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[api] Eliminate heap allocation in process_batch_ using stack-allocated PacketInfo array (#9703)

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
This commit is contained in:
J. Nick Koston 2025-07-20 12:24:57 -10:00 committed by GitHub
parent e474a33abd
commit 5511d61dba
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2 changed files with 30 additions and 9 deletions
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27
esphome/components/api/api_connection.cpp
View File

@ -1671,6 +1671,10 @@ ProtoWriteBuffer APIConnection::allocate_batch_message_buffer(uint16_t size) {
}
void APIConnection::process_batch_() {
// Ensure PacketInfo remains trivially destructible for our placement new approach
static_assert(std::is_trivially_destructible<PacketInfo>::value,
"PacketInfo must remain trivially destructible with this placement-new approach");
if (this->deferred_batch_.empty()) {
this->flags_.batch_scheduled = false;
return;
@ -1708,9 +1712,12 @@ void APIConnection::process_batch_() {
return;
}
// Pre-allocate storage for packet info
std::vector<PacketInfo> packet_info;
packet_info.reserve(num_items);
size_t packets_to_process = std::min(num_items, MAX_PACKETS_PER_BATCH);
// Stack-allocated array for packet info
alignas(PacketInfo) char packet_info_storage[MAX_PACKETS_PER_BATCH * sizeof(PacketInfo)];
PacketInfo *packet_info = reinterpret_cast<PacketInfo *>(packet_info_storage);
size_t packet_count = 0;
// Cache these values to avoid repeated virtual calls
const uint8_t header_padding = this->helper_->frame_header_padding();
@ -1742,8 +1749,8 @@ void APIConnection::process_batch_() {
// The actual message data follows after the header padding
uint32_t current_offset = 0;
// Process items and encode directly to buffer
for (size_t i = 0; i < this->deferred_batch_.size(); i++) {
// Process items and encode directly to buffer (up to our limit)
for (size_t i = 0; i < packets_to_process; i++) {
const auto &item = this->deferred_batch_[i];
// Try to encode message
// The creator will calculate overhead to determine if the message fits
@ -1757,7 +1764,11 @@ void APIConnection::process_batch_() {
// Message was encoded successfully
// payload_size is header_padding + actual payload size + footer_size
uint16_t proto_payload_size = payload_size - header_padding - footer_size;
packet_info.emplace_back(item.message_type, current_offset, proto_payload_size);
// Use placement new to construct PacketInfo in pre-allocated stack array
// This avoids default-constructing all MAX_PACKETS_PER_BATCH elements
// Explicit destruction is not needed because PacketInfo is trivially destructible,
// as ensured by the static_assert in its definition.
new (&packet_info[packet_count++]) PacketInfo(item.message_type, current_offset, proto_payload_size);
// Update tracking variables
items_processed++;
@ -1783,8 +1794,8 @@ void APIConnection::process_batch_() {
}
// Send all collected packets
APIError err =
this->helper_->write_protobuf_packets(ProtoWriteBuffer{&this->parent_->get_shared_buffer_ref()}, packet_info);
APIError err = this->helper_->write_protobuf_packets(ProtoWriteBuffer{&this->parent_->get_shared_buffer_ref()},
std::span<const PacketInfo>(packet_info, packet_count));
if (err != APIError::OK && err != APIError::WOULD_BLOCK) {
on_fatal_error();
ESP_LOGW(TAG, "%s: Batch write failed %s errno=%d", this->get_client_combined_info().c_str(), api_error_to_str(err),

12
esphome/components/api/api_connection.h
View File

@ -19,7 +19,17 @@ namespace api {
// Keepalive timeout in milliseconds
static constexpr uint32_t KEEPALIVE_TIMEOUT_MS = 60000;
// Maximum number of entities to process in a single batch during initial state/info sending
static constexpr size_t MAX_INITIAL_PER_BATCH = 20;
// This was increased from 20 to 24 after removing the unique_id field from entity info messages,
// which reduced message sizes allowing more entities per batch without exceeding packet limits
static constexpr size_t MAX_INITIAL_PER_BATCH = 24;
// Maximum number of packets to process in a single batch (platform-dependent)
// This limit exists to prevent stack overflow from the PacketInfo array in process_batch_
// Each PacketInfo is 8 bytes, so 64 * 8 = 512 bytes, 32 * 8 = 256 bytes
#if defined(USE_ESP32) || defined(USE_HOST)
static constexpr size_t MAX_PACKETS_PER_BATCH = 64; // ESP32 has 8KB+ stack, HOST has plenty
#else
static constexpr size_t MAX_PACKETS_PER_BATCH = 32; // ESP8266/RP2040/etc have smaller stacks
#endif
class APIConnection : public APIServerConnection {
public: