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FastBufferWriter and FastBufferReader

The serialization and deserialization is done via FastBufferWriter and FastBufferReader. These have methods for serializing individual types and methods for serializing packed numbers, but in particular provide a high-performance method called WriteValue()/ReadValue() (for Writers and Readers, respectively) that can extremely quickly write an entire unmanaged struct to a buffer.

There's a trade-off of CPU usage vs bandwidth in using this: Writing individual fields is slower (especially when it includes operations on unaligned memory), but allows the buffer to be filled more efficiently, both because it avoids padding for alignment in structs, and because it allows you to use BytePacker.WriteValuePacked()/ByteUnpacker.ReadValuePacked() and BytePacker.WriteValueBitPacked()/ByteUnpacker.ReadValueBitPacked(). The difference between these two is that the BitPacked variants pack more efficiently, but they reduce the valid range of values. See the section below for details on packing.

Example

struct ExampleStruct
{
private float f;
private bool b;
private int i;
void Serialize(FastBufferWriter writer);
}

In this example struct, Serialize can be implemented in two ways:

void Serialize(FastBufferWriter writer)
{
if(!writer.TryBeginWrite(sizeof(float) + sizeof(bool) + sizeof(i)))
{
throw new OverflowException("Not enough space in the buffer");
}
writer.WriteValue(f);
writer.WriteValue(b);
writer.WriteValue(i);
}
void Serialize(FastBufferWriter writer)
{
if(!writer.TryBeginWrite(sizeof(ExampleStruct)))
{
throw new OverflowException("Not enough space in the buffer");
}
writer.WriteValue(this);
}

This creates efficiently packed data in the message, and can be further optimized by using BytePacker.WriteValuePacked() and BytePacker.WriteValueBitPacked(), but it has two downsides:

  • First, it involves more method calls and more instructions, making it slower.
  • Second, that it creates a greater opportunity for the serialize and deserialize code to become misaligned, since they must contain the same operations in the same order.

You can also use a hybrid approach if you have a few values that will need to be packed and several that won't:

struct ExampleStruct
{
struct Embedded
{
private byte a;
private byte b;
private byte c;
private byte d;
}
public Embedded embedded;
public float f;
public short i;
void Serialize(FastBufferWriter writer)
{
writer.WriteValue(embedded);
BytePacker.WriteValuePacked(writer, f);
BytePacker.WriteValuePacked(writer, i);
}
}

This allows the four bytes of the embedded struct to be rapidly serialized as a single action, then adds the compacted data at the end, resulting in better bandwidth usage than serializing the whole struct as-is, but better performance than serializing it one byte at a time.

FastBufferWriter and FastBufferReader#

FastBufferWriter and FastBufferReader are replacements for the old NetworkWriter and NetworkReader. For those familiar with the old classes, there are some key differences:

  • FastBufferWriter uses WriteValue() as the name of the method for all types except INetworkSerializable types, which are serialized through WriteNetworkSerializable()
  • FastBufferReader similarly uses ReadValue() for all types except INetworkSerializable (which is read through ReadNetworkSerializable), with the output changed from a return value to an out parameter to allow for method overload resolution to pick the correct value.
  • FastBufferWriter and FastBufferReader outsource packed writes and reads to BytePacker and ByteUnpacker, respectively.
  • FastBufferWriter and FastBufferReader are structs, not classes. This means they can be constructed and destructed without GC allocations.
  • FastBufferWriter and FastBufferReader both use the same allocation scheme as Native Containers, allowing the internal buffers to be created and resized without creating any garbage and with the use of Allocator.Temp or Allocator.TempJob.
  • FastBufferReader can be instantiated using Allocator.None to operate on an existing buffer with no allocations and no copies.
  • Neither FastBufferReader nor FastBufferWriter inherits from nor contains a Stream.
  • FastBufferReader and FastBufferWriter are heavily optimized for speed, using aggressive inlining and unsafe code to achieve the fastest possible buffer storage and retrieval.
  • FastBufferReader and FastBufferWriter use unsafe typecasts and UnsafeUtility.MemCpy operations on byte* values, achieving native memory copy performance with no need to iterate or do bitwise shifts and masks.
  • FastBufferReader and FastBufferWriter are intended to make data easier to debug - one such thing to support will be a #define MLAPI_FAST_BUFFER_UNPACK_ALL that will disable all packing operations to make the buffers for messages that use them easier to read.
  • FastBufferReader and FastBufferWriter do not support runtime type discovery - there is no WriteObject or ReadObject implementation. All types must be known at compile time. This is to avoid garbage and boxing allocations.

A core benefit of NativeArray<byte> is that it offers access to the allocation scheme of Allocator.TempJob. This uses a special type of allocation that is nearly as fast as stack allocation and involves no GC overhead, while being able to persist for a few frames. In general they are rarely if ever needed for more than a frame, but this does provide a very efficient option for creating buffers as needed, which avoids the need to use a pool for them. The only downside is that buffers created this way must be manually disposed after use, as they're not garbage collected.

