我假设将实现不同接口的对象分成几个列表，然后对这些列表进行迭代，比将所有对象都转储到单个列表然后通过instanceof进行切换要快得多。例如。这个：
ArrayList<Visible> visibles = new ArrayList<>();
ArrayList<Highlightable> highlightables = new ArrayList<>();
ArrayList<Selectable> selectables = new ArrayList<>();
// populate the lists
// Visible is an interface, Highlightable is also interface (extends Visible),
// Selectable extends Highlightable
// All interfaces have 3 concrete subclasses each,
// to test situations when JVM is not able to optimize too much due to small number of classes
for (Visible e : visibles) {
vsum += e.visibleValue();
}
for (Highlightable e : highlightables) {
vsum += e.visibleValue();
hsum += e.highlightableValue();
}
for (Selectable e : selectables) {
vsum += e.visibleValue();
hsum += e.highlightableValue();
ssum += e.selectableValue();
}
应该比
更快ArrayList<Visible> visibles = new ArrayList<>();
// populate the list
for (Visible e : visibles) {
if (e instanceof Selectable) {
vsum += e.visibleValue();
hsum += ((Selectable) e).highlightableValue();
ssum += ((Selectable) e).selectableValue();
} else if (e instanceof Highlightable) {
vsum += e.visibleValue();
hsum += ((Highlightable) e).highlightableValue();
} else {
vsum += e.visibleValue();
}
}
但是似乎并非如此：
Main.separateLists thrpt 30 1546.898 ± 32.312 ops/s
Main.singleListAndInstanceof thrpt 30 1673.733 ± 29.804 ops/s
我为下面的基准添加了完整的源代码。
instanceof版本更快的原因是什么？即使我们假设isntanceof指令是免费的，但两个版本的性能至少应相等（因为它们仍会添加元素以列出并迭代它们）。
package test;
import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.annotations.Fork;
import org.openjdk.jmh.annotations.Measurement;
import org.openjdk.jmh.annotations.Warmup;
import java.util.ArrayList;
import java.util.Random;
public class Main {
public static void main(String[] args) throws Exception {
org.openjdk.jmh.Main.main(args);
}
@Benchmark
@Warmup(iterations = 5, time = 1)
@Measurement(iterations = 15, time = 1)
@Fork(value = 2)
public static long separateLists() {
ArrayList<Visible> visibles = new ArrayList<>(3_500);
ArrayList<Highlightable> highlightables = new ArrayList<>(3_500);
ArrayList<Selectable> selectables = new ArrayList<>(3_500);
Random random = new Random();
for (int i = 0; i < 10_000; i++) {
switch (random.nextInt(9)) {
case 0:
visibles.add(new Visible1(i));
break;
case 1:
highlightables.add(new Highlightable1(i));
break;
case 2:
selectables.add(new Selectable1(i));
break;
case 3:
visibles.add(new Visible2(i));
break;
case 4:
highlightables.add(new Highlightable2(i));
break;
case 5:
selectables.add(new Selectable2(i));
break;
case 6:
visibles.add(new Visible3(i));
break;
case 7:
highlightables.add(new Highlightable3(i));
break;
case 8:
selectables.add(new Selectable3(i));
break;
}
}
long listSize = visibles.size() + highlightables.size() + selectables.size();
long vsum = 0;
long hsum = 0;
long ssum = 0;
for (Visible e : visibles) {
vsum += e.visibleValue();
}
for (Highlightable e : highlightables) {
vsum += e.visibleValue();
hsum += e.highlightableValue();
}
for (Selectable e : selectables) {
vsum += e.visibleValue();
hsum += e.highlightableValue();
ssum += e.selectableValue();
}
return listSize + vsum * hsum * ssum;
}
@Benchmark
@Warmup(iterations = 5, time = 1)
@Measurement(iterations = 15, time = 1)
@Fork(value = 2)
