[英]Why ConcurrentHashMap::putIfAbsent is faster than ConcurrentHashMap::computeIfAbsent?
使用 ConcurrentHashMap,我發現computeIfAbsent 比putIfAbsent 慢兩倍。 這里是簡單的測試:
import java.util.ArrayList;
import java.util.List;
import java.util.UUID;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;
public class Test {
public static void main(String[] args) throws Exception {
String[] keys = {"a1", "a2", "a3", "a4", "a5", "a6", "a7", "a8", "a9", "a0", "a01", "a02", "a03", "a04", "a05", "a06", "a07", "a08", "a09", "a00"};
System.out.println("Test case 1");
long time = System.currentTimeMillis();
testCase1(keys);
System.out.println("ExecutionTime: " + String.valueOf(System.currentTimeMillis() - time));
System.out.println("Test case 2");
time = System.currentTimeMillis();
testCase2(keys);
System.out.println("ExecutionTime: " + String.valueOf(System.currentTimeMillis() - time));
System.out.println("Test case 3");
time = System.currentTimeMillis();
testCase3(keys);
System.out.println("ExecutionTime: " + String.valueOf(System.currentTimeMillis() - time));
}
public static void testCase1(String[] keys) throws InterruptedException {
ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();
List<Thread> threads = new ArrayList<>();
for (String key : keys) {
Thread thread = new Thread(() -> map.computeIfAbsent(key, s -> {
System.out.println(key);
String result = new TestRun().compute();
System.out.println("Computing finished for " + key);
return result;
}));
thread.start();
threads.add(thread);
}
for (Thread thread : threads) {
thread.join();
}
}
public static void testCase2(String[] keys) throws InterruptedException {
List<Thread> threads = new ArrayList<>();
for (String key : keys) {
Thread thread = new Thread(() -> {
System.out.println(key);
new TestRun().compute();
System.out.println("Computing finished for " + key);
});
thread.start();
threads.add(thread);
}
for (Thread thread : threads) {
thread.join();
}
}
public static void testCase3(String[] keys) throws InterruptedException {
ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();
List<Thread> threads = new ArrayList<>();
for (String key : keys) {
Thread thread = new Thread(() -> {
Callable<String> c = () -> {
System.out.println(key);
String result = new TestRun().compute();
System.out.println("Computing finished for " + key);
return result;
};
try {
map.putIfAbsent(key, c.call());
} catch (Exception e) {
e.printStackTrace(System.out);
}
});
thread.start();
threads.add(thread);
}
for (Thread thread : threads) {
thread.join();
}
}
}
class TestRun {
public String compute() {
try {
Thread.currentThread().sleep(5000);
} catch (Exception e) {
e.printStackTrace(System.out);
}
return UUID.randomUUID().toString();
}
}
在我的筆記本電腦上運行這個測試,testCase1(它使用 computeIfAbsent())執行時間是 10068ms,testCase2(它執行相同的東西但沒有將它包裝到 computeIfAbsent())執行時間是 5009ms(當然它有點變化,但是主要趨勢是這樣)。 最有趣的是 testCase3 - 它與 testCase1 幾乎相同(除了使用 putIfAbsent() 而不是 computeIfAbsent()),但它的執行速度要快兩倍(testCase3 為 5010 毫秒,而 testCase1 為 10068 毫秒)。
查看源代碼,對於computeIfAbsent() 和putVal()(在引擎蓋下的putIfAbsent() 中使用)幾乎相同。
有人知道是什么導致線程執行時間不同嗎?
您遇到了記錄在案的功能:
在計算過程中,其他線程對該映射的一些嘗試更新操作可能會被阻塞,因此計算應該簡短且簡單,並且不得嘗試更新該映射的任何其他映射。
computeIfAbsent 檢查密鑰是否存在並鎖定地圖的某些部分。 然后它調用函子並將結果放入映射中(如果返回值不為空)。 只有在此之后地圖的這一部分才會被解鎖。
另一方面,test3總是調用c.call(),計算結束后調用putIfAbsent。
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