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Posted to issues@kylin.apache.org by GitBox <gi...@apache.org> on 2022/10/11 11:41:36 UTC

[GitHub] [kylin] liuzhao-lz commented on a diff in pull request #1994: improve performance of getInstanceFromEnv

liuzhao-lz commented on code in PR #1994:
URL: https://github.com/apache/kylin/pull/1994#discussion_r992210473


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core-common/src/main/java/org/apache/kylin/common/KylinConfig.java:
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@@ -65,7 +65,7 @@ public class KylinConfig extends KylinConfigBase {
     public static final String KYLIN_CONF = "KYLIN_CONF";
 
     // static cached instances
-    private static KylinConfig SYS_ENV_INSTANCE = null;
+    private static volatile KylinConfig SYS_ENV_INSTANCE = null;

Review Comment:
   非 volatile 变量的读取会从主存拷到寄存器，thread1 & thread2 进入 sychronized 前SYS_ENV_INSTANCE被拷贝到各自的寄存器，假如 thread2 先执行了 sychronized 内的代码，thread2执行完后 thread1 由于SYS_ENV_INSTANCE 拷贝到寄存器，第二次判空可能结果为 true，就会再一次执行 sychronized 内的代码



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