public static long singleListAndInstanceof() {
ArrayList<Visible> visibles = new ArrayList<>(10_000);
Random random = new Random();
for (int i = 0; i < 10_000; i++) {
switch (random.nextInt(9)) {
case 0:
visibles.add(new Visible1(i));
break;
case 1:
visibles.add(new Highlightable1(i));
break;
case 2:
visibles.add(new Selectable1(i));
break;
case 3:
visibles.add(new Visible2(i));
break;
case 4:
visibles.add(new Highlightable2(i));
break;
case 5:
visibles.add(new Selectable2(i));
break;
case 6:
visibles.add(new Visible3(i));
break;
case 7:
visibles.add(new Highlightable3(i));
break;
case 8:
visibles.add(new Selectable3(i));
break;
}
}
long listSize = visibles.size();
long vsum = 0;
long hsum = 0;
long ssum = 0;
for (Visible e : visibles) {
if (e instanceof Selectable) {
vsum += e.visibleValue();
hsum += ((Selectable) e).highlightableValue();
ssum += ((Selectable) e).selectableValue();
} else if (e instanceof Highlightable) {
vsum += e.visibleValue();
hsum += ((Highlightable) e).highlightableValue();
} else {
vsum += e.visibleValue();
}
}
return listSize + vsum * hsum * ssum;
}
}
abstract class Visible {
abstract int visibleValue();
}
abstract class Highlightable extends Visible {
abstract int highlightableValue();
}
abstract class Selectable extends Highlightable {
abstract int selectableValue();
}
class Visible1 extends Visible {
private int v;
Visible1(int v) {
this.v = v;
}
@Override int visibleValue() { return v; }
}
class Highlightable1 extends Highlightable {
private int v;
Highlightable1(int v) {
this.v = v;
}
@Override int visibleValue() { return v*2; }
@Override int highlightableValue() { return v*3; }
}
class Selectable1 extends Selectable {
private int v;
Selectable1(int v) {
this.v = v;
}
@Override int visibleValue() { return v*4; }
@Override int highlightableValue() { return v*5; }
@Override int selectableValue() { return v*6; }
}
class Visible2 extends Visible {
private int v;
Visible2(int v) {
this.v = v;
}
@Override int visibleValue() { return v*7; }
}
class Highlightable2 extends Highlightable {
private int v;
Highlightable2(int v) {
this.v = v;
}
@Override int visibleValue() { return v*8; }
@Override int highlightableValue() { return v*9; }
}
class Selectable2 extends Selectable {
private int v;
Selectable2(int v) {
this.v = v;
}
@Override int visibleValue() { return v*10; }
@Override int highlightableValue() { return v*11; }
@Override int selectableValue() { return v*12; }
}
class Visible3 extends Visible {
private int v;
Visible3(int v) {
this.v = v;
}
@Override int visibleValue() { return v*13; }
}
class Highlightable3 extends Highlightable {
private int v;
Highlightable3(int v) {
this.v = v;
}
@Override int visibleValue() { return v*14; }
@Override int highlightableValue() { return v*15; }
}
class Selectable3 extends Selectable {
private int v;
Selectable3(int v) {
this.v = v;
}
@Override int visibleValue() { return v*16; }
@Override int highlightableValue() { return v*17; }
@Override int selectableValue() { return v*18; }
}
基准输出：
Main.separateLists thrpt 600 1690.522 ± 6.570 ops/s
Main.singleListAndInstanceof thrpt 600 1751.375 ± 4.368 ops/s
Main.separateLists:L1-dcache-load-misses:u thrpt 2 2298.258 #/op
