AW: Questions on tomcat heap usage and garbage collection (avoiding OutOfMemoryError exceptions)

[Ralph Einfeldt <ra...@uptime-isc.de>]

Some answers:

- AFAIK the only 'memory leak' in tomcat is, that every JSP class stays
in memory
  after it is loaded.
- Find out or guess the amount of memory you need to hold all your
classes
- Find out or guess the amount of memory you need to all session data
for all 
  concurrent sessions.
  Keep in mind that the session will be alive for some time (default: 30
minutes 
  after the last action in the action). This can be a reason for tomcat
to bail 
  out, if you use the wrong tool to do the stress test, where each
request 
  creates a new session (given a load of 10 requests per second you can
have up 
  to some 20.000 concurrent sessions).
  (Tomcat needs some memory for each session too)
- The default setting can be to low if you have many JSP's and/or data
in sessions
- Try to find out if any of your software or any library that you use
has memory leaks
- If an OutOfMemoryError happens, all 'processes' of the vm die, because
  what you see are not processes, but threads that share the same
memory.

> -----Ursprüngliche Nachricht-----
> Von: Bang, Steinar [mailto:Steinar.bang@tandbergtv.com]
> Gesendet: Mittwoch, 24. Oktober 2001 09:21
> An: 'tomcat-user@jakarta.apache.org'
> Betreff: Questions on tomcat heap usage and garbage 
> collection (avoiding
> OutOfMemoryError exceptions)
> 
> 
> Platform:	Intel PIII 797.499MHz, 256MB RAM
> 		Debian Woody GNU/Linux,
> 		kernel 2.2.19
> 		Blackdown J2SDK 1.3.1
> 		apache 1.3.19
> 		tomcat 3.2.3
> 
> When using httperf[1] to stresstest JSPs on the above 
> platform, I'm running into the OutOfMemoryError 
> exception problem:
> 	<http://www.jguru.com/faq/view.jsp?EID=64889>
> 
> The solution suggested in the URL above, is to increase
> the heap size, but this is just pushing the problem
> ahead.  The important thing for us, is that tomcat doesn't die.
> 
> If this is a problem for high load sites, JSP technology
> would be unusable, and obviously it isn't.  So what
> do others do?
> 
> Possible alternatives:
>  1. increase the heap size to something astronomical,
>     and just hope for the best? 
>  2. tune apache/tomcat to restart more often, so that
>     the max heap size aren't reached before the tomcat
>     processes are restarted?
>  3. catch the exception?
>  4. use some kind of watchdog to restart tomcat?
> 
> Question 3 raises more questions:
>  3a. What do you do when you catch the exception?
>      There isn't much you _can_ do if you don't have
>      any memory left.  Maybe force a garbage collection
>      and then re-run the request?
>  3b. Why do all the tomcat processes connected to
>      apache processes die, if one of them gets an
>      OutOfMemoryError exception
> 
> Which brings us to question 2: tuning apache/tomcat:
> When reading the documentation, I thought that there
> was supposed to be a single tomcat process, serving
> all requests.  However top reports a lot of tomcat
> processes, when I'm stresstesting the server, which
> leads me to belive that there is a separate tomcat
> process for each apache server process running.
> Is this correct?
> 
> In any case; based on this assumption, I reduced
> the number of currently running apache processes
> from the default 150 to 20, with dramatically
> changed behaviour: instead of running with a 20%
> load, the machine was running at an 80%-90% load
> with less physical memory used (restarting processes
> is a lot of work).
> 
> However, even with the high load, the end result
> was a lot more stable.  Watching the tomcat processes
> in top, I could watch them grow up to around 82MB,
> before there suddenly was a lot of processes with
> around 19MB.
> 
> It wasn't completely stable though.  I set up
> httperf to do 1 million requests, before I left
> last night, and it only got to 51927 before tomcat
> died.
> 
> I run httperf from a different machine (PIII 800MHz),
> using the command:
> 	httperf --server=no-video2 --port=80 --uri=/tps-sba/Welcome.jsp
> --rate 150 --num-conns=1000000 --num-calls=1
> (--num-conns varies.  I have mostly been using 
> 10000).
> 
> 
> The memory usage observed in top, also raises some
> questions related to my question 1:
>  1a. Why does tomcat processes always start at around
>      19MB, no matter what I've set the initial heap
>      size to be?
>  1b. Why does the initial heap size seem to matter
>      for my stability if the processes always start
>      at 19MB?  Is it all in my head?
> 
> I'm interested in all direct experiences, ideas,
> guesses, answers, etc.
> 
> Thanx!
> 
> 
> - Steinar
> 
> 
> [1] <http://freshmeat.net/projects/httperf>
> 
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