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0109: Communication Buffers

SEED-0109: Communication Buffers

Status: Open for Comments Intent Approved Last Call Accepted Rejected

Proposal Date: 2023-08-28

CL: pwrev/168357

Author: Taylor Cramer

Facilitator: Erik Gilling

Summary

This SEED proposes that Pigweed adopt a standard buffer type for network-style communications. This buffer type will be used in the new sockets API (see the recently-accepted Communications SEED 107 for more details), and will be carried throughout the communications stack as-appropriate.

Top-Level Usage Examples

Sending a Proto Into a Socket

Allocator& msg_allocator = socket.Allocator();
size_t size = EncodedProtoSize(some_proto);
std::optional<pw::MultiBuf> buffer = msg_allocator.Allocate(size);
if (!buffer) { return; }
EncodeProto(some_proto, *buffer);
socket.Send(std::move(*buffer));

Socket s provide an allocator which should be used to create a pw::MultiBuf. The data to be sent is then filled into this buffer before being sent. Socket must accept pw::MultiBuf s which were not created by their own Allocator, but this may incur a performance penalty.

Zero-Copy Fragmentation

The example above has a hidden feature: zero-copy fragmentation! The socket can provide a pw::MultiBuf which is divided up into separate MTU-sized chunks, each of which has reserved space for headers and/or footers:

std::optional<pw::MultiBuf> Allocate(size_t size) {
  if (size == 0) { return pw::MultiBuf(); }
  size_t data_per_chunk = mtu_size_ - header_size_;
  size_t num_chunks = 1 + ((size - 1) / data_per_chunk);
  std::optional<pw::MultiBuf> buffer = pw::MultiBuf::WithCapacity(num_chunks);
  if (!buffer) { return std::nullopt; }
  for (size_t i = 0; i < num_chunks; i++) {
    // Note: we could allocate smaller than `mtu_size_` for the final
    // chunk. This is ommitted here for brevity.
    std::optional<pw::MultiBuf::Chunk> chunk = internal_alloc_.Allocate(mtu_size_);
    if (!chunk) { return std::nullopt; }
    // Reserve the header size by discarding the front of each buffer.
    chunk->DiscardFront(header_size_);
    buffer->Chunks()[i] = std::move(chunk);
  }
  return *buffer;
}

This code reserves header_size_ bytes at the beginning of each Chunk. When the caller writes into this memory and then passes it back to the socket, these bytes can be claimed for the header using the ClaimPrefix function.

One usecase that seems to demand the ability to fragment like this is breaking up SOCK_SEQPACKET messages which (at least on Unix / Linux) may be much larger than the MTU size: up to SO_SNDBUF (see this man page).

Multiplexing Incoming Data

[[nodiscard]] bool SplitAndSend(pw::MultiBuf buffer) {
  std::optional<std::array<pw::MultiBuf, 2>> buffers =
    std::move(buffer).Split(split_index);
  if (!buffers) { return false; }
  socket_1_.Send(std::move(buffers->at(0)));
  socket_2_.Send(std::move(buffers->at(1)));
  return true;
}

Incoming buffers can be split without copying, and the results can be forwarded to multiple different Socket s, RPC services or clients.

Motivation

Today, a Pigweed communications stack typically involves a number of different buffers.

pw_rpc users, for example, frequently use direct-memory access (DMA) or other system primitives to read data into a buffer, apply some link-layer protocol such as HDLC which copies data into a second buffer, pass the resulting data into pw_rpc which parses it into its own buffer. Multiple sets of buffers are present on the output side as well. Between DMA in and DMA out, it’s easy for data to pass through six or more different buffers.

These independent buffer systems introduce both time and space overhead. Aside from the additional CPU time required to move the data around, users need to ensure that all of the different buffer pools involved along the way have enough space reserved to contain the entire message. Where caching is present, moving the memory between locations may create an additional delay by thrashing between memory regions.

Proposal

pw::buffers::MultiBuf is a handle to a buffer optimized for use within Pigweed’s communications stack. It provides efficient, low-overhead buffer management, and serves as a standard type for passing data between drivers, TCP/IP implementations, RPC 2.0, and transfer 2.0.

A single MultiBuf is a list of Chunk s of data. Each Chunk points to an exclusively-owned portion of a reference-counted allocation. MultiBuf s can be easily split, joined, prefixed, postfixed, or infixed without needing to copy the underlying data.

The memory pointed to by Chunk s is typically allocated from a pool provided by a Socket. This allows the Socket to provide backpressure to callers, and to ensure that memory is placed in DMA or shared memory regions as-necessary.