Main.singleListAndInstanceof:L1-dcache-load-misses:u thrpt 2 627.451 #/op
Main.separateLists:L1-dcache-loads:u thrpt 2 1217756.290 #/op
Main.singleListAndInstanceof:L1-dcache-loads:u thrpt 2 1135982.650 #/op
Main.separateLists:L1-icache-load-misses:u thrpt 2 113.599 #/op
Main.singleListAndInstanceof:L1-icache-load-misses:u thrpt 2 99.896 #/op
Main.separateLists:L1-icache-loads:u thrpt 2 656048.382 #/op
Main.singleListAndInstanceof:L1-icache-loads:u thrpt 2 694074.004 #/op
Main.separateLists:LLC-load-misses:u thrpt 2 872.681 #/op
Main.singleListAndInstanceof:LLC-load-misses:u thrpt 2 355.666 #/op
Main.separateLists:LLC-loads:u thrpt 2 12036.496 #/op
Main.singleListAndInstanceof:LLC-loads:u thrpt 2 7445.434 #/op
Main.separateLists:LLC-stores:u thrpt 2 15277.223 #/op
Main.singleListAndInstanceof:LLC-stores:u thrpt 2 10649.517 #/op
Main.separateLists:branch-misses:u thrpt 2 22463.763 #/op
Main.singleListAndInstanceof:branch-misses:u thrpt 2 29940.958 #/op
Main.separateLists:branches:u thrpt 2 254018.586 #/op
Main.singleListAndInstanceof:branches:u thrpt 2 275450.951 #/op
Main.separateLists:cycles:u thrpt 2 1988517.839 #/op
Main.singleListAndInstanceof:cycles:u thrpt 2 1921584.057 #/op
Main.separateLists:dTLB-load-misses:u thrpt 2 66.212 #/op
Main.singleListAndInstanceof:dTLB-load-misses:u thrpt 2 64.442 #/op
Main.separateLists:dTLB-loads:u thrpt 2 1217929.340 #/op
Main.singleListAndInstanceof:dTLB-loads:u thrpt 2 1135799.981 #/op
Main.separateLists:iTLB-load-misses:u thrpt 2 4.179 #/op
Main.singleListAndInstanceof:iTLB-load-misses:u thrpt 2 3.876 #/op
Main.separateLists:iTLB-loads:u thrpt 2 656595.175 #/op
Main.singleListAndInstanceof:iTLB-loads:u thrpt 2 693913.010 #/op
Main.separateLists:instructions:u thrpt 2 2273646.245 #/op
Main.singleListAndInstanceof:instructions:u thrpt 2 2045332.939 #/op
Main.separateLists:stalled-cycles-backend:u thrpt 2 773671.154 #/op
Main.singleListAndInstanceof:stalled-cycles-backend:u thrpt 2 619477.824 #/op
Main.separateLists:stalled-cycles-frontend:u thrpt 2 184604.485 #/op
Main.singleListAndInstanceof:stalled-cycles-frontend:u thrpt 2 271938.450 #/op
Main.separateLists:·gc.alloc.rate thrpt 600 217.266 ± 0.846 MB/sec
Main.singleListAndInstanceof:·gc.alloc.rate thrpt 600 222.747 ± 0.556 MB/sec
Main.separateLists:·gc.alloc.rate.norm thrpt 600 202181.035 ± 2.986 B/op
Main.singleListAndInstanceof:·gc.alloc.rate.norm thrpt 600 200083.395 ± 4.720 B/op
Main.separateLists:·gc.churn.PS_Eden_Space thrpt 600 217.792 ± 3.841 MB/sec
Main.singleListAndInstanceof:·gc.churn.PS_Eden_Space thrpt 600 223.528 ± 4.973 MB/sec
Main.separateLists:·gc.churn.PS_Eden_Space.norm thrpt 600 202704.197 ± 3508.997 B/op
Main.singleListAndInstanceof:·gc.churn.PS_Eden_Space.norm thrpt 600 200804.794 ± 4414.457 B/op
Main.separateLists:·gc.churn.PS_Survivor_Space thrpt 600 0.095 ± 0.008 MB/sec
Main.singleListAndInstanceof:·gc.churn.PS_Survivor_Space thrpt 600 0.091 ± 0.008 MB/sec
Main.separateLists:·gc.churn.PS_Survivor_Space.norm thrpt 600 88.896 ± 7.778 B/op
Main.singleListAndInstanceof:·gc.churn.PS_Survivor_Space.norm thrpt 600 81.693 ± 7.269 B/op
Main.separateLists:·gc.count thrpt 600 2440.000 counts
Main.singleListAndInstanceof:·gc.count thrpt 600 2289.000 counts
