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From Benjamin Reed <br...@yahoo-inc.com>
Subject RE: Recommended session timeout
Date Thu, 26 Feb 2009 22:28:08 GMT
just a quick sanity check. are you sure your memory is not overcommitted? in other words you
aren't swapping. since the gc does a bunch of random memory accesses if you swap at all things
will go very slow.

From: Joey Echeverria [joey42@gmail.com]
Sent: Thursday, February 26, 2009 1:31 PM
To: zookeeper-user@hadoop.apache.org
Subject: Re: Recommended session timeout

I've answered the questions you asked previously below, but I thought
I would open with the actual culprit now that we found it. When I said
loading data before, what I was talking about was sending data via
Thrift to the machine that was getting disconnected from zookeeper.
This turned out to be the problem. Too much data was being sent in
short span of time and this caused memory pressure on the heap. This
increased the fraction of the time that the GC had to run to keep up.
During a 143 second test, the GC was running for 33 seconds.

We found this by running tcpdump on both the machine running the
ensemble server and the machine connecting to zookeeper as a client.
We deduced it wasn't a network (lost packet) issue, as we never saw
unmatched packets in our tests. What did see were "long" 2-7 second
pauses with no packets being sent. We first attempted to up the
priority of the zookeeper threads to see if that would help. When it
didn't, we started monitoring the GC time. We don't have a work around
yet, other than sending data in smaller batches and  using a longer

Thanks for all your help!


> As an experiment try increasing the timeout to say 30 seconds and re-run
> your tests. Any change?

30 seconds and higher works fine.

> "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)

The ensemble config file looks as follows:



> 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.

That makes some sense, but it might be worth tweaking that parameter
based on sessionTimeout since 1 second can easily be 10-20% of

> 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 problem doesn't appear to be on the server end as max latency
never went above 5ms. Also, no messages are shown as queued.

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