Garbage Collector question?

[yogesh@xxxxxxxxx (Yogesh Pandeya)]

Hi Shrihari,
When you start a VM, it gets some initial memory from the system. It =
differs from VM to VM and version to version. That is the default size =
of the VM's memory. You can change the initial memory using -Xms option =
while starting the VM.
Now whenever you do a "new", it is a *potential* trigger for gc, but not =
always neccessary. when the VM starts running out of memory because of =
object creation, it tries to first collect some garbage and allocate =
memory to objects. If it still requires more memory, it gets it from the =
system(underlying OS) and runs with higher memory. this newly acquired =
memory is not returned back to the system till VM runs. You can check =
this by using runtime total memory and free memory. You will find the =
total memory increasing as you create more objects without assigning it =
to null.=20

The idea behind object pool is that if you know that you are going to =
create large number of objects again and again for some processing, you =
create a pool (or bucket) of objects at the time of initialization. =
during run time you take the objects from the bucket, use them for your =
purpose, and return them to the bucket. For this you have a pool manager =
which is initialized in the begining and you request the manager to =
create objects (you will need down the line during run time) during =
initialization. During run time you request those objects from the =
manager, initialize them as needed, and return them to the manager after =
use. This saves a lot of you time for object creation. The objects =
created in the pool are not temporary. They satay with you for the =
application lifetime unless you have methods in the manager to =
specifically destroy them.(setting them to null)

Hope this helps you to understand things.
Cheers,
Yogesh
-----Original Message-----
From:	Shrihari.S.C [SMTP:sriharis@xxxxxxxxxxxxxxxxxxxx]
Sent:	Tuesday, November 30, 1999 4:06 PM
Cc:	Java Mail List
Subject:	RE: Garbage Collector question?

Hi Yogesh,
	Could u be more clear on how JVM gets more memory if the gc does n't =
get u
space.
Can temporary objects in the object pool be reused and if so how to do =
that?
		Regards
			Shrihari

> -----Original Message-----
> From: Yogesh Pandeya [mailto:yogesh@xxxxxxxxx]
> Sent: Tuesday, November 30, 1999 9:15 PM
> To: 'Shrihari.S.C'; Java Mail List
> Subject: RE: Garbage Collector question?
>
>
> Hi Srihari,
> Its not always the case that gc runs as a low priority thread.
> When the JVM runs short of memory, gc runs at high priority to
> reclaim space. If it could not get space by gc, it(JVM) gets more
> memory from the system.(correct me Gurus if I am wrong)
>
> About temperory objects....
> If you are declaring an object handle locally, and assigning new
> object to it, it will be gabage collected if it goes out of
> scope. If you are declaring the handle at the top level, and
> allocate a new object to it, it will hang around with you unless
> you assign it to null. This kind of memory leaks are programer's
> creation and he/she has to take care of it.
> If you need to create many objects locally many times, its always
> a good idea to use object pool as it will improve the performance
> significantly as "new" is a costly affair.
> Does this make sense or I am missing somthing???
> regards,
> Yogesh
>
>
> -----Original Message-----
> From:	Shrihari.S.C [SMTP:sriharis@xxxxxxxxxxxxxxxxxxxx]
> Sent:	Tuesday, November 30, 1999 2:41 PM
> To:	Java Mail List
> Cc:	yogesh@xxxxxxxxx
> Subject:	RE: Garbage Collector question?
>
> Hi Yogesh,
> 	The System.gc() is a low priority thread that cleans those
> set of objects
> that are not from the primordial class loader , but from the custom =
class
> loader and those which has been assigned null. But considering
> the fact that
> the class contains too many objects that have been temporarily created
> because of some intermediate computation and have not been assigned =
null,
> before which the computation is exited, the System.gc() siezes to work
> because of improper set of unreferenced objects to be
> deallocated. What do u
> think can be set of other options left out to see this problem ?
> 			Regards
> 								Shrihari
> > -----Original Message-----
> > From: Yogesh Pandeya [mailto:yogesh@xxxxxxxxx]
> > Sent: Tuesday, November 30, 1999 7:32 PM
> > To: 'magesh@xxxxxxxxxxxxxxxx'; 	 punam
> > chordia
> > Subject: RE: Garbage Collector question?
> >
> >
> >
> > Hi Magesh,
> > Another sequence on gc!!!! Anyway.....
> >
> > Putting System.gc() doesn't guarantee that it will run when you
> > want to. Its VM's prerogative to run gc or not. It is guaranteed
> > run at the point you put System.gc(), only if you run the
> > application with -noclassgc (or -Xnoclassgc) option.
> > Other point to note is, if you are creating objects temporarily,
> > you can use an object pool and reuse your objects. There is good
> > article this month's Javaworld.
> > Hope it helps,
> > cheers,
> > Yogesh
> >
> >      -----Original Message-----
> > From:	magesh@xxxxxxxxxxxxxxxx [SMTP:magesh@xxxxxxxxxxxxxxxx]
> > Sent:	Tuesday, November 30, 1999 1:23 PM
> > To:	advanced-java@xxxxxxxxxxxxxxxx; punam chordia
> > Subject:	RE: Garbage Collector question?
> >
> >
> >
> > Hi,
> >
> > Does this guarantee that the System.gc() will run without
> getting context
> > switched
> > because it has a low priority??
> >
> > thanks,
> > mmp.
> >
> >
> >
> >
> >
> > punam chordia <punamac@xxxxxxx> on 11/30/99 02:44:03 AM
> >
> > To:   Mageshkumar Maruthapillai/Technology Providers/lk
> > cc:
> > Subject:  RE: Garbage Collector question?
> >
> >
> >
> >
> > We also had a similar problem.
> > U can run garbage collector explicitly by writing the following line =
of
> > code,
> > System.gc();
> > I hope it helps you.
> > Punam Chordia
> > Software Engineer
> > ITL Infosys, Pune.
> > Ph. 91-020-647420/1 Extension: 1046
> > > -----Original Message-----
> > > From:   magesh@xxxxxxxxxxxxxxxx [SMTP:magesh@xxxxxxxxxxxxxxxx]
> > > Sent:   Tuesday, November 30, 1999 PM 01:06
> > > To:     advanced-java@xxxxxxxxxxxxxxxx
> > > Subject:     Garbage Collector question?
> > >
> > >
> > >
> > > Hi,
> > >
> > >     My server code is consuming a lot of memory dur to a large =
number
> > > of objects being created temporarily.
> > >
> > > I want to be able to get a reference to the garbage collector
> > > of the virtual machine so that I can give it a high priority to it
> > > very short intervals intermittently.
> > >
> > >
> > > Is there a way to get the reference to the garbage collector?
> > >
> > >
> > > any help is appreciated.
> > >
> > > thanks,
> > > mmp
> > >
> > >
> > >
> > > ---
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> >
> >
> >
> >
> >
> > ---
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>


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