Recycle serialization buffers on transmission

docs/design.md

@@ -434,3 +434,20 @@ Thus, there is no direct implementation of actions in `rmw_zenoh_cpp`.
 ## Security
 
 TBD
+
+## Environment variables
+
+### `RMW_ZENOH_BUFFER_POOL_MAX_SIZE_BYTES`
+
+The RMW recycles serialization buffers on transmission using a buffer pool with bounded memory
+usage.
+These buffers are returned to the pool - without being deallocated - once they cross the
+network boundary in host-to-host communication, or after transmission in inter-process
+communication, or upon being consumed by subscriptions in intra-process communication, etc. 
+
+When the total size of the allocated buffers within the pool exceeds
+`RMW_ZENOH_BUFFER_POOL_MAX_SIZE_BYTES`, serialization buffers are allocated using the system
+allocator and moved to Zenoh; no recycling is performed in this case to prevent the buffer pool from
+growing uncontrollably. 
+
+The default value of `RMW_ZENOH_BUFFER_POOL_MAX_SIZE_BYTES` is 16MB; this value was chosen since it is roughly the size of the cache in a modern CPU.

rmw_zenoh_cpp/CMakeLists.txt

@@ -47,6 +47,7 @@ add_library(rmw_zenoh_cpp SHARED
   src/detail/type_support_common.cpp
   src/detail/zenoh_config.cpp
   src/detail/zenoh_utils.cpp
+  src/detail/buffer_pool.cpp
   src/rmw_event.cpp
   src/rmw_get_network_flow_endpoints.cpp
   src/rmw_get_node_info_and_types.cpp

rmw_zenoh_cpp/src/detail/buffer_pool.cpp

@@ -0,0 +1,104 @@
+// Copyright 2025 Open Source Robotics Foundation, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "buffer_pool.hpp"
+
+#include <cstddef>
+#include <cstdlib>
+#include <mutex>
+#include <vector>
+
+#include "rcutils/allocator.h"
+#include "rcutils/env.h"
+#include "logging_macros.hpp"
+
+namespace rmw_zenoh_cpp
+{
+///=============================================================================
+BufferPool::BufferPool()
+: buffers_(), mutex_()
+{
+  const char * env_value;
+  const char * error_str = rcutils_get_env("RMW_ZENOH_BUFFER_POOL_MAX_SIZE_BYTES", &env_value);
+  if (error_str != nullptr) {
+    RMW_ZENOH_LOG_ERROR_NAMED(
+      "rmw_zenoh_cpp",
+      "Unable to read maximum buffer pool size, falling back to default.");
+    max_size_ = DEFAULT_MAX_SIZE;
+  } else if (strcmp(env_value, "") == 0) {
+    max_size_ = DEFAULT_MAX_SIZE;
+  } else {
+    max_size_ = std::atoll(env_value);
+  }
+  size_ = 0;
+}
+
+///=============================================================================
+BufferPool::~BufferPool()
+{
+  rcutils_allocator_t allocator = rcutils_get_default_allocator();
+
+  for (Buffer & buffer : buffers_) {
+    allocator.deallocate(buffer.data, allocator.state);
+  }
+}
+
+///=============================================================================
+BufferPool::Buffer BufferPool::allocate(size_t size)
+{
+  std::lock_guard<std::mutex> guard(mutex_);
+
+  rcutils_allocator_t allocator = rcutils_get_default_allocator();
+
+  if (buffers_.empty()) {
+    if (size_ + size > max_size_) {
+      return {};
+    } else {
+      size_ += size;
+    }
+    uint8_t * data = static_cast<uint8_t *>(allocator.allocate(size, allocator.state));
+    if (data == nullptr) {
+      return {};
+    } else {
+      return Buffer {data, size};
+    }
+  } else {
+    Buffer buffer = buffers_.back();
+    buffers_.pop_back();
+    if (buffer.size < size) {
+      size_t size_diff = size - buffer.size;
+      if (size_ + size_diff > max_size_) {
+        return {};
+      }
+      uint8_t * data = static_cast<uint8_t *>(allocator.reallocate(
+          buffer.data, size, allocator.state));
+      if (data == nullptr) {
+        return {};
+      }
+      size_ += size_diff;
+      buffer.data = data;
+      buffer.size = size;
+    }
+    return buffer;
+  }
+}
+
+///=============================================================================
+void BufferPool::deallocate(BufferPool::Buffer buffer)
+{
+  std::lock_guard<std::mutex> guard(mutex_);
+  buffers_.push_back(buffer);
+}
+
+}  // namespace rmw_zenoh_cpp

