文档章节

JAVA常见问题其一:Why is it faster to process a sorted array than an unsorted array?

-_-struggle
 -_-struggle
发布于 2016/08/08 12:17
字数 865
阅读 3
收藏 0

You are the operator of a junction and you hear a train coming. You have no idea which way it is supposed to go. You stop the train to ask the driver which direction they want. And then you set the switch appropriately.

Trains are heavy and have a lot of inertia. So they take forever to start up and slow down.

Is there a better way? You guess which direction the train will go!

  • If you guessed right, it continues on.
  • If you guessed wrong, the captain will stop, back up, and yell at you to flip the switch. Then it can restart down the other path.

If you guess right every time, the train will never have to stop.
If you guess wrong too often, the train will spend a lot of time stopping, backing up, and restarting.

 

Consider an if-statement:

You are a processor and you see a branch. You have no idea which way it will go. What do you do? You halt execution and wait until the previous instructions are complete. Then you continue down the correct path.

Modern processors are complicated and have long pipelines. So they take forever to "warm up" and "slow down".

Is there a better way? You guess which direction the branch will go!

  • If you guessed right, you continue executing.
  • If you guessed wrong, you need to flush the pipeline and roll back to the branch. Then you can restart down the other path.

If you guess right every time, the execution will never have to stop.
If you guess wrong too often, you spend a lot of time stalling, rolling back, and restarting.

This is branch prediction. I admit it's not the best analogy since the train could just signal the direction with a flag. But in computers, the processor doesn't know which direction a branch will go until the last moment.

So how would you strategically guess to minimize the number of times that the train must back up and go down the other path? You look at the past history! If the train goes left 99% of the time, then you guess left. If it alternates, then you alternate your guesses. If it goes one way every 3 times, you guess the same...

In other words, you try to identify a pattern and follow it. This is more or less how branch predictors work.

Most applications have well-behaved branches. So modern branch predictors will typically achieve >90% hit rates. But when faced with unpredictable branches with no recognizable patterns, branch predictors are virtually useless.

Further reading: "Branch predictor" article on Wikipedia.

 

As hinted from above, the culprit is this if-statement:

if (data[c] >= 128)
    sum += data[c];

Notice that the data is evenly distributed between 0 and 255. When the data is sorted, roughly the first half of the iterations will not enter the if-statement. After that, they will all enter the if-statement.

This is very friendly to the branch predictor since the branch consecutively goes the same direction many times. Even a simple saturating counter will correctly predict the branch except for the few iterations after it switches direction.

Quick visualization:

T = branch taken
N = branch not taken

data[] = 0, 1, 2, 3, 4, ... 126, 127, 128, 129, 130, ... 250, 251, 252, ...
branch = N  N  N  N  N  ...   N    N    T    T    T  ...   T    T    T  ...

       = NNNNNNNNNNNN ... NNNNNNNTTTTTTTTT ... TTTTTTTTTT  (easy to predict)

However, when the data is completely random, the branch predictor is rendered useless because it can't predict random data. Thus there will probably be around 50% misprediction. (no better than random guessing)

data[] = 226, 185, 125, 158, 198, 144, 217, 79, 202, 118,  14, 150, 177, 182, 133, ...
branch =   T,   T,   N,   T,   T,   T,   T,  N,   T,   N,   N,   T,   T,   T,   N  ...

       = TTNTTTTNTNNTTTN ...   (completely random - hard to predict)

So what can be done?

If the compiler isn't able to optimize the branch into a conditional move, you can try some hacks if you are willing to sacrifice readability for performance.

Replace:

if (data[c] >= 128)
    sum += data[c];

with:

int t = (data[c] - 128) >> 31;
sum += ~t & data[c];

This eliminates the branch and replaces it with some bitwise operations.

(Note that this hack is not strictly equivalent to the original if-statement. But in this case, it's valid for all the input values of data[].)