Creating and Disposing FastBufferWriters and FastBufferReaders#

To create your own FastBufferWriters and FastBufferReaders, it's important to note that struct default/parameterless constructors cannot be removed or overridden, but FastBufferWriter and FastBufferReader require constructor behavior to be functional.

FastBufferWriter always owns its internal buffer and must be constructed with an initial size, an allocator, and a maximum size. If the maximum size is not provided or is less than or equal to the initial size, the FastBufferWriter cannot expand.

FastBufferReader can be constructed to either own its buffer or reference an existing one via Allocator.None. Not all types are compatible with Allocator.None - only byte*, NativeArray<byte>, and FastBufferWriter input types can provide Allocator.None. You can obtain a byte* from a byte[] using the following method:

byte[] byteArray;
fixed(byte* bytePtr = byteArray)
{
// use bytePtr here
}

It's important to note with Allocator.None that the FastBufferReader will be directly referencing a position in memory, which means the FastBufferReader must not live longer than the input buffer it references - and if the input buffer is a byte[], the FastBufferReader must not live longer than the fixed() statement, because outside of that statement, the garbage collector is free to move that memory, which will cause random and unpredictable errors.

Regardless which allocator you use (including Allocator.None), FastBufferWriter and FastBufferReader must always have Dispose() called on them when you're done with them. The best practice is to use them within using blocks.

Bounds Checking#

For performance reasons, by default, FastBufferReader and FastBufferWriter do not do bounds checking on each write. Rather, they require the use of specific bounds checking functions - TryBeginRead(int amount) and TryBeginWrite(int amount), respectively. This improves performance by allowing you to verify the space exists for the multiple values in a single call, rather than doing that check on every single operation.

info

In editor mode and development builds, calling these functions records a watermark point, and any attempt to read or write past the watermark point will throw an exception. This ensures these functions are used properly, while avoiding the performance cost of per-operation checking in production builds. In production builds, attempting to read or write past the end of the buffer will cause undefined behavior, likely program instability and/or crashes.

For convenience, every WriteValue() and ReadValue() method has an equivalent WriteValueSafe() and ReadValueSafe() that does bounds checking for you, throwing OverflowException if the boundary is exceeded. Additionally, some methods, such as arrays (where the amount of data being read can't be known until the size value is read) and INetworkSerializable values (where the size can't be predicted outside the implementation) will always do bounds checking internally.

Bitwise Reading and Writing#

Writing values in sizes measured in bits rather than bytes comes with a cost

  • First, it comes with a cost of having to track bitwise lengths and convert them to bytewise lenghts.
  • Second, it comes with a cost of having to remember to add padding after your bitwise writes and reads to ensure the next bytewise write or read functions correctly, and to make sure the buffer length includes any trailing bits.

To address that, FastBufferReader and FastBufferWriter do not, themselves, have bitwise operations. When needed, however, you can create a BitWriter or BitReader instance, which is used ensure that no unaligned bits are left at the end - from the perspective of FastBufferReader and FastBufferWriter, only bytes are allowed. BitWriter and BitReader operate directly on the underlying buffer, so calling non-bitwise operations within a bitwise context is an error (and will raise an exception in non-production builds).

FastBufferWriter writer = new FastBufferWriter(256, Allocator.TempJob);
using(var bitWriter = writer.EnterBitwiseContext())
{
bitWriter.WriteBit(a);
bitWriter.WriteBits(b, 5);
} // Dispose automatically adds 2 more 0 bits to pad to the next byte.

Packing#

Packing values is done using the utility classes BytePacker and ByteUnpacker. These generally offer two different ways of packing values:

  • BytePacker.WriteValuePacked()/ByteUnpacker.ReadValuePacked() are the most versatile. They can write any range of values that fit into the type, and also have special built-in methods for many common Unity types that can automatically pack the values contained within.

  • BytePacker.WriteValueBitPacked()/ByteUnpacker.ReadValueBitPacked() offer tighter/more optimal packing (the data in the buffer will never exceed sizeof(type), which can happen with very large values using WriteValuePacked(), and will usually be one byte smaller than with WriteValuePacked() except for values <= 240, which will be one byte with both methods), but come with the limitations that they can only be used on integral types, and they use some bits of the type to encode length information, meaning that they reduce the usable size of the type. The sizes allowed by these functions are as follows:

    TypeUsable Size
    short14 bits + sign bit (-16,384 to 16,383)
    ushort15 bits (0 to 32,767)
    int29 bits + sign bit (-536,870,912 to 536,870,911)
    uint30 bits (0 to 1,073,741,824)
    long60 bits + sign bit (-1,152,921,504,606,846,976 to 1,152,921,504,606,846,975)
    ulong61 bits (0 to 2,305,843,009,213,693,952)