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From "Paulex Yang (JIRA)" <j...@apache.org>
Subject [jira] Created: (HARMONY-38) com.ibm.platform.struct.AbstractMemorySpy doesn't actually autoFree native memory
Date Mon, 23 Jan 2006 11:33:09 GMT
com.ibm.platform.struct.AbstractMemorySpy doesn't actually autoFree native memory

         Key: HARMONY-38
         URL: http://issues.apache.org/jira/browse/HARMONY-38
     Project: Harmony
        Type: Bug
  Components: Classlib  
    Reporter: Paulex Yang
    Priority: Critical

com.ibm.platform.struct.IMemorySpy is a memory watcher, and the AbstractMemorySpy is a default
implementation working with nio direct buffers to free the native memories when direct buffer
instances are garbage collected. But currently its auto free donsn't work.  

Look inside the orphanedMemory(Object obj) method, the memory is freed only if the local variable
wrapper can be found from the memoryInUse, a Map which records the native memories used by
direct buffers, but actually it will be removed when autoFree(PlatfromAddress) is invoked.
Checking the autoFree() reference, I found that all DirectByteBuffer will invoke autoFree()
in constructor, which means no memory will be freed. 

A simple fix:
modify the Ln 94 in autoFree() from:
        wrapper = (AddressWrapper) memoryInUse.remove(address);
        wrapper = (AddressWrapper) memoryInUse.get(address);
and it should work.

The steps to reproduce the bug:

1. Add the following line in the the Ln109, which indicates the memory is auto freed by AbstractMemorySpy
         System.out.println("MemorySpy autofree:"+wrapper.shadow);

2. Run the test below:

public class MemorySpyTest {
    public static void main(String[] args) throws Exception {
        for (int i = 0; i < 1000; i++) {
                ByteBuffer buffer = ByteBuffer.allocateDirect(1024);
                buffer = null;
            }catch(Exception e){

Before modification: application exits silently.

After modification, hundres of lines printed out, like this:
MemorySpy autofree:PlatformAddress[9310024]
MemorySpy autofree:PlatformAddress[9311056]
MemorySpy autofree:PlatformAddress[9312088]
MemorySpy autofree:PlatformAddress[9313120]
MemorySpy autofree:PlatformAddress[9314152]
MemorySpy autofree:PlatformAddress[9315184]
MemorySpy autofree:PlatformAddress[9316216]
MemorySpy autofree:PlatformAddress[9317248]

Another possible pitfall in AbstractMemorySpy is it uses WeakReference and ReferenceQueue
to get notification when the buffers are ready to be garbage collected. But it is not so safe
yet to free the native memory, because the weakly-reachable object has its chance to be resurrected
to strongly-reachable, if only some finalize() method doing this is executed. A safer solution
is to use PhantomReference instead, because phantom-reachable object has no chance to survive.
The only concern is the PhantomReference should be explicitly cleared by application, but
that's OK because the orphanedMemory() method is a perfect candidate to do this. 

I will attach the patch soon.

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