vagetablechicken commented on issue #3273: fragment mem tracker doesn't be unregistered from parent URL: https://github.com/apache/incubator-doris/issues/3273#issuecomment-610717223 > @vagetablechicken > I'm not sure what is your mean by "the top level is needed". Could you please explain more carefully? OK. We can regard mem trackers as nodes. _There are many trees, cause we may create a mem tracker as a no-parent node. But we don't need to pay attention on them._ Focus on one plan fragment, the root mem tracker is exec_env_mem_tracker(level 0), and the query_mem_tracker & fragment_mem_tracker are the root node's children(level 1). These can be called top-level nodes. And the low-level nodes, such as scan node's mem_tracker, sink node's mem_tracker...(level >=2) Low-level mem trackers will be destroyed when plan finished. And the top-level mem trackers will be destroyed too. For example, the fragment_mem_tracker, we don't need to worry about dangling pointers(child trackers) in it, because it will be destroyed when plan fragment finished, But the fragment_mem_tracker's pointer in exec_env_mem_tracker is never delete. If we use exec_env_mem_tracker and visit its children, we may access the deallocated memory.
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