Main.separateLists:·gc.time thrpt 600 4501.000 ms
Main.singleListAndInstanceof:·gc.time thrpt 600 4236.000 ms
更新：以下是基准代码和结果，其中阵列设置代码已提取到单独的方法中并从测量中删除。如预期的那样，instanceof在这种情况下速度较慢-上述差异可能与列表设置中的分支预测问题有关。 （尽管它们也很有趣，但他们可能应该单独提出一个问题）
package test;
import org.openjdk.jmh.annotations.*;
import java.util.ArrayList;
import java.util.Random;
public class Main {
public static void main(String[] args) throws Exception {
org.openjdk.jmh.Main.main(args);
}
@State(Scope.Thread)
public static class SeparateListsState {
public ArrayList<Visible> visibles;
public ArrayList<Highlightable> highlightables;
public ArrayList<Selectable> selectables;
@Setup(Level.Invocation)
public void doSetup() {
visibles = new ArrayList<>();
highlightables = new ArrayList<>();
selectables = new ArrayList<>();
Random random = new Random(9698426994L + 8879);
for (int i = 0; i < 10_000; i++) {
switch (random.nextInt(9)) {
case 0:
visibles.add(new Visible1(i));
break;
case 1:
highlightables.add(new Highlightable1(i));
break;
case 2:
selectables.add(new Selectable1(i));
break;
case 3:
visibles.add(new Visible2(i));
break;
case 4:
highlightables.add(new Highlightable2(i));
break;
case 5:
selectables.add(new Selectable2(i));
break;
case 6:
visibles.add(new Visible3(i));
break;
case 7:
highlightables.add(new Highlightable3(i));
break;
case 8:
selectables.add(new Selectable3(i));
break;
}
}
}
}
@Benchmark
@BenchmarkMode(Mode.Throughput)
@Warmup(iterations = 5, time = 1)
@Measurement(iterations = 150, time = 1)
@Fork(value = 2)
public static long separateLists(SeparateListsState state) {
long vsum = 0;
long hsum = 0;
long ssum = 0;
for (Visible e : state.visibles) {
vsum += e.visibleValue();
}
for (Highlightable e : state.highlightables) {
vsum += e.visibleValue();
hsum += e.highlightableValue();
}
for (Selectable e : state.selectables) {
vsum += e.visibleValue();
hsum += e.highlightableValue();
ssum += e.selectableValue();
}
return vsum * hsum * ssum;
}
@State(Scope.Thread)
public static class SingleListAndInstanceofState {
public ArrayList<Visible> visibles;
@Setup(Level.Invocation)
public void doSetup() {
visibles = new ArrayList<>();
Random random = new Random(9698426994L + 8879);
for (int i = 0; i < 10_000; i++) {
switch (random.nextInt(9)) {
case 0:
visibles.add(new Visible1(i));
break;
case 1:
visibles.add(new Highlightable1(i));
break;
case 2:
visibles.add(new Selectable1(i));
break;
case 3:
visibles.add(new Visible2(i));
break;
case 4:
visibles.add(new Highlightable2(i));
break;
case 5:
visibles.add(new Selectable2(i));
break;
case 6:
visibles.add(new Visible3(i));
break;
case 7:
visibles.add(new Highlightable3(i));
break;
case 8:
visibles.add(new Selectable3(i));
break;
}
}
}
}
@Benchmark
@BenchmarkMode(Mode.Throughput)
@Warmup(iterations = 5, time = 1)
@Measurement(iterations = 150, time = 1)
@Fork(value = 2)
public static long singleListAndInstanceof(SingleListAndInstanceofState state) {
long vsum = 0;
long hsum = 0;
long ssum = 0;
for (Visible e : state.visibles) {
if (e instanceof Selectable) {
vsum += e.visibleValue();
hsum += ((Selectable) e).highlightableValue();
ssum += ((Selectable) e).selectableValue();
} else if (e instanceof Highlightable) {
vsum += e.visibleValue();
