http: implement slab allocation for HTTP header parsing

PR-URL: #61375 Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com> Reviewed-By: Anna Henningsen <anna@addaleax.net> Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
aduh95 · Feb 10, 2026 · Jan 28, 2026 · Feb 2, 2026 · Feb 2, 2026 · Feb 2, 2026
commit 9383ac4ab7c9f981d2bcdcdfbc8016de9888b42b
diff --git a/benchmark/http/bench-parser-fragmented.js b/benchmark/http/bench-parser-fragmented.js
@@ -0,0 +1,67 @@
+'use strict';
+
+const common = require('../common');
+
+const bench = common.createBenchmark(main, {
+  len: [8, 16],
+  frags: [2, 4, 8],
+  n: [1e5],
+}, {
+  flags: ['--expose-internals', '--no-warnings'],
+});
+
+function main({ len, frags, n }) {
+  const { HTTPParser } = common.binding('http_parser');
+  const REQUEST = HTTPParser.REQUEST;
+  const kOnHeaders = HTTPParser.kOnHeaders | 0;
+  const kOnHeadersComplete = HTTPParser.kOnHeadersComplete | 0;
+  const kOnBody = HTTPParser.kOnBody | 0;
+  const kOnMessageComplete = HTTPParser.kOnMessageComplete | 0;
+
+  function processHeaderFragmented(fragments, n) {
+    const parser = newParser(REQUEST);
+
+    bench.start();
+    for (let i = 0; i < n; i++) {
+      // Send header in fragments
+      for (const frag of fragments) {
+        parser.execute(frag, 0, frag.length);
+      }
+      parser.initialize(REQUEST, {});
+    }
+    bench.end(n);
+  }
+
+  function newParser(type) {
+    const parser = new HTTPParser();
+    parser.initialize(type, {});
+
+    parser.headers = [];
+
+    parser[kOnHeaders] = function() { };
+    parser[kOnHeadersComplete] = function() { };
+    parser[kOnBody] = function() { };
+    parser[kOnMessageComplete] = function() { };
+
+    return parser;
+  }
+
+  // Build the header
+  let header = `GET /hello HTTP/1.1\r\nContent-Type: text/plain\r\n`;
+
+  for (let i = 0; i < len; i++) {
+    header += `X-Filler${i}: ${Math.random().toString(36).substring(2)}\r\n`;
+  }
+  header += '\r\n';
+
+  // Split header into fragments
+  const headerBuf = Buffer.from(header);
+  const fragSize = Math.ceil(headerBuf.length / frags);
+  const fragments = [];
+
+  for (let i = 0; i < headerBuf.length; i += fragSize) {
+    fragments.push(headerBuf.slice(i, Math.min(i + fragSize, headerBuf.length)));
+  }
+
+  processHeaderFragmented(fragments, n);
+}
diff --git a/src/node_http_parser.cc b/src/node_http_parser.cc
@@ -122,72 +122,116 @@ class BindingData : public BaseObject {
   SET_MEMORY_INFO_NAME(BindingData)
 };
 
-// helper class for the Parser
-struct StringPtr {
-  StringPtr() {
-    on_heap_ = false;
-    Reset();
-  }
+class Parser;
+
+class StringPtrAllocator {
+ public:
+  // Memory impact: ~8KB per parser (66 StringPtr × 128 bytes).
+  static constexpr size_t kSlabSize = 8192;
 
+  StringPtrAllocator() = default;
 
-  ~StringPtr() {
-    Reset();
+  // Allocate memory from the slab. Returns nullptr if full.
+  char* TryAllocate(size_t size) {
+    if (length_ + size > kSlabSize) {
+      return nullptr;
+    }
+    char* ptr = buffer_ + length_;
+    length_ += size;
+    return ptr;
+  }
+
+  // Check if pointer is within this allocator's buffer.
+  bool Contains(const char* ptr) const {
+    return ptr >= buffer_ && ptr < buffer_ + kSlabSize;
   }
 
+  // Reset allocator for new message.
+  void Reset() { length_ = 0; }
 
-  // If str_ does not point to a heap string yet, this function makes it do
+ private:
+  char buffer_[kSlabSize];
+  size_t length_ = 0;
+};
+
+struct StringPtr {
+  StringPtr() = default;
+  ~StringPtr() { Reset(); }
+
+  StringPtr(const StringPtr&) = delete;
+  StringPtr& operator=(const StringPtr&) = delete;
+
+  // If str_ does not point to owned storage yet, this function makes it do
   // so. This is called at the end of each http_parser_execute() so as not
   // to leak references. See issue #2438 and test-http-parser-bad-ref.js.
-  void Save() {
-    if (!on_heap_ && size_ > 0) {
-      char* s = new char[size_];
-      memcpy(s, str_, size_);
-      str_ = s;
-      on_heap_ = true;
+  void Save(StringPtrAllocator* allocator) {
+    if (str_ == nullptr || on_heap_ ||
+        (allocator != nullptr && allocator->Contains(str_))) {
+      return;
     }
+    // Try allocator first, fall back to heap
+    if (allocator != nullptr) {
+      char* ptr = allocator->TryAllocate(size_);
+      if (ptr != nullptr) {
+        memcpy(ptr, str_, size_);
+        str_ = ptr;
+        return;
+      }
+    }
+    char* s = new char[size_];
+    memcpy(s, str_, size_);
+    str_ = s;
+    on_heap_ = true;
   }
 
