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I need to understand below behavior of sql server queries

System has 4 CPUs with hyper thread ratio of 20 making 80 logical processors count on my sql server instance.

Currently MAXDOP is 0 and ctop is 5

This is a read only instance acting as stand by for LS where all reporting is done.

Lot of queries coming in seems to show running with degree of parallelism 64.

Questions: .1. why degree of parallelism is not 80 when no of logical processors is 80?

2. Is my understanding correct that if there are 6 slow queries running over an hour all showing degree of parallelism 64 would be expected to slow as they might be hitting same threads and thus showing as cx packet waits. Hence I believe changing MAXDOP to say 16 or 8 by testing should give them their own threads to complete?

Thanks