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From "Xiao-Feng Li" <xiaofeng...@gmail.com>
Subject Re: [VM]How to trigue GC while free native memory is low.
Date Fri, 02 Feb 2007 09:05:48 GMT
Leo, I can't fully understand your problem and proposal. I am working
in DRLVM GC component, I want to understand how GC can tackle the
issue you meet.

You said the native memory is allocated (probably with malloc) for
byte buffer. I wonder how the process goes. I assume it's in native
code. Then you use malloc to allocate a block of memory, and put some
Java object (byte buffer) into it? Is this what you do? I wonder how
do you manipulate Java object placement in native code; and if the
native memory block is out of Java heap, do you expect GC to manage
it?

You said when GC reclaims those byte buffer object, the native memory
block can be freed in native code. How do you know if an object is
reclaimed by GC or not?

Thanks,
xiaofeng

On 2/2/07, Leo Li <liyilei1979@gmail.com> wrote:
> Hi, all:
>     After applying patch3073, I added the support for direct byte buffer in
> nio module as spec requires. But now here exists a problem: The native
> memory allocated to the byte buffer can be released when gc releases the
> byte buffer object, however, if the native heap is full while java heap
> still has space, gc will not be triggered.
>     It seems that RI will start gc before native memory heap is depleted and
> thus prevents out-of-memory error.
>
>    Then our work focuses on:
>    1. When gc is required.
>    2. Trigger a gc.
>    The first one requires that we get support from operating system, since
> the memory allocated in the native code, for example by malloc, is out of
> the control of java VM.( I have ever thought of counting the used memory in
> hymemory_allocate, but the plan fails since hymemory_free will not report
> how much space is released.)
>    The second one needs the help from VM. System.gc() is not so reliable as
> spec says, so it is necessary to have a internal channel to notify VM to
> start gc.
>
>     One solution, I think, is to let a monitor thread in VM to check whether
> OS physical memory is low. For example, the QueryMemoryResourceNotification
> of win32 API is a candidate. Although the interrupt model is more effective:
> win32 SDK provides a CreateMemoryResourceNotification to get a handler on
> which the monitor thread can wait, maybe on other platforms, OS does not
> supply such a convenience. So the monitor thread in the VM might have to
> check the OS resource once for a while and if necessary the monitor thread
> will call a GC.
>
>    My suggestion is first to add some function to monitor memory in portlib,
> since it is highly related to platforms and in portlib there has been
> some useful tools.(Thanks Mark to point out that.) On the other hand, we can
> negotiate a channel to trigger gc in VM. Actually I am not an expert on VM
> since I am not sure whether there has been some monitor thread and the load
> on performance if such a monitor thread is added to the VM...
>
>
> --
> Leo Li
> China Software Development Lab, IBM
>
>

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