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From Christopher Schultz <>
Subject Re: Tomcat: client disconnects?
Date Thu, 15 Jan 2009 16:20:53 GMT
Hash: SHA1


pichels wrote:
> We are experiencing users having issues connecting to our RH server and
> getting disconnected with errors in their browsers that state there is a DNS
> error or page cannot be displayed.

Is this a browser error message, or something coming from the Tomcat side?

> Users connect to Apache then Tomcat serves up our JSP's that sometimes
> accesses specific MySQL server DB's.

Are you using a connection pool? If so, what is the configuration? If
not, how are you connecting to the database (and is there a limit on the
number of db connections you allow)?

> Most of the pages seems to be index.jsp or index.php lately for both
> websites(These are websites internal - our company web servers).

MediaWiki uses index.php as it's front-controller, so it's no surprise
that you'd be getting many errors with that page in particular (because
requests always go through that page). Is index.jsp similar to index.php
in that it handles a great percentage of your servlet-server traffic?

> In Tomcat: it is a mess of exceptions that our developers try and keep
> cleaned up. ( We do not use Log4j)

Would you care to post some of those? What are the most popular
exceptions? What seem to be the worst-sounding?

> In Mysql: there are lots of slow queries I've been logging - > 1 sec.

Are these mostly coming from your webapp or MediaWiki? Have you
investigated these? Often, slow queries arise from inadequate or stale
indexes. Try running REPAIR TABLE [tablename] on a table that sees a lot
of slow queries. That should update the indexes in case you recently
bulk-loaded a lot of data. If that's not the problem (and it's unlikely
that it /is/ the problem), you should start running EXPLAIN on some of
those queries to see if you have appropriate indexes on your tables.

> The errors that occur are random browser disconnects - urls that end or
> start with *.jsp/ or *.php/ pages.

Something like SocketException: Connection closed? If that's the case,
the exceptions themselves are not a problem, but could be indicative of
a larger problem: poor performance causes people to hit STOP on their
browsers and re-try requests.

> We have also tried using WinXP host files and using an IP address to bypass
> DNS altogether - no luck.

Are you actually performing DNS lookups?

Two things might be happening if DNS is really your problem:

1. Your DNS server is slow. Mitigations are a) get a
   faster/closer/extra DNS server or b) disable DNS lookups

2. Your DNS settings are misconfigured (e.g. wrong ip address,
   etc.). Check your DNS configuration.

> Problem around 4:15PM - Jan 9, 2009:

What was the problem?

> Problem around 5:05PM - Jan 9, 2009:
> http://wsi/Qoe/index.jsp 
> (same thing....Page Cannot Be Displayed)  

Again... what was the problem? Look in catalina.out or some other log
file in your Tomcat's logs directory.

> Our MysqL server has a tendancy to ramp up or the load avg increases using
> massive CPU cycles - I restart Tomcat and the system usually calms down -
> Mysql included.

This is why I was asking about connection pooling: it looks like Tomcat
is murdering your DB. This could be due to too many connections to
MySQL, overly high query volume, or sloppy code in your application
leaking database resources.

> We are currently using tcpdump on the server & Wireshark on the affected
> clients to look at the packet level of my connections to the webserver.

Have you observed anything interesting using these tools?

> I know more details are probably needed - this is a complex problem.
> And only seems to happen in the am right when users login to their machines
> and start up their browsers and then happens at the end of the day as well.

Maybe you are just getting more peak traffic than your setup can handle.
 Have you performed any formal load testing?

- -chris
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