java为什么不解决空轮询,netty解决空轮询bug

最新推荐文章于 2025-09-25 22:08:33 发布

转载最新推荐文章于 2025-09-25 22:08:33 发布 · 417 阅读

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本文深入探讨了Java NIO中Selector在无结果时仍被唤醒导致的空轮询问题，CPU占用达到100%的情况。分析了Selector的select方法实现，包括超时判断、重试机制以及自动重建Selector的阈值。当空轮询次数超过预设阈值时，系统会重建Selector以避免性能问题。这一机制对于理解和优化Java NIO性能至关重要。

selector在没有结果的情况下，依然被唤醒，导致一直空轮询，cpu100%

直接定位到NioEventLoop

@Override

protected void run() {

for (;;) {

try {

switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {

case SelectStrategy.CONTINUE:

continue;

case SelectStrategy.SELECT:

select(wakenUp.getAndSet(false));

if (wakenUp.get()) {

selector.wakeup();

}

default:

// fallthrough

}

cancelledKeys = 0;

needsToSelectAgain = false;

final int ioRatio = this.ioRatio;

if (ioRatio == 100) {

try {

processSelectedKeys();

} finally {

// Ensure we always run tasks.

runAllTasks();

}

} else {

final long ioStartTime = System.nanoTime();

try {

processSelectedKeys();

} finally {

// Ensure we always run tasks.

final long ioTime = System.nanoTime() - ioStartTime;

runAllTasks(ioTime * (100 - ioRatio) / ioRatio);

}

} catch (Throwable t) {

handleLoopException(t);

}

try {

if (isShuttingDown()) {

closeAll();

if (confirmShutdown()) {

return;

}

} catch (Throwable t) {

handleLoopException(t);

}

第二个case，SelectStrategy.SELECT ,其值为-1。跟进调用的select方法。

private void select(boolean oldWakenUp) throws IOException {

Selector selector = this.selector;

try {

int selectCnt = 0;

long currentTimeNanos = System.nanoTime();

long selectDeadLineNanos = currentTimeNanos + delayNanos(currentTimeNanos);

for (;;) {

long timeoutMillis = (selectDeadLineNanos - currentTimeNanos + 500000L) / 1000000L;

if (timeoutMillis <= 0) {

if (selectCnt == 0) {

selector.selectNow();

selectCnt = 1;

}

break;

}

if (hasTasks() && wakenUp.compareAndSet(false, true)) {

selector.selectNow();

selectCnt = 1;

break;

}

int selectedKeys = selector.select(timeoutMillis);

selectCnt ++;

if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || hasTasks() || hasScheduledTasks()) {

break;

}

if (Thread.interrupted()) {

if (logger.isDebugEnabled()) {

logger.debug("Selector.select() returned prematurely because " +

"Thread.currentThread().interrupt() was called. Use " +

"NioEventLoop.shutdownGracefully() to shutdown the NioEventLoop.");

}

selectCnt = 1;

break;

}

long time = System.nanoTime();

if (time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos) {

// timeoutMillis elapsed without anything selected.

selectCnt = 1;

} else if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 &&

selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {

// The selector returned prematurely many times in a row.

// Rebuild the selector to work around the problem.

logger.warn(

"Selector.select() returned prematurely {} times in a row; rebuilding Selector {}.",

selectCnt, selector);

rebuildSelector();

selector = this.selector;

// Select again to populate selectedKeys.

selector.selectNow();

selectCnt = 1;

break;

}

currentTimeNanos = time;

}

if (selectCnt > MIN_PREMATURE_SELECTOR_RETURNS) {

if (logger.isDebugEnabled()) {

logger.debug("Selector.select() returned prematurely {} times in a row for Selector {}.",

selectCnt - 1, selector);

}

} catch (CancelledKeyException e) {

if (logger.isDebugEnabled()) {

logger.debug(CancelledKeyException.class.getSimpleName() + " raised by a Selector {} - JDK bug?",

selector, e);

}

// Harmless exception - log anyway

}

上述代码中在轮询之前，先定义当前时间currentTimeNanos。接着计算出一个执行最少需要的时间timeoutMillis。每次对selectCnt做++操作。

在后面的代码中通过

time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos

进行判断，如果到达执行到最少时间，则seletCnt重置为1。一旦到达SELECTOR_AUTO_REBUILD_THRESHOLD这个阀值，就需要重建selector来解决这个问题。这个阀值默认是512。