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我试图找出特定情况下的最佳容量和负载系数。我想我明白了它的要点,但我仍然会感谢比我更博学的人的确认。 :)
如果我知道我的 HashMap 将填满以包含 100 个对象,并且大部分时间将花费 100 个对象,那么我猜测最佳值是初始容量 100 和负载因子 1?或者我需要容量 101,还是有其他问题?
编辑:好的,我留出了几个小时并做了一些测试。结果如下:
import java.util.HashMap;
import java.util.Map;
public class HashMapTest {
// capacity - numbers high as 10000000 require -mx1536m -ms1536m JVM parameters
public static final int CAPACITY = 10000000;
public static final int ITERATIONS = 10000;
// set to false to print put performance, or to true to print get performance
boolean doIterations = false;
private Map<Integer, String> cache;
public void fillCache(int capacity) {
long t = System.currentTimeMillis();
for (int i = 0; i <= capacity; i++)
cache.put(i, "Value number " + i);
if (!doIterations) {
System.out.print(System.currentTimeMillis() - t);
System.out.print("\t");
}
}
public void iterate(int capacity) {
long t = System.currentTimeMillis();
for (int i = 0; i <= ITERATIONS; i++) {
long x = Math.round(Math.random() * capacity);
String result = cache.get((int) x);
}
if (doIterations) {
System.out.print(System.currentTimeMillis() - t);
System.out.print("\t");
}
}
public void test(float loadFactor, int divider) {
for (int i = 10000; i <= CAPACITY; i+= 10000) {
cache = new HashMap<Integer, String>(i, loadFactor);
fillCache(i / divider);
if (doIterations)
iterate(i / divider);
}
System.out.println();
}
public static void main(String[] args) {
HashMapTest test = new HashMapTest();
// fill to capacity
test.test(0.75f, 1);
test.test(1, 1);
test.test(1.25f, 1);
// fill to half capacity
test.test(0.75f, 2);
test.test(1, 2);
test.test(1.25f, 2);
}
}
最佳答案
好的,为了让这件事停下来,我创建了一个测试应用程序来运行几个场景并获得一些结果的可视化。以下是测试的完成方式:
equals
方法只使用 ID,所以没有键映射覆盖另一个。 Object
。 package hashmaptest;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
public class HashMapTest {
private static final List<Result> results = new ArrayList<Result>();
public static void main(String[] args) throws IOException {
//First entry of each array is the sample collection size, subsequent entries
//are the hash limits
final int[][] sampleSizesAndHashLimits = new int[][] {
{100, 50, 90, 100},
{1000, 500, 900, 990, 1000},
{100000, 10000, 90000, 99000, 100000}
};
final double[] initialCapacityFactors = new double[] {0.5, 0.75, 1.0, 1.25, 1.5, 2.0};
final float[] loadFactors = new float[] {0.5f, 0.75f, 1.0f, 1.25f};
//Doing a warmup run to eliminate JIT influence
for(int[] sizeAndLimits : sampleSizesAndHashLimits) {
int size = sizeAndLimits[0];
for(int i = 1; i < sizeAndLimits.length; ++i) {
int limit = sizeAndLimits[i];
for(double initCapacityFactor : initialCapacityFactors) {
for(float loadFactor : loadFactors) {
runTest(limit, size, initCapacityFactor, loadFactor);
}
}
}
}
results.clear();
//Now for the real thing...
for(int[] sizeAndLimits : sampleSizesAndHashLimits) {
int size = sizeAndLimits[0];
for(int i = 1; i < sizeAndLimits.length; ++i) {
int limit = sizeAndLimits[i];
for(double initCapacityFactor : initialCapacityFactors) {
for(float loadFactor : loadFactors) {
runTest(limit, size, initCapacityFactor, loadFactor);
}
}
}
}
Collections.sort(results);
for(final Result result : results) {
result.printSummary();
}
// ResultVisualizer.visualizeResults(results);
}
private static void runTest(final int hashLimit, final int sampleSize,
final double initCapacityFactor, final float loadFactor) {
final int initialCapacity = (int)(sampleSize * initCapacityFactor);
System.out.println("Running test for a sample collection of size " + sampleSize
+ ", an initial capacity of " + initialCapacity + ", a load factor of "
+ loadFactor + " and keys with a hash code limited to " + hashLimit);
System.out.println("====================");
double hashOverload = (((double)sampleSize/hashLimit) - 1.0) * 100.0;
System.out.println("Hash code overload: " + hashOverload + "%");
//Generating our sample key collection.
