[GitHub] [incubator-doris] vagetablechicken commented on issue #3273: fragment mem tracker doesn't be unregistered from parent

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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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