lib: optimize FixedQueue by reusing one segment

benchmark/util/fixed-queue.js

@@ -0,0 +1,18 @@
+'use strict';
+
+const common = require('../common');
+
+const bench = common.createBenchmark(main, {
+  n: [1e5],
+}, { flags: ['--expose-internals'] });
+
+function main({ n }) {
+  const FixedQueue = require('internal/fixed_queue');
+  const queue = new FixedQueue();
+  bench.start();
+  for (let i = 0; i < n; i++)
+    queue.push(i);
+  for (let i = 0; i < n; i++)
+    queue.shift();
+  bench.end(n);
+}

lib/internal/fixed_queue.js

@@ -49,6 +49,11 @@ const kMask = kSize - 1;
 // | undefined |                               |   item    |
 // +-----------+                               +-----------+
 //
+// Implementation detail: To reduce allocations, a single spare
+// FixedCircularBuffer may be kept for reuse. When a segment empties, it
+// is stored as the spare, and when the head fills, the spare is linked
+// in. This does not change the active list structure or queue semantics.
+//
 // Adding a value means moving `top` forward by one, removing means
 // moving `bottom` forward by one. After reaching the end, the queue
 // wraps around.
@@ -74,36 +79,46 @@ class FixedCircularBuffer {
   }
 
   push(data) {
-    this.list[this.top] = data;
-    this.top = (this.top + 1) & kMask;
+    const top = this.top;
+    this.list[top] = data;
+    this.top = (top + 1) & kMask;
   }
 
   shift() {
-    const nextItem = this.list[this.bottom];
+    const bottom = this.bottom;
+    const nextItem = this.list[bottom];
     if (nextItem === undefined)
       return null;
-    this.list[this.bottom] = undefined;
-    this.bottom = (this.bottom + 1) & kMask;
+    this.list[bottom] = undefined;
+    this.bottom = (bottom + 1) & kMask;
     return nextItem;
   }
 }
 
 module.exports = class FixedQueue {
   constructor() {
     this.head = this.tail = new FixedCircularBuffer();
+    this._spare = null;
   }
 
   isEmpty() {
     return this.head.isEmpty();
   }
 
   push(data) {
-    if (this.head.isFull()) {
-      // Head is full: Creates a new queue, sets the old queue's `.next` to it,
-      // and sets it as the new main queue.
-      this.head = this.head.next = new FixedCircularBuffer();
+    let head = this.head;
+    if (head.isFull()) {
+      // Head is full: reuse the spare buffer if available.
+      // Otherwise create a new one. Link it and advance head.
+      let nextBuf = this._spare;
+      if (nextBuf !== null) {
+        this._spare = null;
+      } else {
+        nextBuf = new FixedCircularBuffer();
+      }
+      this.head = head = head.next = nextBuf;
     }
-    this.head.push(data);
+    head.push(data);
   }
 
   shift() {
@@ -112,7 +127,8 @@ module.exports = class FixedQueue {
     if (tail.isEmpty() && tail.next !== null) {
       // If there is another queue, it forms the new tail.
       this.tail = tail.next;
-      tail.next = null;
+      // Overwrite any existing spare
+      this._spare = tail;
     }
     return next;
   }