>> Adding in core-libs-dev as there's nothing related to hotspot directly 
>> here.
>
> Correction, this is of course leading to a proposed change in hotspot to 

> implement the new Unsafe methods and perform the native memory tracking. 


Hah, I wrote the same thing in a parallel reply. Jinx. :)

- Adam

> Of course we already have NMT so the obvious question is how this will 
> fit in with NMT?
>
> David

It will add granularity to Native Memory Tracking, allowing people
to tell, at a glance, how much of the allocated native memory has been
used for Direct Byte Buffers. This makes native memory OOM
debugging easier.

Think of it as an NMT upgrade.

Here's an example of what the output should look like:

https://developer.ibm.com/answers/questions/288697/why-does-nativememinfo-in-javacore-show-incorrect.html?sort=oldest

- Adam

>
>> David
>> 
>> On 14/02/2018 9:32 PM, Adam Farley8 wrote:
>>> Hi All,
>>>
>>> Currently, diagnostic core files generated from OpenJDK seem to lump 
all
>>> of the
>>> native memory usages together, making it near-impossible for someone 
to
>>> figure
>>> out *what* is using all that memory in the event of a memory leak.
>>>
>>> The OpenJ9 VM has a feature which allows it to track the allocation of
>>> native
>>> memory for Direct Byte Buffers (DBBs), and to supply that information 
>>> into
>>> the
>>> cores when they are generated. This makes it a *lot* easier to find 
out
>>> what is using
>>> all that native memory, making memory leak resolution less like some 
dark
>>> art, and
>>> more like logical debugging.
>>>
>>> To use this feature, there is a native method referenced in 
Unsafe.java.
>>> To open
>>> up this feature so that any VM can make use of it, the java code below
>>> sets the
>>> stage for it. This change starts letting people call DBB-specific 
methods
>>> when
>>> allocating native memory, and getting into the habit of using it.
>>>
>>> Thoughts?
>>>
>>> Best Regards
>>>
>>> Adam Farley
>>>
>>> P.S. Code:
>>>
>>> diff --git
>>> a/src/java.base/share/classes/java/nio/Direct-X-Buffer.java.template
>>> b/src/java.base/share/classes/java/nio/Direct-X-Buffer.java.template
>>> --- 
a/src/java.base/share/classes/java/nio/Direct-X-Buffer.java.template
>>> +++ 
b/src/java.base/share/classes/java/nio/Direct-X-Buffer.java.template
>>> @@ -85,7 +85,7 @@
>>>                   // Paranoia
>>>                   return;
>>>               }
>>> -            UNSAFE.freeMemory(address);
>>> +            UNSAFE.freeDBBMemory(address);
>>>               address = 0;
>>>               Bits.unreserveMemory(size, capacity);
>>>           }
>>> @@ -118,7 +118,7 @@
>>>           long base = 0;
>>>           try {
>>> -            base = UNSAFE.allocateMemory(size);
>>> +            base = UNSAFE.allocateDBBMemory(size);
>>>           } catch (OutOfMemoryError x) {
>>>               Bits.unreserveMemory(size, cap);
>>>               throw x;
>>> diff --git a/src/java.base/share/classes/jdk/internal/misc/Unsafe.java
>>> b/src/java.base/share/classes/jdk/internal/misc/Unsafe.java
>>> --- a/src/java.base/share/classes/jdk/internal/misc/Unsafe.java
>>> +++ b/src/java.base/share/classes/jdk/internal/misc/Unsafe.java
>>> @@ -632,6 +632,26 @@
>>>       }
>>>       /**
>>> +     * Allocates a new block of native memory for DirectByteBuffers, 
of
>>> the
>>> +     * given size in bytes.  The contents of the memory are
>>> uninitialized;
>>> +     * they will generally be garbage.  The resulting native pointer 
>>> will
>>> +     * never be zero, and will be aligned for all value types.  
Dispose
>>> of
>>> +     * this memory by calling {@link #freeDBBMemory} or resize it 
with
>>> +     * {@link #reallocateDBBMemory}.
>>> +     *
>>> +     * @throws RuntimeException if the size is negative or too large
>>> +     *                          for the native size_t type
>>> +     *
>>> +     * @throws OutOfMemoryError if the allocation is refused by the
>>> system
>>> +     *
>>> +     * @see #getByte(long)
>>> +     * @see #putByte(long, byte)
>>> +     */
>>> +    public long allocateDBBMemory(long bytes) {
>>> +        return allocateMemory(bytes);
>>> +    }
>>> +
>>> +    /**
>>>        * Resizes a new block of native memory, to the given size in 
>>> bytes.
>>> The
>>>        * contents of the new block past the size of the old block are
>>>        * uninitialized; they will generally be garbage.  The resulting
>>> native
>>> @@ -687,6 +707,27 @@
>>>       }
>>>       /**
>>> +     * Resizes a new block of native memory for DirectByteBuffers, to 

>>> the
>>> +     * given size in bytes.  The contents of the new block past the 
size
>>> of
>>> +     * the old block are uninitialized; they will generally be 
garbage.
>>> The
>>> +     * resulting native pointer will be zero if and only if the 
>>> requested
>>> size
>>> +     * is zero.  The resulting native pointer will be aligned for all
>>> value
>>> +     * types.  Dispose of this memory by calling {@link 
#freeDBBMemory},
>>> or
>>> +     * resize it with {@link #reallocateDBBMemory}.  The address 
passed
>>> to
>>> +     * this method may be null, in which case an allocation will be
>>> performed.
>>> +     *
>>> +     * @throws RuntimeException if the size is negative or too large
>>> +     *                          for the native size_t type
>>> +     *
>>> +     * @throws OutOfMemoryError if the allocation is refused by the
>>> system
>>> +     *
>>> +     * @see #allocateDBBMemory
>>> +     */
>>> +    public long reallocateDBBMemory(long address, long bytes) {
>>> +        return reallocateMemory(address, bytes);
>>> +    }
>>> +
>>> +    /**
>>>        * Sets all bytes in a given block of memory to a fixed value
>>>        * (usually zero).
>>>        *
>>> @@ -918,6 +959,17 @@
>>>           checkPointer(null, address);
>>>       }
>>> +    /**
>>> +     * Disposes of a block of native memory, as obtained from {@link
>>> +     * #allocateDBBMemory} or {@link #reallocateDBBMemory}.  The 
address
>>> passed
>>> +     * to this method may be null, in which case no action is taken.
>>> +     *
>>> +     * @see #allocateDBBMemory
>>> +     */
>>> +    public void freeDBBMemory(long address) {
>>> +        freeMemory(address);
>>> +    }
>>> +
>>>       /// random queries
>>>       /**
>>>
>>> Unless stated otherwise above:
>>> IBM United Kingdom Limited - Registered in England and Wales with 
number
>>> 741598.
>>> Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 

>>> 3AU
>>>

Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 
741598. 
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU

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