final List<Key> keys = generateSamples(hashLimit, sampleSize);
//Generating our value collection
final List<Object> values = generateValues(sampleSize);
final HashMap<Key, Object> map = new HashMap<Key, Object>(initialCapacity, loadFactor);
final long startPut = System.nanoTime();
for(int i = 0; i < sampleSize; ++i) {
map.put(keys.get(i), values.get(i));
}
final long endPut = System.nanoTime();
final long putTime = endPut - startPut;
final long averagePutTime = putTime/(sampleSize/10);
System.out.println("Time to map all keys to their values: " + putTime + " ns");
System.out.println("Average put time per 10 entries: " + averagePutTime + " ns");
final long startGet = System.nanoTime();
for(int i = 0; i < sampleSize; ++i) {
map.get(keys.get(i));
}
final long endGet = System.nanoTime();
final long getTime = endGet - startGet;
final long averageGetTime = getTime/(sampleSize/10);
System.out.println("Time to get the value for every key: " + getTime + " ns");
System.out.println("Average get time per 10 entries: " + averageGetTime + " ns");
System.out.println("");
final Result result =
new Result(sampleSize, initialCapacity, loadFactor, hashOverload, averagePutTime, averageGetTime, hashLimit);
results.add(result);
//Haha, what kind of noob explicitly calls for garbage collection?
System.gc();
try {
Thread.sleep(200);
} catch(final InterruptedException e) {}
}
private static List<Key> generateSamples(final int hashLimit, final int sampleSize) {
final ArrayList<Key> result = new ArrayList<Key>(sampleSize);
for(int i = 0; i < sampleSize; ++i) {
result.add(new Key(i, hashLimit));
}
return result;
}
private static List<Object> generateValues(final int sampleSize) {
final ArrayList<Object> result = new ArrayList<Object>(sampleSize);
for(int i = 0; i < sampleSize; ++i) {
result.add(new Object());
}
return result;
}
private static class Key {
private final int hashCode;
private final int id;
Key(final int id, final int hashLimit) {
//Equals implies same hashCode if limit is the same
//Same hashCode doesn't necessarily implies equals
this.id = id;
this.hashCode = id % hashLimit;
}
@Override
public int hashCode() {
return hashCode;
}
@Override
public boolean equals(final Object o) {
return ((Key)o).id == this.id;
}
}
static class Result implements Comparable<Result> {
final int sampleSize;
final int initialCapacity;
final float loadFactor;
final double hashOverloadPercentage;
final long averagePutTime;
final long averageGetTime;
final int hashLimit;
Result(final int sampleSize, final int initialCapacity, final float loadFactor,
final double hashOverloadPercentage, final long averagePutTime,
final long averageGetTime, final int hashLimit) {
this.sampleSize = sampleSize;
this.initialCapacity = initialCapacity;
this.loadFactor = loadFactor;
this.hashOverloadPercentage = hashOverloadPercentage;
this.averagePutTime = averagePutTime;
this.averageGetTime = averageGetTime;
this.hashLimit = hashLimit;
}
@Override
public int compareTo(final Result o) {
final long putDiff = o.averagePutTime - this.averagePutTime;
final long getDiff = o.averageGetTime - this.averageGetTime;
return (int)(putDiff + getDiff);
}
void printSummary() {
System.out.println("" + averagePutTime + " ns per 10 puts, "
+ averageGetTime + " ns per 10 gets, for a load factor of "
+ loadFactor + ", initial capacity of " + initialCapacity
+ " for " + sampleSize + " mappings and " + hashOverloadPercentage
+ "% hash code overload.");
}
}
}
package hashmaptest;
import hashmaptest.HashMapTest.Result;
import java.awt.Color;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.text.DecimalFormat;
import java.text.NumberFormat;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import javax.imageio.ImageIO;
public class ResultVisualizer {
private static final Map<Integer, Map<Integer, Set<Result>>> sampleSizeToHashLimit =
new HashMap<Integer, Map<Integer, Set<Result>>>();
private static final DecimalFormat df = new DecimalFormat("0.00");
static void visualizeResults(final List<Result> results) throws IOException {
final File tempFolder = new File("C:\\temp");
final File baseFolder = makeFolder(tempFolder, "hashmap_tests");
long bestPutTime = -1L;
long worstPutTime = 0L;
long bestGetTime = -1L;
long worstGetTime = 0L;
for(final Result result : results) {
final Integer sampleSize = result.sampleSize;
final Integer hashLimit = result.hashLimit;
final long putTime = result.averagePutTime;
final long getTime = result.averageGetTime;
if(bestPutTime == -1L || putTime < bestPutTime)
bestPutTime = putTime;
if(bestGetTime <= -1.0f || getTime < bestGetTime)
bestGetTime = getTime;
if(putTime > worstPutTime)
worstPutTime = putTime;
if(getTime > worstGetTime)