In-Memory Layout

This diagram shows an example in-memory relationship between two buffers: - Buffer1 references one chunks from region A. - Buffer2 references two chunk from regions A and B.

graph TB; Buffer1 --> Chunk1A Chunk1A -- "[0..64]" --> MemoryRegionA(Memory Region A) Chunk1A --> ChunkRegionTrackerA Buffer2 --> Chunk2A & Chunk2B Chunk2A --> ChunkRegionTrackerA Chunk2A -- "[64..128]" --> MemoryRegionA(Memory Region A) Chunk2B -- "[0..128]" --> MemoryRegionB Chunk2B --> ChunkRegionTrackerB

API

The primary API is as follows:

/// An object that manages a single allocated region which is referenced
/// by one or more chunks.
class ChunkRegionTracker {
 public:
  ChunkRegionTracker(ByteSpan);

  /// Creates the first ``Chunk`` referencing a whole region of memory.
  /// This must only be called once per ``ChunkRegionTracker``.
  Chunk ChunkForRegion();

 protected:
  pw::Mutex lock();

  /// Destroys the `ChunkRegionTracker`.
  ///
  /// Typical implementations will call `std::destroy_at(this)` and then
  /// free the memory associated with the tracker.
  virtual void Destroy();

  /// Defines the entire span of the region being managed. ``Chunk`` s
  /// referencing this tracker may not expand beyond this region
  /// (or into one another).
  ///
  /// This region must not change for the lifetime of this
  /// ``ChunkRegionTracker``.
  virtual ByteSpan region();

 private:
  /// Used to manage the internal linked-list of ``Chunk`` s that allows
  /// chunks to know whether or not they can expand to fill neighboring
  /// regions of memory.
  pw::Mutex lock_;
};

/// A handle to a contiguous refcounted slice of data.
///
/// Note: this Chunk may acquire a ``pw::sync::Mutex`` during destruction, and
/// so must not be destroyed within ISR context.
class Chunk {
 public:
  Chunk();
  Chunk(ChunkRegionTracker&);
  Chunk(Chunk&& other) noexcept;
  Chunk& operator=(Chunk&& other);
  ~Chunk();
  std::byte* data();
  const std::byte* data() const;
  ByteSpan span();
  ConstByteSpan span() const;
  size_t size() const;

  std::byte& operator[](size_t index);
  std::byte operator[](size_t index) const;

  /// Decrements the reference count on the underlying chunk of data and empties
  /// this handle so that `span()` now returns an empty (zero-sized) span.
  ///
  /// Does not modify the underlying data, but may cause it to be
  /// released if this was the only remaining ``Chunk`` referring to it.
  /// This is equivalent to ``{ Chunk _unused = std::move(chunk_ref); }``
  void Release();

  /// Attempts to add `prefix_bytes` bytes to the front of this buffer by
  /// advancing its range backwards in memory. Returns `true` if the
  /// operation succeeded.
  ///
  /// This will only succeed if this `Chunk` points to a section of a chunk
  /// that has unreferenced bytes preceeding it. For example, a `Chunk`
  /// which has been shrunk using `DiscardFront` can then be re-expanded using
  /// `ClaimPrefix`.
  ///
  /// Note that this will fail if a preceding `Chunk` has claimed this
  /// memory using `ClaimSuffix`.
  [[nodiscard]] bool ClaimPrefix(size_t prefix_bytes);

  /// Attempts to add `suffix_bytes` bytes to the back of this buffer by
  /// advancing its range forwards in memory. Returns `true` if the
  /// operation succeeded.
  ///
  /// This will only succeed if this `Chunk` points to a section of a chunk
  /// that has unreferenced bytes following it. For example, a `Chunk`
  /// which has been shrunk using `Truncate` can then be re-expanded using
  /// `ClaimSuffix`.
  ///
  /// Note that this will fail if a following `Chunk` has claimed this
  /// memory using `ClaimPrefix`.
  [[nodiscard]] bool ClaimSuffix(size_t suffix_bytes);

  /// Shrinks this handle to refer to the data beginning at offset ``new_start``.
  ///
  /// Does not modify the underlying data.
  void DiscardFront(size_t new_start);

  /// Shrinks this handle to refer to data in the range ``begin..<end``.
  ///
  /// Does not modify the underlying data.
  void Slice(size_t begin, size_t end);

  /// Shrinks this handle to refer to only the first ``len`` bytes.
  ///
  /// Does not modify the underlying data.
  void Truncate(size_t len);