hsum += ((Highlightable) e).highlightableValue();
} else {
vsum += e.visibleValue();
}
}
return vsum * hsum * ssum;
}
}
abstract class Visible {
abstract int visibleValue();
}
abstract class Highlightable extends Visible {
abstract int highlightableValue();
}
abstract class Selectable extends Highlightable {
abstract int selectableValue();
}
class Visible1 extends Visible {
private int v;
Visible1(int v) {
this.v = v;
}
@Override int visibleValue() { return v; }
}
class Highlightable1 extends Highlightable {
private int v;
Highlightable1(int v) {
this.v = v;
}
@Override int visibleValue() { return v*2; }
@Override int highlightableValue() { return v*3; }
}
class Selectable1 extends Selectable {
private int v;
Selectable1(int v) {
this.v = v;
}
@Override int visibleValue() { return v*4; }
@Override int highlightableValue() { return v*5; }
@Override int selectableValue() { return v*6; }
}
class Visible2 extends Visible {
private int v;
Visible2(int v) {
this.v = v;
}
@Override int visibleValue() { return v*7; }
}
class Highlightable2 extends Highlightable {
private int v;
Highlightable2(int v) {
this.v = v;
}
@Override int visibleValue() { return v*8; }
@Override int highlightableValue() { return v*9; }
}
class Selectable2 extends Selectable {
private int v;
Selectable2(int v) {
this.v = v;
}
@Override int visibleValue() { return v*10; }
@Override int highlightableValue() { return v*11; }
@Override int selectableValue() { return v*12; }
}
class Visible3 extends Visible {
private int v;
Visible3(int v) {
this.v = v;
}
@Override int visibleValue() { return v*13; }
}
class Highlightable3 extends Highlightable {
private int v;
Highlightable3(int v) {
this.v = v;
}
@Override int visibleValue() { return v*14; }
@Override int highlightableValue() { return v*15; }
}
class Selectable3 extends Selectable {
private int v;
Selectable3(int v) {
this.v = v;
}
@Override int visibleValue() { return v*16; }
@Override int highlightableValue() { return v*17; }
@Override int selectableValue() { return v*18; }
}
结果：
Main.separateLists thrpt 300 4211.552 ± 23.791 ops/s
Main.singleListAndInstanceof thrpt 300 3920.251 ± 15.478 ops/s
Main.separateLists:L1-dcache-load-misses:u thrpt 2 3046.033 #/op
Main.singleListAndInstanceof:L1-dcache-load-misses:u thrpt 2 1089.122 #/op
Main.separateLists:L1-dcache-loads:u thrpt 2 1090745.006 #/op
Main.singleListAndInstanceof:L1-dcache-loads:u thrpt 2 1125243.609 #/op
Main.separateLists:L1-icache-load-misses:u thrpt 2 150.542 #/op
Main.singleListAndInstanceof:L1-icache-load-misses:u thrpt 2 143.304 #/op
Main.separateLists:L1-icache-loads:u thrpt 2 600852.620 #/op
Main.singleListAndInstanceof:L1-icache-loads:u thrpt 2 700771.042 #/op
Main.separateLists:LLC-load-misses:u thrpt 2 1299.520 #/op
Main.singleListAndInstanceof:LLC-load-misses:u thrpt 2 636.764 #/op
Main.separateLists:LLC-loads:u thrpt 2 14408.815 #/op
Main.singleListAndInstanceof:LLC-loads:u thrpt 2 10429.768 #/op
Main.separateLists:LLC-stores:u thrpt 2 18999.178 #/op
Main.singleListAndInstanceof:LLC-stores:u thrpt 2 15370.582 #/op
Main.separateLists:branch-misses:u thrpt 2 22578.062 #/op
Main.singleListAndInstanceof:branch-misses:u thrpt 2 29257.959 #/op
Main.separateLists:branches:u thrpt 2 258026.890 #/op
