Observation of performance degradation of 2.7.2 when running under JDK 8

[chickenlj]

dubbo/dubbo-rpc/dubbo-rpc-api/src/main/java/org/apache/dubbo/rpc/protocol/AsyncToSyncInvoker.java

Line 54 in 501e4db

asyncResult.get();

The above result.get() will eventually delegate to CompletableFuture.get(). I checked the implementation provided in JDK 8, it will first spin-waiting for the countdown of 256 (the eighth power of 2) and then park the thread. I believe it's the unnecessary spin that caused the bad performance.

When running under JDK 11, the performance performs well as the previous versions, I still does not check the difference between these two implementations yet.

Thank @Jeff-Lv for help finding this issue.