-
   void Reset() {
     if (on_heap_) {
       delete[] str_;
       on_heap_ = false;
     }
-
     str_ = nullptr;
     size_ = 0;
   }
 
-
-  void Update(const char* str, size_t size) {
+  void Update(const char* str, size_t size, StringPtrAllocator* allocator) {
     if (str_ == nullptr) {
       str_ = str;
-    } else if (on_heap_ || str_ + size_ != str) {
-      // Non-consecutive input, make a copy on the heap.
-      // TODO(bnoordhuis) Use slab allocation, O(n) allocs is bad.
-      char* s = new char[size_ + size];
-      memcpy(s, str_, size_);
-      memcpy(s + size_, str, size);
-
-      if (on_heap_)
-        delete[] str_;
-      else
-        on_heap_ = true;
+    } else if (on_heap_ ||
+               (allocator != nullptr && allocator->Contains(str_)) ||
+               str_ + size_ != str) {
+      // Non-consecutive input, make a copy
+      const size_t new_size = size_ + size;
+      char* new_str = nullptr;
+
+      // Try allocator first (if not already on heap)
+      if (!on_heap_ && allocator != nullptr) {
+        new_str = allocator->TryAllocate(new_size);
+      }
 
-      str_ = s;
+      if (new_str != nullptr) {
+        memcpy(new_str, str_, size_);
+        memcpy(new_str + size_, str, size);
+        str_ = new_str;
+      } else {
+        // Fall back to heap
+        char* s = new char[new_size];
+        memcpy(s, str_, size_);
+        memcpy(s + size_, str, size);
+        if (on_heap_) delete[] str_;
+        str_ = s;
+        on_heap_ = true;
+      }
     }
     size_ += size;
   }
 
-
   Local<String> ToString(Environment* env) const {
     if (size_ != 0)
       return OneByteString(env->isolate(), str_, size_);
     else
       return String::Empty(env->isolate());
   }
 
-
   // Strip trailing OWS (SPC or HTAB) from string.
   Local<String> ToTrimmedString(Environment* env) {
     while (size_ > 0 && IsOWS(str_[size_ - 1])) {
@@ -196,14 +240,11 @@ struct StringPtr {
     return ToString(env);
   }
 
-
-  const char* str_;
-  bool on_heap_;
-  size_t size_;
+  const char* str_ = nullptr;
+  bool on_heap_ = false;
+  size_t size_ = 0;
 };
 
-class Parser;
-
 struct ParserComparator {
   bool operator()(const Parser* lhs, const Parser* rhs) const;
 };
@@ -259,8 +300,7 @@ class Parser : public AsyncWrap, public StreamListener {
       : AsyncWrap(binding_data->env(), wrap),
         current_buffer_len_(0),
         current_buffer_data_(nullptr),
-        binding_data_(binding_data) {
-  }
+        binding_data_(binding_data) {}
 
   SET_NO_MEMORY_INFO()
   SET_MEMORY_INFO_NAME(Parser)
@@ -278,6 +318,7 @@ class Parser : public AsyncWrap, public StreamListener {
     headers_completed_ = false;
     chunk_extensions_nread_ = 0;
     last_message_start_ = uv_hrtime();
+    allocator_.Reset();
     url_.Reset();
     status_message_.Reset();
 
@@ -308,7 +349,7 @@ class Parser : public AsyncWrap, public StreamListener {
       return rv;
     }
 
-    url_.Update(at, length);
+    url_.Update(at, length, &allocator_);
     return 0;
   }
 
@@ -319,7 +360,7 @@ class Parser : public AsyncWrap, public StreamListener {
       return rv;
     }
 
-    status_message_.Update(at, length);
+    status_message_.Update(at, length, &allocator_);
     return 0;
   }
 
@@ -345,7 +386,7 @@ class Parser : public AsyncWrap, public StreamListener {
     CHECK_LT(num_fields_, kMaxHeaderFieldsCount);
     CHECK_EQ(num_fields_, num_values_ + 1);
 
-    fields_[num_fields_ - 1].Update(at, length);
+    fields_[num_fields_ - 1].Update(at, length, &allocator_);
 
     return 0;
   }
@@ -366,7 +407,7 @@ class Parser : public AsyncWrap, public StreamListener {
     CHECK_LT(num_values_, arraysize(values_));
     CHECK_EQ(num_values_, num_fields_);
 
-    values_[num_values_ - 1].Update(at, length);
+    values_[num_values_ - 1].Update(at, length, &allocator_);
 
     return 0;
   }
@@ -594,15 +635,15 @@ class Parser : public AsyncWrap, public StreamListener {
   }
 
   void Save() {
-    url_.Save();
-    status_message_.Save();
+    url_.Save(&allocator_);
+    status_message_.Save(&allocator_);
 
     for (size_t i = 0; i < num_fields_; i++) {
-      fields_[i].Save();
+      fields_[i].Save(&allocator_);
     }
 
     for (size_t i = 0; i < num_values_; i++) {
-      values_[i].Save();
+      values_[i].Save(&allocator_);
     }
   }
 
@@ -1006,6 +1047,7 @@ class Parser : public AsyncWrap, public StreamListener {
 
 
   llhttp_t parser_;
+  StringPtrAllocator allocator_;             // shared slab for all StringPtrs
   StringPtr fields_[kMaxHeaderFieldsCount];  // header fields
   StringPtr values_[kMaxHeaderFieldsCount];  // header values
   StringPtr url_;