A Java buffer pool memory space is a non-managed memory space that is situated separately from the garbage collector-managed memory. In order to better understand the Java buffer pool memory space, we will first learn the basics of buffer memories.
What Are Buffer Memories?
A buffer is a transit memory location for data to travel from point A to point B in a computing system. Nearly all computing devices use buffer memory, such as SmartPhones, Network devices (Switches, Routers), and computers.
This article will focus on the Java buffer pool memory space that has been managed by Byte Buffer.
Byte Buffer
In Java programming, when using getter and setter functions to manipulate or move data, interacting with external input devices or looking to deal with efficient memory access to files, the byte buffer class is an effective alternative to objects found in traditional Java programming. Additionally, a ByteBuffer is a Java class library for managing buffer memory.
Let’s take a look at both ByteBuffer categories: Direct Byte Buffer and Non-Direct Byte Buffer.
Direct Buffer
Java Direct byte buffers perform their native Input/Output operations directly to the buffer memory without involving any intermediary memory. Using a direct byte buffer, instances could be created by using the ByteBuffer.allocateDirect() method and when the buffer is created, it has a higher allocation and deallocation cost than a non-direct buffer.
Moreover, as mentioned previously, direct buffers and other buffers are not part of the managed garbage collector area, making their impact difficult to detect on applications or system memory. Directly mapping a file to a specific buffer memory region is one way of creating a direct buffer.
Non-Direct Buffer
Java Non-direct byte buffers are created by using ByteBuffer.allocate().
A major difference is the handling of transition data and memory allocation since there could be intermediary memories in its Input and Output operations.
Memory Mapped Buffer
By mapping a region of the file directly into memory, a direct byte buffer may also be created. To put it differently, we can load a specific section of a file into native memory so it can be accessed later. As you can see, it can significantly boost performance if we need to read the content of a file multiple times.
A Big thanks to Memory-mapped files, as it allows subsequent reads to use the data from the memory rather than loading it from the disc every time. Using FileChannel.map(), a MappedByteBuffer can be created.
Moreover, the OS can also flush the buffer directly to the disk when shutting down the system when using memory-mapped files. In addition, the OS can lock a mapped portion of the file from other processes on the machine.
Allocation is Expensive
Direct buffers have the disadvantage of being expensive to allocate. Buffer.allocateDirect() is a relatively slow operation regardless of the buffer’s size. As a result, it is more efficient to either use direct buffers for large and long-lived buffers, or create one large buffer, slice off portions on-demand, and re-use them when no longer required.
Moreover, Slices may not always be of the same size, which can cause problems with slicing. In the initial large byte buffer, fragments may form when objects of different sizes are allocated and released. Direct byte buffers can’t be compacted like Java heap because they’re not collected by the garbage collector.
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Monitoring the Usage of Buffer Pools
If you wish to monitor the amount of direct and mapped byte buffers used by your application, you can easily do it by using VisualVM (with BufferMonitor plugin) or by the Java monitoring tool like Seagence.
Conclusion
We hope you got a clear understanding of what Java buffer pool is in memory space. Considering the high amount of computing resources required to allocate a direct buffer, it is recommended to use a direct buffer for long-term buffer storage compared to a non-direct buffer.
By using the -XX: MaxDirectMemorySize=N flag, you can also limit the amount of direct byte buffer space that an application can allocate. Despite the possibility of doing so, a compelling reason must be given.