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From André Warnier ...@ice-sa.com>
Subject Re: Tomcat stops responding
Date Wed, 22 Dec 2010 14:17:27 GMT
Hi.

K J.Sreekumar wrote:
> Hello Andre
> 
> 
>>>>  TCP    0.0.0.0:8080           0.0.0.0:0              LISTENING
>>>> 5356
>>>>
>>>>  [tomcat6.exe]
>>>>
>>>>
>>> Apart from the above and the other ports in LISTEN state, when "tomcat
>>> freezes", do you have any other ports in the netstat listing, shown as
>>> "CLOSE_WAIT" for example ?
>>> If yes, how many ?
>>>
> These are the ports in the close wait state -

Ok, there do not seem to be a whole bunch of them, which is good.
One some systems, with badly-behaved applications, I have seen hundreds of those, to the 
point of rendering the system totally incapable of accepting new TCP connections of any 
kind.  But that does not appear to be the case here.

> 
>   TCP    127.0.0.1:3450         127.0.0.1:8080         CLOSE_WAIT      1836
> 
>   [httpd.exe]

This is one of the connections between the Apache front-end proxy_http module, and the 
back-end tomcat port 8080 HTTP Connector.
The fact that it is a "CLOSE_WAIT" state indicates that one of the sides has closed its 
connection, but the other has not yet.  It is a normal TCP state, if it remains moderate 
and does not last too long.

One source of problems - as I believe Mark pointed out recently, maybe in another thread -

is when there is a mismatch between the number of connections which the front-end is 
trying to make to Tomcat, and the number of threads available in Tomcat to handle them.

If there are many more client requests than Tomcat threads available to serve them, then a

lot of them will end up in the accept queue of the back-end Tomcat, waiting for a tomcat 
thread to become available to serve them.  At some point, this queue reaches its maximum 
size, and then further requests are being rejected.

There are a whole bunch of parameters allowing you to control this at the httpd level, see

: http://httpd.apache.org/docs/2.2/mod/mod_proxy.html#proxypass
and at the Tomcat level, the Connector attribute "maxThreads".
The defaults are usually fine however, so I would not start experimenting with then until

you know what the problem really is.

...

> 
>   TCP    192.168.103.117:1790   184.84.255.35:80 <http://184.84.255.35/>
> CLOSE_WAIT      6008
> 
>   [jucheck.exe]
> 

As far as i know, the above is the "java update scheduler" service.
Nothing to do with the current problem, but I generally dislike this kind of thing on a 
server, and turn them off.  They use up resources, and ports which you later always wonder

about.  Plus, I don't want any server of mine to decide to update himself, or even pop up

annoying dialogs all the time.
A matter of preference.

> 
> 
> 
>>> Then, the next time Tomcat appears to freeze, try with a browser to access
>> the "freeze" page, as :
>> http://(hostname)/freeze/freeze.html
>> and as
>> http://(hostname):8080/freeze/freeze.html
>>
>> and let's see what happens.
> 
> 
> We have another test application running on tomcat, which also fails to
> respond once tomcat starts ignoring requests; so is the case with tomcat
> manager too.
> 

Right.  What I was trying to do, is to have some application as simple as possible, and 
totally independent of your own webapps.  A simple html page will be served by the Tomcat

embedded "default servlet", using only Tomcat code.
If that one blocks too, then you would know that it has nothing to do with application 
code, extra libraries etc..
Well, not quite, as tomcat could still be blocked by your own apps.
But if the rest blocks, and this does not, then it would be a clear sign that the block is

in your applications.
It is equivalent to the telnet test done before, just a bit easier to use.


> We are trying to replicate this behavior again by directly running tomcat on
> 80; will be posting the observations here.
> 
That's a good idea, to eliminate the front-end http and the proxy connector's impact.

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