  /// Splits this chunk in two, with the second chunk starting at `split_point`.
  std::array<Chunk, 2> Split(size_t split_point) &&;
};

/// A handle to a sequence of potentially non-contiguous refcounted slices of
/// data.
class MultiBuf {
 public:
  struct Index {
    size_t chunk_index;
    size_t byte_index;
  };

  MultiBuf();

  /// Creates a ``MultiBuf`` pointing to a single, contiguous chunk of data.
  ///
  /// Returns ``std::nullopt`` if the ``ChunkList`` allocator is out of memory.
  static std::optional<MultiBuf> FromChunk(Chunk chunk);

  /// Splits the ``MultiBuf`` into two separate buffers at ``split_point``.
  ///
  /// Returns ``std::nullopt`` if the ``ChunkList`` allocator is out of memory.
  std::optional<std::array<MultiBuf, 2>> Split(Index split_point) &&;
  std::optional<std::array<MultiBuf, 2>> Split(size_t split_point) &&;

  /// Appends the contents of ``suffix`` to this ``MultiBuf`` without copying data.
  /// Returns ``false`` if the ``ChunkList`` allocator is out of memory.
  [[nodiscard]] bool Append(MultiBuf suffix);

  /// Discards the first elements of ``MultiBuf`` up to (but not including)
  /// ``new_start``.
  ///
  /// Returns ``false`` if the ``ChunkList`` allocator is out of memory.
  [[nodiscard]] bool DiscardFront(Index new_start);
  [[nodiscard]] bool DiscardFront(size_t new_start);

  /// Shifts and truncates this handle to refer to data in the range
  /// ``begin..<stop``.
  ///
  /// Does not modify the underlying data.
  ///
  /// Returns ``false`` if the ``ChunkList`` allocator is out of memory.
  [[nodiscard]] bool Slice(size_t begin, size_t end);

  /// Discards the tail of this ``MultiBuf`` starting with ``first_index_to_drop``.
  /// Returns ``false`` if the ``ChunkList`` allocator is out of memory.
  [[nodiscard]] bool Truncate(Index first_index_to_drop);
  /// Discards the tail of this ``MultiBuf`` so that only ``len`` elements remain.
  /// Returns ``false`` if the ``ChunkList`` allocator is out of memory.
  [[nodiscard]] bool Truncate(size_t len);

  /// Discards the elements beginning with ``cut_start`` and resuming at
  /// ``resume_point``.
  ///
  /// Returns ``false`` if the ``ChunkList`` allocator is out of memory.
  [[nodiscard]] bool DiscardSegment(Index cut_start, Index resume_point);

  /// Returns an iterable over the ``Chunk``s of memory within this ``MultiBuf``.
  auto Chunks();
  auto Chunks() const;

  /// Returns a ``BidirectionalIterator`` over the bytes in this ``MultiBuf``.
  ///
  /// Note that use of this iterator type may be less efficient than
  /// performing chunk-wise operations due to the noncontiguous nature of
  /// the iterator elements.
  auto begin();
  auto end();

  /// Counts the total number of ``Chunk`` s.
  ///
  /// This function is ``O(n)`` in the number of ``Chunk`` s.
  size_t CalculateNumChunks() const;

  /// Counts the total size in bytes of all ``Chunk`` s combined.
  ///
  /// This function is ``O(n)`` in the number of ``Chunk`` s.
  size_t CalculateTotalSize() const;

  /// Returns an ``Index`` which can be used to provide constant-time access to
  /// the element at ``position``.
  ///
  /// Any mutation of this ``MultiBuf`` (e.g. ``Append``, ``DiscardFront``,
  /// ``Slice``, or ``Truncate``) may invalidate this ``Index``.
  Index IndexAt(size_t position) const;
};


class MultiBufAllocationFuture {
 public:
  Poll<std::optional<Buffer>> Poll(Context&);
};

class MultiBufAllocationFuture {
 public:
  Poll<std::optional<MultiBuf::Chunk>> Poll(Context&);
};

class MultiBufAllocator {
 public:
  std::optional<MultiBuf> Allocate(size_t size);
  std::optional<MultiBuf> Allocate(size_t min_size, size_t desired_size);
  std::optional<MultiBuf::Chunk> AllocateContiguous(size_t size);
  std::optional<MultiBuf::Chunk> AllocateContiguous(size_t min_size, size_t desired_size);

