把循环index的类型由long改为int，让循环前放置Safepoint，效果会不会比Thread.sleep(0)更好？

[somePeopleFireAndWood]

如题 MappedFile 类，507~527行这个循环，把循环的i、j改成long类型。删除掉522行的Thread.sleep(0)。

这么改，关于gc方面的优化，效果会不会更好？代码看着也会更加优雅

[fuyou001]

虽然是sleep 0，但因操作系统时钟精度原因，实际会sleep 几毫秒，不同的系统，精度可能值会不一样； 删除之后，就没有sleep的效果了；

[somePeopleFireAndWood]

虽然是sleep 0，但因操作系统时钟精度原因，实际会sleep 几毫秒，不同的系统，精度可能值会不一样； 删除之后，就没有sleep的效果了；

@fuyou001 请问为什么要有sleep的效果呢？没有sleep的效果不是更好吗？ 另外还有的问题是，如果追求sleep的效果，sleep(0)有没有可能被jit优化啊？

[francisoliverlee]

@foxeatsspicy can you make a test code ?

[sunheyi6]

@foxeatsspicy can you make a test code ?

@francisoliverlee the person doesn't seem to exist，page is 404

[Yong0724]

safepoint，安全点。

[lizhanhui]

虽然是sleep 0，但因操作系统时钟精度原因，实际会sleep 几毫秒，不同的系统，精度可能值会不一样； 删除之后，就没有sleep的效果了；

@fuyou001

No. It is not true. The intention of sleep(0) is to introduce safe points. When there is an ongoing GC, this thread will take fewer milliseconds to reach a safe point. This would be helpful if the available memory runs low. See https://stackoverflow.com/questions/67232199/how-to-reduce-time-taken-on-threads-reaching-safepoint-sync-state

[lizhanhui]

@duhenglucky The pull request is not completed yet. Need more insights involved.

[linlinjie]

continuing to pay attention

[gmshake]

虽然是sleep 0，但因操作系统时钟精度原因，实际会sleep 几毫秒，不同的系统，精度可能值会不一样； 删除之后，就没有sleep的效果了；

@fuyou001

No. It is not true. The intention of sleep(0) is to introduce safe points. When there is an ongoing GC, this thread will take fewer milliseconds to reach a safe point. This would be helpful if the available memory runs low. See https://stackoverflow.com/questions/67232199/how-to-reduce-time-taken-on-threads-reaching-safepoint-sync-state

Maybe Thread.yield() is more clear for the usage?

[kimmking]

实际上，这里的改动无效：for循环里有方法调用，就不存在这个问题。方法调用栈退出的地方，是可以加check safepoint的。

for循环退不出的场景，也可以通过-XX:+UseCountedLoopSafepoints解决，不需要改代码。 @lizhanhui