When is GC invoked?

[David Jenkins]

I have a very small application designed so that the main thread spawns
3 additional threads. All 4 threads then wait for an event from an
Oracle database (message posted on an AQ queue), and are blocked most of
the time. The main thread may be blocked for weeks or months. The 3
spawned threads are much more active, with data to process, objects
created and freed, hourly or more often.

I've found that the application displays symptoms of a memory leak,
accumulating memory without ever releasing it. However, I can expire
the 3 spawned threads through the main thread, and when I do, memory is
immediately garbage collected.

My question is: when is the Java garbage collector invoked, and on which
thread? I've found tons of documentation on what the GC does, but so
far nothing on when it runs. My guess is, based on observing this
application, that the GC runs as part of the main thread, and that if
the main thread is blocked, GC will not occur. However, my boss refuses
to accept this explanation, but I'm sure he'd believe any one of the
subscribers to this forum.

Any suggestions would be greatly appreciated.

David Jenkins

[Harish]

GC runs in a seperate thread, and in sme implementations there are more than
one thread.
here is an artcile on GC..http://java.sun.com/docs/hotspot/gc/

"David Jenkins" <dj********@ade lphia.net> wrote in message
news:KI******** ************@ad elphia.com...

I have a very small application designed so that the main thread spawns 3
additional threads. All 4 threads then wait for an event from an Oracle
database (message posted on an AQ queue), and are blocked most of the time.
The main thread may be blocked for weeks or months. The 3 spawned threads
are much more active, with data to process, objects created and freed,
hourly or more often.

I've found that the application displays symptoms of a memory leak,
accumulating memory without ever releasing it. However, I can expire the
3 spawned threads through the main thread, and when I do, memory is
immediately garbage collected.

My question is: when is the Java garbage collector invoked, and on which
thread? I've found tons of documentation on what the GC does, but so far
nothing on when it runs. My guess is, based on observing this
application, that the GC runs as part of the main thread, and that if the
main thread is blocked, GC will not occur. However, my boss refuses to
accept this explanation, but I'm sure he'd believe any one of the
subscribers to this forum.

Any suggestions would be greatly appreciated.

David Jenkins

[David Jenkins]

Harish wrote:

GC runs in a seperate thread, and in sme implementations there are more than
one thread.
here is an artcile on GC..http://java.sun.com/docs/hotspot/gc/

"David Jenkins" <dj********@ade lphia.net> wrote in message
news:KI******** ************@ad elphia.com...

Thanks, this helps. I'll have to read it more carefully, but I'm still
wondering: if the GC runs on a separate thread, how does this thread
know when to perform GC? Is it perhaps event-driven, invoked whenever
an object is released? Or when one of the object spaces fills? In
either case, how is it signaled? Sorry to belabor the issue, I just
haven't found any document that describes this part of the process.

[Harish]

not sure about the excat way it is done. but this is how i would see it:
the GC thread(s) gets signaled by the JVM. it doesnot get invoked for every
object release.
its not preiodical either. GC AFAIK is done on a demand basis.
The thread(s) is signaled using synhronized objects.(using Object.notify)
And I guess its triggered when one of the memory space gets full.

"David Jenkins" <dj********@ade lphia.net> wrote in message
news:fs******** ************@ad elphia.com...

Harish wrote:

GC runs in a seperate thread, and in sme implementations there are more
than one thread.
here is an artcile on GC..http://java.sun.com/docs/hotspot/gc/

"David Jenkins" <dj********@ade lphia.net> wrote in message
news:KI******** ************@ad elphia.com...

Thanks, this helps. I'll have to read it more carefully, but I'm still
wondering: if the GC runs on a separate thread, how does this thread know
when to perform GC? Is it perhaps event-driven, invoked whenever an
object is released? Or when one of the object spaces fills? In either
case, how is it signaled? Sorry to belabor the issue, I just haven't
found any document that describes this part of the process.

[Casey Hawthorne]

"Many garbage collection algorithms suspend all threads before they
run. This ensures that when the garbage collector runs, it has
complete access to the memory in the heap and can perform its tasks
safely without the threat of being preempted by another thread. When
the garbage collector is done with its work, all threads that it
suspended are resumed."
page 23 "Practical Java" Peter Haggar 2000
I'm wondering if the blocked main thread (or one or all three of its
child threads) cannot be suspended by the GC for some reason while its
three child threads are active?

--
Regards,
Casey

[Joseph Dionne]

Casey Hawthorne wrote:

"Many garbage collection algorithms suspend all threads before they
run. This ensures that when the garbage collector runs, it has
complete access to the memory in the heap and can perform its tasks
safely without the threat of being preempted by another thread. When
the garbage collector is done with its work, all threads that it
suspended are resumed."
page 23 "Practical Java" Peter Haggar 2000
I'm wondering if the blocked main thread (or one or all three of its
child threads) cannot be suspended by the GC for some reason while its
three child threads are active?

--
Regards,
Casey

Knowing little of what your app does, my comments may not help.

Java GC is great, but you can defeat the GC, and have memory leaks in
Java. If you are using some Collection class to contain your objects
and then passing references to other classes that have Collections too,
your objects may may still have valid references, and will not be
garbage collected.

joseph