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From Joey Echeverria <joe...@gmail.com>
Subject Re: Recommended session timeout
Date Tue, 24 Feb 2009 20:30:01 GMT
Thanks for all the info. I'll have to run the tests tomorrow as I
don't have access to my cluster right now.

One piece of clarification, the data loading isn't data being loaded
into zk. The data is being loaded into the system we're developing
which uses zk to support distributed operations. I mentioned the data
loading as it increased the burden on the network.

I did run some tests with a longer sessionTimeout (60s) and I didn't
see any disconnects.

I'll let you know how my testing goes tomorrow.



On Tue, Feb 24, 2009 at 6:14 PM, Patrick Hunt <phunt@apache.org> wrote:
> Joey Echeverria wrote:
>> Thanks for the link to the documentation. I've been running tests with
>> a 5 second session timeout and disconnect events appear frequent. The
>> network they're operating on is generally quite, but the disconnects
>> to correlate with an increase in activity (e.g. loading data into the
>> system).
> As an experiment try increasing the timeout to say 30 seconds and re-run
> your tests. Any change?
> "loading data" - could you explain a bit more about what you mean by this?
> If you are able to provide enough information for us to replicate we could
> try it out (also provide info on your ensemble configuration as Mahadev
> suggested)
>> Does this seem normal to you or does it imply a potential
>> configuration problem on my network?
> Not enough info at this time to speculate. Can you provide the configs for
> at least 1 server in the ensemble (I'm assuming they are all pretty much the
> same)
>> On a related topic, I was reading the 3.1 client source code,
>> particularly the reconnect source, and noticed that the client sleeps
>> for up to 1 second before trying to reconnect. This seems excessive
>> and with a 5 second session timeout leads to more frequent session
>> expirations. Almost every time it sleeps for more than about 800 ms, a
>> disconnect is followed by an expiration.
> You are referring to startConnect in SendThread?
> We randomly sleep up to 1 second to ensure that the clients don't all storm
> the server(s) after a bounce.
> I suspect that the following is happening:
> Your client(s) is sending information to the server, the server has 1 or
> more outstanding requests from the client. You mentioned "loading data", at
> some point the server flushes the data to disk, it could be that this flush
> takes a significant amount of time. As there is communication btw the server
> and client (client sent a request, server is responding) there will be no
> heatbeating server->client going on while the request is outstanding (the
> server will not send a heartbeat because of the request being in progress).
> As a result the client doesn't see a response for a potentially long period
> of time (because of the flush).
> Try using the stat & srst commands detailed here:
> http://hadoop.apache.org/zookeeper/docs/current/zookeeperAdmin.html#sc_zkCommands
> 1) configure your test client to connect to 1 server in the ensemble
> 2) run the srst command on that server
> 3) run your client test
> 4) run the stat command on that server
> 5) if the test takes some time, run the stat a few times during the test
>  to get more data points
> The stat command will give you min/avg/max latency for requests to the
> server. If max latency goes above your timeout then you will see the
> disconnect on the client. This indicates that the server is probably to
> blame (vs say networking issues, which we see alot). Let us know the results
> of this test.
> Btw, latency can be effected by a number of factors:
> http://hadoop.apache.org/zookeeper/docs/current/zookeeperAdmin.html#sc_commonProblems
> 1) make sure the server JDK is not swapping (etc...) as this will kill
> latency
> 2) is the host(s) running the ZK server on dedicated devices (cpu/mem/disk)
> or sharing resources with other applications?
> 3) are you using a dedicated transaction log device (drive)? This is
> critical for low-latency & high throughput of the ensemble.
> Patrick
>> Is this a bug, or desirable behavior?
>> Thanks,
>> -Joey
>> On Mon, Feb 23, 2009 at 10:37 PM, Patrick Hunt <phunt@apache.org> wrote:
>>> The latest docs (3.1.0 has some updates to that section) can be found
>>> here:
>>> http://hadoop.apache.org/zookeeper/docs/r3.1.0/zookeeperProgrammers.html#ch_zkSessions
>>> Patrick
>>> Mahadev Konar wrote:
>>>> Hi Joey,
>>>>  here is a link to information on session timeouts.
>>>> http://hadoop.apache.org/zookeeper/docs/r3.0.1/zookeeperProgrammers.html#ch_
>>>> zkSessions
>>>>  The session timeouts depends on how sensitive you want your application
>>>> to
>>>> be. A very low session timeout like (1-2 seconds) might lead to your
>>>> application being very sensitive to events like minor network problems
>>>> etc.,
>>>> a higher values of say (30 seconds) on the other hand might lead to slow
>>>> detection of client failures -- example one of the zookeeper client
>>>> which
>>>> has ephemeral node goes down, in this case the ephemeral nodes will only
>>>> go
>>>> away after session timeout.
>>>> I have seen some users using 10-15 seconds of session timeout, but you
>>>> should use as per your application requirements.
>>>> Hope this helps.
>>>> mahadev
>>>> On 2/22/09 3:09 AM, "Joey Echeverria" <joey42@gmail.com> wrote:
>>>>> Is there a recommended session timeout? Does it change based on the
>>>>> ensemble size?
>>>>> Thanks,
>>>>> -Joey

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