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Posted to users@tomcat.apache.org by Mike Seda <mi...@lillegroup.com> on 2015/01/21 20:59:35 UTC
DisableExplicitGC
Is it advisable to have the following JVM option set in Tomcat 7 (with
Java 7)?
-XX:+DisableExplicitGC
Or will this prevent Tomcat itself from calling System.gc() when it
needs to?
Mike
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Re: DisableExplicitGC
Posted by Christopher Schultz <ch...@christopherschultz.net>.
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André,
On 1/21/15 3:13 PM, André Warnier wrote:
> Mike Seda wrote:
>> Is it advisable to have the following JVM option set in Tomcat 7
>> (with Java 7)? -XX:+DisableExplicitGC
>>
>> Or will this prevent Tomcat itself from calling System.gc() when
>> it needs to?
>>
>
> Tomcat is probably not doing that anyway (I'll let someone else
> answer that part), but there might be some monitoring/control
> application running, which could be using that.
>
>
> The reason I am thinking of this, is that I have a couple of
> installations where because of an unruly webapp leaving tons of
> connections open in CLOSE_WAIT state, I am using a jmxsh script
> regularly, to provoke such GC's to clean them up. If I set the
> option above, that script would probably fail, and the system would
> slowly get to an inoperative state.
I'm interested... if you add -XX:+DisableExplicitGC and try your
JMX-based gc-caller, does it still invoke the gc?
I know that sounds stupid, but I wonder if the JMX bean's "gc" method
just calls System.gc() or if it uses something under the covers that
will still work.
- -chris
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Re: DisableExplicitGC
Posted by André Warnier <aw...@ice-sa.com>.
Mike Seda wrote:
> Is it advisable to have the following JVM option set in Tomcat 7 (with
> Java 7)?
> -XX:+DisableExplicitGC
>
> Or will this prevent Tomcat itself from calling System.gc() when it
> needs to?
>
Tomcat is probably not doing that anyway (I'll let someone else answer that part), but
there might be some monitoring/control application running, which could be using that.
The reason I am thinking of this, is that I have a couple of installations where because
of an unruly webapp leaving tons of connections open in CLOSE_WAIT state, I am using a
jmxsh script regularly, to provoke such GC's to clean them up.
If I set the option above, that script would probably fail, and the system would slowly
get to an inoperative state.
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Re: DisableExplicitGC
Posted by Mark Thomas <ma...@apache.org>.
On 21/01/2015 19:59, Mike Seda wrote:
> Is it advisable to have the following JVM option set in Tomcat 7 (with
> Java 7)?
> -XX:+DisableExplicitGC
That should be fine. Apps really should not be calling that.
> Or will this prevent Tomcat itself from calling System.gc() when it
> needs to?
Tomcat only calls it in two places - both diagnostic tools. It will
never get called in normal operation.
Mark
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Re: DisableExplicitGC
Posted by Daniel Mikusa <dm...@pivotal.io>.
On Wed, Jan 21, 2015 at 3:51 PM, Christopher Schultz <
chris@christopherschultz.net> wrote:
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> Hash: SHA256
>
> Daniel,
>
> On 1/21/15 3:16 PM, Daniel Mikusa wrote:
> > On Wed, Jan 21, 2015 at 2:59 PM, Mike Seda
> > <mi...@lillegroup.com> wrote:
> >
> >> Is it advisable to have the following JVM option set in Tomcat 7
> >> (with Java 7)? -XX:+DisableExplicitGC
> >>
> >
> > It may or may not make a difference. Some questions like these
> > come to mind...
> >
> > - How would you feel if one of the applications you deployed
> > called System.gc()? - How about if it called it a lot? or at a
> > really inconvenient time? - How much do you trust your apps or
> > their developers? - Does one of your apps rely on that
> > functionality for some reason?
>
> Our application does not call System.gc automatically, but we have
> that ability through an admin screen where we can say "do GC now", and
> we call it. Sometimes when we have a heap that's been high for a while
> - -- say, tripping our monitoring software a lot -- we go in there and
> hit the GC button to see what happens. Usually, heap usage goes down
> to 25% of what it had been before... it's just that the JVM hadn't
> done a full GC in a while because it didn't need to.
>
I've done that too, but initiated the GC through jvisualvm.
I was wondering if DisableExplicitGC would prevent that as well, so I did a
quick test and it seemed to prevent it. Guess that's something to consider
too.
Dan
>
> >> Or will this prevent Tomcat itself from calling System.gc() when
> >> it needs to?
> >
> > I really doubt Tomcat would call that.
>
> Tomcat calls System.gc() in only one place --
> StandardHost.findReloadedContextMemoryLeaks -- which is mostly a
> debugging tool.
>
> - -chris
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> To unsubscribe, e-mail: users-unsubscribe@tomcat.apache.org
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>
>
Re: DisableExplicitGC
Posted by Christopher Schultz <ch...@christopherschultz.net>.
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Hash: SHA256
Daniel,
On 1/21/15 3:16 PM, Daniel Mikusa wrote:
> On Wed, Jan 21, 2015 at 2:59 PM, Mike Seda
> <mi...@lillegroup.com> wrote:
>
>> Is it advisable to have the following JVM option set in Tomcat 7
>> (with Java 7)? -XX:+DisableExplicitGC
>>
>
> It may or may not make a difference. Some questions like these
> come to mind...
>
> - How would you feel if one of the applications you deployed
> called System.gc()? - How about if it called it a lot? or at a
> really inconvenient time? - How much do you trust your apps or
> their developers? - Does one of your apps rely on that
> functionality for some reason?
Our application does not call System.gc automatically, but we have
that ability through an admin screen where we can say "do GC now", and
we call it. Sometimes when we have a heap that's been high for a while
- -- say, tripping our monitoring software a lot -- we go in there and
hit the GC button to see what happens. Usually, heap usage goes down
to 25% of what it had been before... it's just that the JVM hadn't
done a full GC in a while because it didn't need to.
>> Or will this prevent Tomcat itself from calling System.gc() when
>> it needs to?
>
> I really doubt Tomcat would call that.
Tomcat calls System.gc() in only one place --
StandardHost.findReloadedContextMemoryLeaks -- which is mostly a
debugging tool.
- -chris
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Re: DisableExplicitGC
Posted by Daniel Mikusa <dm...@pivotal.io>.
On Wed, Jan 21, 2015 at 2:59 PM, Mike Seda <mi...@lillegroup.com> wrote:
> Is it advisable to have the following JVM option set in Tomcat 7 (with
> Java 7)?
> -XX:+DisableExplicitGC
>
It may or may not make a difference. Some questions like these come to
mind...
- How would you feel if one of the applications you deployed called
System.gc()?
- How about if it called it a lot? or at a really inconvenient time?
- How much do you trust your apps or their developers?
- Does one of your apps rely on that functionality for some reason?
>
> Or will this prevent Tomcat itself from calling System.gc() when it
> needs to?
>
I really doubt Tomcat would call that.
Dan