Benchmarks: Core i7 920 @ 3.5 GHz

C++ - Visual Studio 2010 - x64 Release

//  Branch - Random
seconds = 11.777

//  Branch - Sorted
seconds = 2.352

//  Branchless - Random
seconds = 2.564

//  Branchless - Sorted
seconds = 2.587

Java - Netbeans 7.1.1 JDK 7 - x64

//  Branch - Random
seconds = 10.93293813

//  Branch - Sorted
seconds = 5.643797077

//  Branchless - Random
seconds = 3.113581453

//  Branchless - Sorted
seconds = 3.186068823

Observations:

  • With the Branch: There is a huge difference between the sorted and unsorted data.
  • With the Hack: There is no difference between sorted and unsorted data.
  • In the C++ case, the hack is actually a tad slower than with the branch when the data is sorted.

A general rule of thumb is to avoid data-dependent branching in critical loops. (such as in this example)

 

本文转载自:http://stackoverflow.com/questions/11227809/why-is-it-faster-to-process-a-sorted-array-than-an-unsor

共有 人打赏支持
-_-struggle
粉丝 3
博文 210
码字总数 50914
作品 0
苏州
后端工程师
67. Two Sum II - Input array is sorted - LeetCode

Question 167. Two Sum II - Input array is sorted Solution 题目大意:和Two Sum一样,这里给出的数组是有序的 思路:target - nums[i],这样就实现了降维了 Java实现: 别人的实现: https://l...

yysue
08/16
0
0
153. Find Minimum in Rotated Sorted Array-LeetCode

Question 153. Find Minimum in Rotated Sorted Array Solution 题目大意:给一个按增序排列的数组,其中有一段错位了[1,2,3,4,5,6]变成[4,5,6,1,2,3],把1求出来 思路:遍历,如果当前元素比...

yysue
07/07
0
0
bytebuffer vs c++ malloc

Java equivalents of malloc(), new, free() and delete (ctd) Continued from our introduction to memory management operators in C/C++ and Java. A Java equivalent of the malloc() fu......

不道归来
2017/10/23
0
0
MyBatis传入参数与parameterType

传入简单类型 Java代码: MAPPER : 2. 传入List JAVA代码: 单独传入list时,foreach中的collection必须是list,不不管变量的具体名称是什么。比如这里变量名为idList, collection却是list。 ...

为了美好的明天
05/17
0
0
Top 10 Websites for Advanced Level Java Developers

Stackoverflow Stackoverflow.com is probably the most popular website in the programming world. There are millions of good questions and answers. Learning an API or a programming......

perfectspr
2014/12/11
0
0

没有更多内容

加载失败,请刷新页面

加载更多

Spring IOC 之 注册 BeanDefinition

获取 Document 对象后,会根据该对象和 Resource 资源对象调用 registerBeanDefinitions() 方法,开始注册 BeanDefinitions 首先调用 createBeanDefinitionDocumentReader() 方法实例化 Bean...

职业搬砖20年
7分钟前
0
0
Pycahrm左边出现一些bookmark的取消方法

pycharm 左边出一个对号或则一个0,放上去提示Bookmark 取消方法:按F11

鹏灬
10分钟前
0
0
JDK1.8使用Dubbo时需注意

Dubbo自带的很多包都比较旧了,其中的javassist在JDK1.8上运行会报错 而且错误通常比较诡异,javassist是编辑和创建Java字节码的类库,常见的错误会从spring中报出 解决办法: <dependency>...

火龙战士
12分钟前
0
0
Java 对Oracle数据库中的 BLOB类型 插入图片

package test;import java.io.BufferedInputStream;import java.io.FileInputStream;import java.io.PrintStream;import java.sql.Connection;import java.sql.DriverManager;......

青峰Jun19er
14分钟前
0
0
awk复习

10月22日任务 复习 awk调用外部变量 -v参数设置内部变量调用外部变量 [root@test awk]# a=44[root@test awk]# echo "ABCD" | awk -v GET_A=$a '{print GET_A}'44 更复杂的应用: [root@...

robertt15
22分钟前
0
0

没有更多内容

加载失败,请刷新页面

加载更多

返回顶部
顶部