From 55ab1cf8b42c7bf73d636f9de4a5d1b70f7945ed Mon Sep 17 00:00:00 2001
From: Evgeny Medvedev <medvedev1088@gmail.com>
Date: Tue, 6 Mar 2018 01:06:38 +0700
Subject: [PATCH 1/2] Fix OutOfMemory in read stream
 https://github.com/ethereumjs/merkle-patricia-tree/issues/36

---
 baseTrie.js                | 26 +++++++++++++++++------
 prioritizedTaskExecutor.js | 43 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 63 insertions(+), 6 deletions(-)
 create mode 100644 prioritizedTaskExecutor.js

diff --git a/baseTrie.js b/baseTrie.js
index 1e1066f..9080031 100644
--- a/baseTrie.js
+++ b/baseTrie.js
@@ -7,6 +7,7 @@ const ethUtil = require('ethereumjs-util')
 const semaphore = require('semaphore')
 const TrieNode = require('./trieNode')
 const ReadStream = require('./readStream')
+const PrioritizedTaskExecutor = require('./prioritizedTaskExecutor')
 const matchingNibbleLength = require('./util').matchingNibbleLength
 const doKeysMatch = require('./util').doKeysMatch
 const callTogether = require('./util').callTogether
@@ -461,6 +462,11 @@ Trie.prototype._walkTrie = function (root, onNode, onDone) {
     })
   })
 
+  // the maximum pool size should be high enough to utilise the parallelizability of reading nodes from disk and
+  // low enough to utilize the prioritisation of node lookup.
+  var maxPoolSize = 500
+  var taskExecutor = new PrioritizedTaskExecutor(maxPoolSize)
+
   function processNode (nodeRef, node, key, cb) {
     if (!node) return cb()
     if (aborted) return cb()
@@ -490,17 +496,25 @@ Trie.prototype._walkTrie = function (root, onNode, onDone) {
           var keyExtension = childData[0]
           var childRef = childData[1]
           var childKey = key.concat(keyExtension)
-          self._lookupNode(childRef, function (childNode) {
-            processNode(childRef, childNode, childKey, cb)
+          var priority = childKey.length
+          taskExecutor.execute(priority, function (taskCallback) {
+            self._lookupNode(childRef, function (childNode) {
+              taskCallback()
+              processNode(childRef, childNode, childKey, cb)
+            })
           })
         }, cb)
       },
       only: function (childIndex) {
         var childRef = node.getValue(childIndex)
-        self._lookupNode(childRef, function (childNode) {
-          var childKey = key.slice()
-          childKey.push(childIndex)
-          processNode(childRef, childNode, childKey, cb)
+        var childKey = key.slice()
+        childKey.push(childIndex)
+        var priority = childKey.length
+        taskExecutor.execute(priority, function (taskCallback) {
+          self._lookupNode(childRef, function (childNode) {
+            taskCallback()
+            processNode(childRef, childNode, childKey, cb)
+          })
         })
       }
     }
diff --git a/prioritizedTaskExecutor.js b/prioritizedTaskExecutor.js
new file mode 100644
index 0000000..e6457d3
--- /dev/null
+++ b/prioritizedTaskExecutor.js
@@ -0,0 +1,43 @@
+module.exports = PrioritizedTaskExecutor
+
+/**
+ * Executes tasks up to maxPoolSize at a time, other items are put in a priority queue.
+ * @class PrioritizedTaskExecutor
+ * @param {Number} maxPoolSize The maximum size of the pool
+ * @prop {Number} maxPoolSize The maximum size of the pool
+ * @prop {Number} currentPoolSize The current size of the pool
+ * @prop {Array} queue The task queue
+ */
+function PrioritizedTaskExecutor (maxPoolSize) {
+  this.maxPoolSize = maxPoolSize
+  this.currentPoolSize = 0
+  this.queue = []
+}
+
+/**
+ * Executes the task.
+ * @param {Number} priority The priority of the task
+ * @param {Function} task The function that accepts the callback, which must be called upon the task completion.
+ */
+PrioritizedTaskExecutor.prototype.execute = function (priority, task) {
+  var self = this
+
+  if (self.currentPoolSize < self.maxPoolSize) {
+    self.currentPoolSize++
+    task(function () {
+      self.currentPoolSize--
+      if (self.queue.length > 0) {
+        self.queue.sort(function (a, b) {
+          return b.priority - a.priority
+        })
+        var item = self.queue.shift()
+        self.execute(item.priority, item.task)
+      }
+    })
+  } else {
+    self.queue.push({
+      priority: priority,
+      task: task
+    })
+  }
+}

From 6731209be025bcc9585c2b7adcde0d97058a7273 Mon Sep 17 00:00:00 2001
From: Evgeny Medvedev <medvedev1088@gmail.com>
Date: Thu, 8 Mar 2018 04:33:16 +0700
Subject: [PATCH 2/2] Add tests for PrioritizedTaskExecutor

---
 test/prioritizedTaskExecutor.js | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)
 create mode 100644 test/prioritizedTaskExecutor.js

diff --git a/test/prioritizedTaskExecutor.js b/test/prioritizedTaskExecutor.js
new file mode 100644
index 0000000..bdd8e8d
--- /dev/null
+++ b/test/prioritizedTaskExecutor.js
@@ -0,0 +1,23 @@
+const PrioritizedTaskExecutor = require('../prioritizedTaskExecutor.js')
+const tape = require('tape')
+const taskExecutor = new PrioritizedTaskExecutor(2)
+
+tape('prioritized task executor test', function (t) {
+  var tasks = [1, 2, 3, 4]
+  var callbacks = []
+  var executionOrder = []
+  tasks.forEach(function (task) {
+    taskExecutor.execute(task, function (cb) {
+      executionOrder.push(task)
+      callbacks.push(cb)
+    })
+  })
+
+  callbacks.forEach(function (callback) {
+    callback()
+  })
+
+  var expectedExecutionOrder = [1, 2, 4, 3]
+  t.deepEqual(executionOrder, expectedExecutionOrder)
+  t.end()
+})