Main.singleListAndInstanceof:branches:u thrpt 2 284911.889 #/op
Main.separateLists:cycles:u thrpt 2 1915774.770 #/op
Main.singleListAndInstanceof:cycles:u thrpt 2 1974841.023 #/op
Main.separateLists:dTLB-load-misses:u thrpt 2 101.573 #/op
Main.singleListAndInstanceof:dTLB-load-misses:u thrpt 2 99.982 #/op
Main.separateLists:dTLB-loads:u thrpt 2 1090174.103 #/op
Main.singleListAndInstanceof:dTLB-loads:u thrpt 2 1129185.929 #/op
Main.separateLists:iTLB-load-misses:u thrpt 2 4.432 #/op
Main.singleListAndInstanceof:iTLB-load-misses:u thrpt 2 3.955 #/op
Main.separateLists:iTLB-loads:u thrpt 2 600301.665 #/op
Main.singleListAndInstanceof:iTLB-loads:u thrpt 2 703339.482 #/op
Main.separateLists:instructions:u thrpt 2 1974603.052 #/op
Main.singleListAndInstanceof:instructions:u thrpt 2 2040460.093 #/op
Main.separateLists:stalled-cycles-backend:u thrpt 2 808914.974 #/op
Main.singleListAndInstanceof:stalled-cycles-backend:u thrpt 2 685615.056 #/op
Main.separateLists:stalled-cycles-frontend:u thrpt 2 186013.216 #/op
Main.singleListAndInstanceof:stalled-cycles-frontend:u thrpt 2 272207.204 #/op
Main.separateLists:·gc.alloc.rate thrpt 300 346.891 ± 1.166 MB/sec
Main.singleListAndInstanceof:·gc.alloc.rate thrpt 300 358.297 ± 0.614 MB/sec
Main.separateLists:·gc.alloc.rate.norm thrpt 300 310744.294 ± 0.107 B/op
Main.singleListAndInstanceof:·gc.alloc.rate.norm thrpt 300 328992.302 ± 0.110 B/op
Main.separateLists:·gc.churn.PS_Eden_Space thrpt 300 349.387 ± 14.305 MB/sec
Main.singleListAndInstanceof:·gc.churn.PS_Eden_Space thrpt 300 360.039 ± 9.075 MB/sec
Main.separateLists:·gc.churn.PS_Eden_Space.norm thrpt 300 313154.953 ± 13018.012 B/op
Main.singleListAndInstanceof:·gc.churn.PS_Eden_Space.norm thrpt 300 330629.833 ± 8345.712 B/op
Main.separateLists:·gc.churn.PS_Survivor_Space thrpt 300 0.092 ± 0.012 MB/sec
Main.singleListAndInstanceof:·gc.churn.PS_Survivor_Space thrpt 300 0.094 ± 0.011 MB/sec
Main.separateLists:·gc.churn.PS_Survivor_Space.norm thrpt 300 82.348 ± 10.661 B/op
Main.singleListAndInstanceof:·gc.churn.PS_Survivor_Space.norm thrpt 300 86.465 ± 10.417 B/op
Main.separateLists:·gc.count thrpt 300 1196.000 counts
Main.singleListAndInstanceof:·gc.count thrpt 300 1235.000 counts
Main.separateLists:·gc.time thrpt 300 2178.000 ms
Main.singleListAndInstanceof:·gc.time thrpt 300 2355.000 ms
答案 0 :(得分：7)
正如NoDataFound在评论中所建议的那样，您不仅在比较遍历列表的性能，还在比较列表填充方法。您需要将代码的这一部分放入设置方法中-否则，您可能会对三个ArrayList实例上的大小调整操作（以及其他事情）产生影响。
您还应该放弃使用Random来填充列表，或者至少在两个实现中使用相同的种子实例化它-否则，您将不会在两个实现中创建元素的可重复顺序。 / p>
答案 1 :(得分：5)
核心点是：您的测量有缺陷。您的问题的第一个版本不仅测量了不同的“事物”，而且即使更新的问题也有一个大问题，10_000的样本量太小了！
那根本不是一个合理的样本量。至少不是一个人。您应该检查一下10K，50K，100K，100万，...循环迭代的结果。
您知道，您犯了许多人使用Java的错误：他们认为事物的源代码上的这种或那种结构决定了性能。
这只是部分正确。您会看到，真正的性能提升是基于JVM中的JIT编译器根据运行时收集的性能分析信息在运行时所做的无数优化而产生的。
我认为，JIT启动并将字节码转换为高度优化的机器代码的默认阈值类似于90K（？）方法调用/循环迭代。
请让它陷入：除非您的代码执行了100K或更大的规模，否则JIT甚至不会认为您的代码“值得优化”。但是随后它将开始起作用，这可能会对整体性能产生巨大影响。然后，您在源代码中添加的内容可能不再重要。
因此，没有一个度量可以告诉您什么是“最佳”解决方案。而是取决于您经历的迭代次数。
因此，真正的答案是：Java性能基准测试非常困难，您必须非常努力地做事以及从结果中得出的结论。
除此之外，真正的真实答案是：性能是一个奢侈品问题。当然，应该避免愚蠢的错误，这些错误会毫无用处地消耗CPU周期。但是除此之外，您的主要目标始终是编写易于阅读和理解的简单代码。然后，当您发现“感觉很慢”或“这不符合我们的SLA”时，则您仔细定义实验以测量发生的情况，从而确定导致性能问题的那段代码。仅作记录：您知道JIT可以优化最佳代码...惊喜：简单直接的代码，看起来像90％的优秀Java程序员倾向于编写代码。