From 1ced82cc812b830cf755d2d300351ea92a0dc9a2 Mon Sep 17 00:00:00 2001
From: Aaron Myers <atm@apache.org>
Date: Sat, 29 Sep 2012 00:59:04 +0000
Subject: [PATCH] HADOOP-8866. SampleQuantiles#query is O(N^2) instead of O(N).
 Contributed by Andrew Wang.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1391711 13f79535-47bb-0310-9956-ffa450edef68
---
 hadoop-common-project/hadoop-common/CHANGES.txt            | 3 +++
 .../org/apache/hadoop/metrics2/util/SampleQuantiles.java   | 7 +++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/hadoop-common-project/hadoop-common/CHANGES.txt b/hadoop-common-project/hadoop-common/CHANGES.txt
index 979b9410fc..40970210d0 100644
--- a/hadoop-common-project/hadoop-common/CHANGES.txt
+++ b/hadoop-common-project/hadoop-common/CHANGES.txt
@@ -280,6 +280,9 @@ Release 2.0.3-alpha - Unreleased
 
   OPTIMIZATIONS
 
+    HADOOP-8866. SampleQuantiles#query is O(N^2) instead of O(N). (Andrew Wang
+    via atm)
+
   BUG FIXES
 
     HADOOP-8795. BASH tab completion doesn't look in PATH, assumes path to
diff --git a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/util/SampleQuantiles.java b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/util/SampleQuantiles.java
index c0b14cc21e..97685b4fe4 100644
--- a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/util/SampleQuantiles.java
+++ b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/util/SampleQuantiles.java
@@ -210,9 +210,12 @@ private long query(double quantile) throws IOException {
     int rankMin = 0;
     int desired = (int) (quantile * count);
 
+    ListIterator<SampleItem> it = samples.listIterator();
+    SampleItem prev = null;
+    SampleItem cur = it.next();
     for (int i = 1; i < samples.size(); i++) {
-      SampleItem prev = samples.get(i - 1);
-      SampleItem cur = samples.get(i);
+      prev = cur;
+      cur = it.next();
 
       rankMin += prev.g;