  MultiBufAllocationFuture AllocateAsync(size_t size);
  MultiBufAllocationFuture AllocateAsync(size_t min_size, size_t desired_size);
  MultiBufChunkAllocationFuture AllocateContiguousAsync(size_t size);
  MultiBufChunkAllocationFuture AllocateContiguousAsync(size_t min_size, size_t desired_size);

 protected:
  virtual std::optional<MultiBuf> DoAllocate(size_t min_size, size_t desired_size);
  virtual std::optional<MultiBuf::Chunk> DoAllocateContiguous(size_t min_size, size_t desired_size);

  /// Invoked by the ``MultiBufAllocator`` to signal waiting futures that buffers of
  /// the provided sizes may be available for allocation.
  void AllocationAvailable(size_t size_available, size_t contiguous_size_available);
};

The Chunk s in the prototype are stored in fallible dynamically-allocated arrays, but they could be stored in vectors of a fixed maximum size. The Chunk s cannot be stored as an intrusively-linked list because this would require per-Chunk overhead in the underlying buffer, and there may be any number of Chunk s within the same buffer.

Multiple Chunk s may not refer to the same memory, but they may refer to non-overlapping sections of memory within the same region. When one Chunk within a region is deallocated, a neighboring chunk may expand to use its space.

Vectorized Structure

The most significant design choices made in this API is supporting vectorized IO via a list of Chunk s. While this does carry an additional overhead, it is strongly motivated by the desire to carry data throughout the communications stack without needing a copy. By carrying a list of Chunk s, MultiBuf allows data to be prefixed, suffixed, infixed, or split without incurring the overhead of a separate allocation and copy.

Managing Allocations with MultiBufAllocator and ChunkRegionTracker

pw::MultiBuf is not associated with a concrete allocator implementation. Instead, it delegates the creation of buffers to implementations of the MultiBufAllocator base class. This allows different allocator implementations to vend out pw::MultiBuf s that are optimized for use with a particular communications stack.

For example, a communications stack which runs off of shared memory or specific DMA’able regions might choose to allocate memory out of those regions to allow for zero-copy writes.

Additionally, custom allocator implementations can reserve headers, footers, or even split a pw::MultiBuf into multiple chunks in order to provide zero-copy fragmentation.

Deallocation of these regions is managed through the ChunkRegionTracker. When no more Chunk s associated with a ChunkRegionTracker exist, the ChunkRegionTracker receives a Destroy call to release both the region and the ChunkRegionTracker.

The MultiBufAllocator can place the ChunkRegionTracker wherever it wishes, including as a header or footer for the underlying region allocation. This is not required, however, as memory in regions like shared or DMA’able memory might be limited, in which case the ChunkRegionTracker can be stored elsewhere.

Compatibility With Existing Communications Interfaces

lwIP

Lightweight IP stack (lwIP) is a TCP/IP implementation designed for use on small embedded systems. Some Pigweed platforms may choose to use lwIP as the backend for their Socket implementations, so it’s important that pw::MultiBuf interoperate efficiently with their native buffer type.

lwIP has its own buffer type, pbuf optimized for use with zero-copy applications. pbuf represents buffers as linked lists of reference-counted pbuf s which each have a pointer to their payload, a length, and some metadata. While this does not precisely match the representation of pw::MultiBuf, it is possible to construct a pbuf list which points at the various chunks of a pw::MultiBuf without needing to perform a copy of the data.

Similarly, a pw::MultiBuf can refer to a pbuf by using each pbuf as a “Chunk region”, holding a reference count on the region’s pbuf so long as any Chunk continues to reference the data referenced by that buffer.

Existing Solutions

Linux’s sk_buff

Linux has a similar buffer structure called sk_buff. It differs in a few significant ways:

It provides separate head, data, tail, and end pointers. Other scatter-gather fragments are supplied using the frags[] structure.

Separately, it has a frags_list of IP fragments which is created via calls to ip_push_pending_frames. Fragments are supplied by the frags[] payload in addition to the skb_shared_info.frag_list pointing to the tail.

sk_buff reference-counts not only the underlying chunks of memory, but also the sk_buff structure itself. This allows for clones of sk_buff without manipulating the reference counts of the individual chunks. We anticipate that cloning a whole pw::buffers::MultiBuf will be uncommon enough that it is better to keep these structures single-owner (and mutable) rather than sharing and reference-counting them.

FreeBSD’s mbuf

FreeBSD uses a design called mbuf which interestingly allows data within the middle of a buffer to be given a specified alignment, allowing data to be prepended within the same buffer. This is similar to the structure of Chunk, which may reference data in the middle of an allocated buffer, allowing prepending without a copy.