rmw_zenoh_cpp/src/detail/buffer_pool.hpp

@@ -0,0 +1,60 @@
+// Copyright 2025 Open Source Robotics Foundation, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef DETAIL__BUFFER_POOL_HPP_
+#define DETAIL__BUFFER_POOL_HPP_
+
+#include <cstddef>
+#include <cstdlib>
+#include <mutex>
+#include <vector>
+
+#include "rcutils/allocator.h"
+#include "rcutils/env.h"
+#include "logging_macros.hpp"
+
+namespace rmw_zenoh_cpp
+{
+///=============================================================================
+class BufferPool
+{
+public:
+  struct Buffer
+  {
+    uint8_t * data;
+    size_t size;
+  };
+
+  BufferPool();
+
+  ~BufferPool();
+
+  Buffer allocate(size_t size);
+
+  void deallocate(Buffer buffer);
+
+private:
+  std::vector<Buffer> buffers_;
+  std::mutex mutex_;
+  size_t max_size_;
+  size_t size_;
+  // NOTE(fuzzypixelz): Pooled buffers are recycled with the expectation that they would reside in
+  // cache, thus this this value should be comparable to the size of a modern CPU cache. The default
+  // value (16 MiB) is relatively conservative as CPU cache sizes range from a few MiB to a few
+  // hundred MiB.
+  const size_t DEFAULT_MAX_SIZE = 16 * 1024 * 1024;
+};
+}  // namespace rmw_zenoh_cpp
+
+#endif  // DETAIL__BUFFER_POOL_HPP_

rmw_zenoh_cpp/src/detail/rmw_context_impl_s.hpp

@@ -24,6 +24,7 @@
 
 #include "graph_cache.hpp"
 #include "rmw_node_data.hpp"
+#include "buffer_pool.hpp"
 
 #include "rmw/ret_types.h"
 #include "rmw/types.h"
@@ -92,6 +93,9 @@ struct rmw_context_impl_s final
   // Forward declaration
   class Data;
 
+  // Pool of serialization buffers.
+  rmw_zenoh_cpp::BufferPool serialization_buffer_pool;
+
 private:
   std::shared_ptr<Data> data_{nullptr};
 };

rmw_zenoh_cpp/src/detail/rmw_publisher_data.cpp

@@ -20,6 +20,7 @@
 #include <cinttypes>
 #include <memory>
 #include <mutex>
+#include <optional>
 #include <string>
 #include <utility>
 #include <vector>
@@ -216,24 +217,33 @@ rmw_ret_t PublisherData::publish(
     type_support_impl_);
 
   // To store serialized message byte array.
-  char * msg_bytes = nullptr;
-
-  rcutils_allocator_t * allocator = &rmw_node_->context->options.allocator;
+  uint8_t * msg_bytes = nullptr;
+
+  rmw_context_impl_s *context_impl = static_cast<rmw_context_impl_s *>(rmw_node_->data);
+
+  rcutils_allocator_t allocator = rcutils_get_default_allocator();
+
+  // Try to get memory from the serialization buffer pool.
+  BufferPool::Buffer serialization_buffer =
+    context_impl->serialization_buffer_pool.allocate(max_data_length);
+  if (serialization_buffer.data == nullptr) {
+    void * data = allocator.allocate(max_data_length, allocator.state);
+    RMW_CHECK_FOR_NULL_WITH_MSG(
+      data, "failed to allocate serialization buffer", return RMW_RET_BAD_ALLOC);
+    msg_bytes = static_cast<uint8_t *>(data);
+  } else {
+    msg_bytes = serialization_buffer.data;
+  }
 
   auto always_free_msg_bytes = rcpputils::make_scope_exit(
-    [&msg_bytes, allocator]() {
-      if (msg_bytes) {
-        allocator->deallocate(msg_bytes, allocator->state);
+    [&msg_bytes, &allocator, &serialization_buffer]() {
+      if (serialization_buffer.data == nullptr) {
+        allocator.deallocate(msg_bytes, allocator.state);
       }
     });
 
-  // Get memory from the allocator.
-  msg_bytes = static_cast<char *>(allocator->allocate(max_data_length, allocator->state));
-  RMW_CHECK_FOR_NULL_WITH_MSG(
-    msg_bytes, "bytes for message is null", return RMW_RET_BAD_ALLOC);
-
   // Object that manages the raw buffer
-  eprosima::fastcdr::FastBuffer fastbuffer(msg_bytes, max_data_length);
+  eprosima::fastcdr::FastBuffer fastbuffer(reinterpret_cast<char *>(msg_bytes), max_data_length);
 
   // Object that serializes the data
   rmw_zenoh_cpp::Cdr ser(fastbuffer);
@@ -258,10 +268,19 @@ rmw_ret_t PublisherData::publish(
     sequence_number_++, source_timestamp, entity_->copy_gid()).serialize_to_zbytes();
 
   // TODO(ahcorde): shmbuf
-  std::vector<uint8_t> raw_data(
-    reinterpret_cast<const uint8_t *>(msg_bytes),
-    reinterpret_cast<const uint8_t *>(msg_bytes) + data_length);
-  zenoh::Bytes payload(std::move(raw_data));
+  zenoh::Bytes payload;
+  if (serialization_buffer.data == nullptr) {
+    std::vector<uint8_t> raw_data(
+      reinterpret_cast<const uint8_t *>(msg_bytes),
+      reinterpret_cast<const uint8_t *>(msg_bytes) + data_length);
+    payload = zenoh::Bytes(std::move(raw_data));
+  } else {
+    auto deleter = [buffer_pool = &context_impl->serialization_buffer_pool,
+        buffer = serialization_buffer](uint8_t *){
+        buffer_pool->deallocate(buffer);
+      };
+    payload = zenoh::Bytes(msg_bytes, data_length, deleter);
+  }
 
   TRACETOOLS_TRACEPOINT(
     rmw_publish, static_cast<const void *>(rmw_publisher_), ros_message, source_timestamp);