I've also worked with many teams deploying clusters (probably close to > 100 by now), some of which had problems, the suggestions I'm making to > you are based on that experience. > >> Heartbeats should be handled in an isolated queue and a >> dedicated thread. I don't think we need strict ordering keeping >> of heartbeats, do we? > > ZK is purposely architected this way, it is not a mistake/bug. It is a > falicy for a highly available service to respond quickly to a > heartbeat when it cannot service regular requests in a timely fashion. > This is one of the main reasons why heartbeats are handled in this > way. > > Patrick > >>> 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 <tru64...@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 >>>>>> >>>>>> >>>>>> >>>> >>>> >> >> >
