This document summarizes the differences between Netty4 ByteBuf and DirectBuffer in Java, and their performance implications. It introduces Netty4's use of reference counting ByteBuf to avoid the performance costs of PhantomReference and enable manual resource management. Microbenchmarks show Pooled and Unpooled ByteBuf outperform Unpooled DirectBuffer, due to reference counting and buffer pooling avoiding garbage collection overhead.

1. Netty4 ByteBufNetty4 ByteBuf
vsvs
Java DirectBufferJava DirectBuffer
Michael Fong
v0.11 . 1

2. ChangelogChangelog
v 0.1
Introduce DirectBuffer and PhantomReference
include performance impact via microbenchmark
Introduce netty4 ByteBuf
include performance impact via microbenchmark
1 . 2

3. Problem StatementProblem Statement
Why Netty4 introduces reference counting ByteBuf as opposed
to existing PhantomReference Clearner based DirectBuffer
Performance perspective
Automatic / Manual releasing perspective
2

4. DirectByteBuffer DirectByteBuffer
3 . 1

5. Memory LayoutMemory Layout
Off Heap
(Native
Memory)
3 . 2
Direct Buffers can be passed
directly to native OS functions
for performing memory I/O
Java applicaiton still holds an
reference object in heap to
orchestrate I/O operations,
whereas the buffer that holds
the data is held in native
memory

6. Memory LayoutMemory Layout
Off Heap
(Native
Memory)
3 . 2
Direct Buffers can be passed
directly to native OS functions
for performing memory I/O
Java applicaiton still holds an
reference object in heap to
orchestrate I/O operations,
whereas the buffer that holds
the data is held in native
memory

7. Memory LayoutMemory Layout
Off Heap
(Native
Memory)
Heap DirectBuffer Object
3 . 2
Direct Buffers can be passed
directly to native OS functions
for performing memory I/O
Java applicaiton still holds an
reference object in heap to
orchestrate I/O operations,
whereas the buffer that holds
the data is held in native
memory

8. Memory LayoutMemory Layout
Off Heap
(Native
Memory)
malloc()
Heap DirectBuffer Object
3 . 2
Direct Buffers can be passed
directly to native OS functions
for performing memory I/O
Java applicaiton still holds an
reference object in heap to
orchestrate I/O operations,
whereas the buffer that holds
the data is held in native
memory

9. AllocationAllocation
1.
2.
3.
DirectByteBuffer.java
Unsafe.java
unsafe.cpp
3 . 3

10. DeAllocationDeAllocation
1.
2.
3.
DirectByteBuffer.java
Unsafe.java
unsafe.cpp
Cleaner extends
<Object>
DirectByteBuffer.Deallocator
implements Runnable
PhantomReference
3 . 4

11. Phantom ReferencePhantom ReferenceWorks a bit like Finalizer except for a few additional
requirements:
a separate thread to handle reference queue
Need to keep strong references to referent objects.
PhantomReference (or FinalReference) provides
automatic object / reference clean up with performance costs.
RefQueue
Ref<Object>
Heap
Object
Stack
local
reference
[ ]Ref 3 . 5

12. Phantom ReferencePhantom ReferenceWorks a bit like Finalizer except for a few additional
requirements:
a separate thread to handle reference queue
Need to keep strong references to referent objects.
PhantomReference (or FinalReference) provides
automatic object / reference clean up with performance costs.
RefQueue
Ref<Object>
Heap
Object
Stack[ ]Ref 3 . 5

13. Phantom ReferencePhantom ReferenceWorks a bit like Finalizer except for a few additional
requirements:
a separate thread to handle reference queue
Need to keep strong references to referent objects.
PhantomReference (or FinalReference) provides
automatic object / reference clean up with performance costs.
RefQueue
Ref<Object>
Heap
Object
Stack[ ]Ref 3 . 5

14. Phantom ReferencePhantom ReferenceWorks a bit like Finalizer except for a few additional
requirements:
a separate thread to handle reference queue
Need to keep strong references to referent objects.
PhantomReference (or FinalReference) provides
automatic object / reference clean up with performance costs.
RefQueue
Ref<Object>
Heap
Object
Stack[ ]Ref 3 . 5

15. Performance ImpactPerformance Impact
[GC [ParNew[ ... [PhantomReference, 5532 refs, 0.0037622 secs] ...
Released: 5468 In use: 38897
Reference processing is a part of Stop-
the-World pause.
too many references may prolong
Stop-the-World pause significantly.
Improve the finalization process
Once phantom reference is
unreachable, it would never be
queued for processing by GC, thus
saving time in reference processing
phase.
3 . 6

16. MicroBenchmarkMicroBenchmark
gc.time (ms) gc.count
GcOnFinalizedObject 37708.000 51.000
GcOnPhantomReference
Object
885.000 2013.000
GcOnHeapObject 859.000 1911.000
Conditions:
1000 object refs recycled
100 iterations
Duo Core Intel Core i7-7500U [ ]Code3 . 7

17. ByteBufByteBuf
4 . 1

18. Types of ByteBufTypes of ByteBuf
A ByteBuf could be categorized from 2 aspects
1. Pooled or Unpooled
2. Heap or DirectBuffer
Buffer pooling introduces ByteBufAllocator
a java variant of jemalloc
buddy memory allocation
ByteBuf Type GC Overhead Reference
Counting
Required
Pooled Heap Buf High Yes
Pooled Direct Buf min Yes
Unpooled Heap Buf high None
Unpooled Direct Buf min None
4 . 2

19. ByteBuf vsByteBuf vs
ByteBufAllocatorByteBufAllocator
4 . 3

20. AllocationAllocation
1. AbstractByteBufAllocator.java
2. UnpooledUnsafeDirectByteBuf.java
3. ByteBuf.java
4.
5.
Unsafe.java
unsafe.cpp
4 . 4

21. DeAllocationDeAllocation
1. AbstractReferenceCountedByteBuf.java
2. UnpooledUnsafeDirectByteBuf.java
3. PlatformDependent.java
4. CleanerJava6.java
5.
6.
Unsafe.java
unsafe.cpp
ByteBuf invokes
DirectByteBuffer.Cleaner.clean() via
Reflection / Unsafe (faster) to release the
native memory region directly.
[ ]Ref4 . 5

22. Problem RevisitedProblem Revisited
Why Netty4 introduces reference counting ByteBuf as opposed
to existing PhantomReference Clearner based DirectBuffer
Reasons:
PhantomReference provides automatic object / reference
clean up with performance costs.
Developer need to manually release ByteBuf each time a
reference finishes its scope.
GC bound overhead
References
]
[ ]
[1
2
public ByteBuf b(ByteBuf buf) {
try {
...
return anAllocatedByteBuf;
} finally {
buf.release();
}
}
4 . 6

23. MicroBenchmarkMicroBenchmark
gc.time (ms) gc.count
Pooled Heap ByteBuf 86 78
Pooled Direct ByteBuf 64 80
Unpooled Heap ByteBuf 1225 484
Unpooled Direct ByteBuf 948 656
Unpooled DirectBuffer 109359 179
Conditions:
buffer size = 1024 bytes
100 iterations
Duo Core Intel Core i7-7500U
[ ]Code4 . 7