heaplimit永远不能超过heaptop，也不能低于使用- Xms指定的初始堆大小。

The heaplimit can never exceed heaptop, and can never go below the initial heap size specified using -xms.

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每当某个堆超出了其配置的大小时，便可以使用溢出缓冲区来满足实例共享内存区内任何堆的峰值需求。

The overflow buffer is used to satisfy peak memory requirements for any heap in the instance Shared memory region whenever a heap exceeds its configured size.

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堆需求不变的应用程序可以通过使用固定堆大小改进GC暂停时间。

Applications with consistent heap requirements might see GC pause time improvements by using a fixed heap size.

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