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why more than 30 gb memory for a reduce task is not recommended?

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why more than 30 gb memory for a reduce task is not recommended?

Contributor

My cluster is having 188 GB ram on data nodes.
I have given 19 GB for map memory and since we have to assign reduce memory just double of map memory so i have assign 38 GB.
But there is this warning which says more than 30 GB is not recommended.
Any reason?