Re: Session timeout and memory
Re: Session timeout and memory
- Subject: Re: Session timeout and memory
- From: Owen McKerrow <email@hidden>
- Date: Wed, 20 Sep 2006 11:17:38 +1000
Hi All,
Interesting thread ! And its very similar to a problem Im having.
We are deploying our App under Tomcat and after several days of
running we get and out of memory exception. so we increased the heap
size. We got a couple more days out of the app. So we increased it
again, this time we last 2 weeks, but once again we have run out of
memory.
This leads to a couple of questions :
1) Does anyone know if there is a memory handling issue of WOApps
under Tomcat ? i.e. it never lets the memory go.
2) When we deploy under JavaMonitor we set our apps to restart
themselves regularly. Why's this ? I had thought it may have been a
carry over from Object-C days just to make sure we clean up any
memory leaks. But under Java, one would image it should be all
garbage collected, so why do we still recommend nightly/weekly
restarts ?
3) Can someone point me at some good doc for learning about memory
profiling of Java app's. Specifically over a long period of time.
Even to figure out how much ram each session is taking and when that
is being garbage collected as per Tom's example below.
Owen McKerrow
WebMaster, emlab
Ph : +61 02 4221 5517
http://emlab.uow.edu.au
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'The test of a first-rate intelligence is the ability to hold two
opposed ideas in the mind at the same time and still be able to
function.'
-F.Scott Fitzgerald,
On 19/09/2006, at 6:29 PM, Tom M. Blenko wrote:
What you are seeing is almost certainly just the way garbage
collection works. When the session times out, the objects it
allocated are available for garbage collection. They are not
collected at that time, just made available for collection. The
garbage collector won't run until it needs to.
The latter is not correct. The garbage collector on HotSpot runs
frequently and a common occasion is when a response has been
returned by a WO application. It appears that it collects on these
occasions against the local store. This makes sense in the
immediate context, you'd probably have to test it against you own
applications to determine whether it makes sense on balance. (I
don't believe you can control this behavior in any case).
The garbage collector doesn't collect all available memory on these
occasions, however. I had a disagreement with someone once about
whether the garbage collector would collect a cycle, which it isn't
difficult to create, e.g., off of a session. We put up a minimal
application on a quiet machine, instrumented the VM, set the
session to timeout after 1 minute, requested one page (which
created the cycle), and waited. The garbage collector ran many
times but it took 10 minutes before the cycle was collected.
Behavior of the garbage collector isn't an issue most of the time.
If the working set is close to the VM memory size, however, it can
cause the VM to thrash and the solution is (usually) to increase
the size of VM memory. It can also happen that you know when a
sizable chunk of memory is being freed and forcing the gc to run
amounts to an optimization, e.g., to reduce paging or total memory
use. You can call gc() to advise the garbage collector to run. You
can call gc() three times in succession to advise it more strongly
to run. Running, in this circumstance, will only cause it to
collect what it chooses, it won't necessarily collect all the
memory available.
Tom
Chuck
On Sep 11, 2006, at 11:18 PM, Ute Hoffmann wrote:
Ok, perhaps I'm looking at the wrong thing. This is the piece of
code which I use to determine the used memory:
Runtime runtime = Runtime.getRuntime();
double nochVoranden = new Double(runtime.totalMemory () -
runtime.freeMemory ()).doubleValue();
So this happens to the logs:
The memory goes up inside of the session. It sometimes drops a
bit but it mainly goes up. So there seems to be a memory leak
somewhere...
Ok, but then the session is terminated or times out. So I would
assume that then the memory is returned as free memory.
So the next session should again show used memory lets say 6 MB.
But when the previous session had a used memory of lets say 50 MB
the next session (created well after the first session is timed
out or terminated) will show 50MB + of used memory.
And yes: There was only one session present in the testcase and
it clearly went up after the first session was timed out without
releasing this used memory.
Do I have to schedule the instances to get rid of this (as it
happens by design) or does it point me to a memory leak in my
code which means whatever holds the memory cannot be released
even after the session is terminated.
Regards
Ute
Anfang der weitergeleiteten E-Mail:
Von: Q <email@hidden>
Datum: Di, 12. Sep 2006 01:43:35 Europe/Berlin
An: Chuck Hill <email@hidden>
Cc: Ute Hoffmann <email@hidden>, webobjects-
email@hidden
Betreff: Re: Session timeout and memory
On 12/09/2006, at 2:26 AM, Chuck Hill wrote:
In addition to garbage collection which has been mentioned, be
careful of what you are looking at. If additional memory is
allocated to the JVM process, it will not be returned to the
operating system until the JVM process terminates. In other
words, the total heap size will never decrease but free memory
on the heap will vary as objects are created and the garbage
collector runs.
Chuck
It should also be pointed out that this is a good thing not a
liability that needs special consideration, because it means
that the JVM doesn't need to constantly malloc and free memory.
Provided there isn't an actual leak causing the JVM size to
continually grow unexpectedly, the virtual memory allocation
shouldn't be something to really worry about, as any vm pages
that go unused for long enough will eventually get paged out by
the OS should the physical memory be required for something
else. Assuming your OS has got enough swap allocated.
-- Seeya...Q
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