worstGetTime = getTime;
Map<Integer, Set<Result>> hashLimitToResults =
sampleSizeToHashLimit.get(sampleSize);
if(hashLimitToResults == null) {
hashLimitToResults = new HashMap<Integer, Set<Result>>();
sampleSizeToHashLimit.put(sampleSize, hashLimitToResults);
}
Set<Result> resultSet = hashLimitToResults.get(hashLimit);
if(resultSet == null) {
resultSet = new HashSet<Result>();
hashLimitToResults.put(hashLimit, resultSet);
}
resultSet.add(result);
}
System.out.println("Best average put time: " + bestPutTime + " ns");
System.out.println("Best average get time: " + bestGetTime + " ns");
System.out.println("Worst average put time: " + worstPutTime + " ns");
System.out.println("Worst average get time: " + worstGetTime + " ns");
for(final Integer sampleSize : sampleSizeToHashLimit.keySet()) {
final File sizeFolder = makeFolder(baseFolder, "sample_size_" + sampleSize);
final Map<Integer, Set<Result>> hashLimitToResults =
sampleSizeToHashLimit.get(sampleSize);
for(final Integer hashLimit : hashLimitToResults.keySet()) {
final File limitFolder = makeFolder(sizeFolder, "hash_limit_" + hashLimit);
final Set<Result> resultSet = hashLimitToResults.get(hashLimit);
final Set<Float> loadFactorSet = new HashSet<Float>();
final Set<Integer> initialCapacitySet = new HashSet<Integer>();
for(final Result result : resultSet) {
loadFactorSet.add(result.loadFactor);
initialCapacitySet.add(result.initialCapacity);
}
final List<Float> loadFactors = new ArrayList<Float>(loadFactorSet);
final List<Integer> initialCapacities = new ArrayList<Integer>(initialCapacitySet);
Collections.sort(loadFactors);
Collections.sort(initialCapacities);
final BufferedImage putImage =
renderMap(resultSet, loadFactors, initialCapacities, worstPutTime, bestPutTime, false);
final BufferedImage getImage =
renderMap(resultSet, loadFactors, initialCapacities, worstGetTime, bestGetTime, true);
final String putFileName = "size_" + sampleSize + "_hlimit_" + hashLimit + "_puts.png";
final String getFileName = "size_" + sampleSize + "_hlimit_" + hashLimit + "_gets.png";
writeImage(putImage, limitFolder, putFileName);
writeImage(getImage, limitFolder, getFileName);
}
}
}
private static File makeFolder(final File parent, final String folder) throws IOException {
final File child = new File(parent, folder);
if(!child.exists())
child.mkdir();
return child;
}
private static BufferedImage renderMap(final Set<Result> results, final List<Float> loadFactors,
final List<Integer> initialCapacities, final float worst, final float best,
final boolean get) {
//[x][y] => x is mapped to initial capacity, y is mapped to load factor
final Color[][] map = new Color[initialCapacities.size()][loadFactors.size()];
for(final Result result : results) {
final int x = initialCapacities.indexOf(result.initialCapacity);
final int y = loadFactors.indexOf(result.loadFactor);
final float time = get ? result.averageGetTime : result.averagePutTime;
final float score = (time - best)/(worst - best);
final Color c = new Color(score, 1.0f - score, 0.0f);
map[x][y] = c;
}
final int imageWidth = initialCapacities.size() * 40 + 50;
final int imageHeight = loadFactors.size() * 40 + 50;
final BufferedImage image =
new BufferedImage(imageWidth, imageHeight, BufferedImage.TYPE_3BYTE_BGR);
final Graphics2D g = image.createGraphics();
g.setColor(Color.WHITE);
g.fillRect(0, 0, imageWidth, imageHeight);
for(int x = 0; x < map.length; ++x) {
for(int y = 0; y < map[x].length; ++y) {
g.setColor(map[x][y]);
g.fillRect(50 + x*40, imageHeight - 50 - (y+1)*40, 40, 40);
g.setColor(Color.BLACK);
g.drawLine(25, imageHeight - 50 - (y+1)*40, 50, imageHeight - 50 - (y+1)*40);
final Float loadFactor = loadFactors.get(y);
g.drawString(df.format(loadFactor), 10, imageHeight - 65 - (y)*40);
}
g.setColor(Color.BLACK);
g.drawLine(50 + (x+1)*40, imageHeight - 50, 50 + (x+1)*40, imageHeight - 15);
final int initialCapacity = initialCapacities.get(x);
g.drawString(((initialCapacity%1000 == 0) ? "" + (initialCapacity/1000) + "K" : "" + initialCapacity), 15 + (x+1)*40, imageHeight - 25);
}
g.drawLine(25, imageHeight - 50, imageWidth, imageHeight - 50);
g.drawLine(50, 0, 50, imageHeight - 25);
g.dispose();
return image;
}
private static void writeImage(final BufferedImage image, final File folder,
final String filename) throws IOException {
final File imageFile = new File(folder, filename);
ImageIO.write(image, "png", imageFile);
}
}
HashMap
的实现,否则结果将无处不在。如果有一点需要注意,那就是默认的初始大小 16 对于除了最小的 map 之外的任何东西都有些愚蠢,因此如果您对大小的顺序有任何想法,请使用设置初始大小的构造函数这将是。 关于java - 固定大小的 HashMap 的最佳容量和负载因子是多少?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/7115445/
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