Breaking Down the MAXDOP Guidance Change

Joe Obbish digs into Microsoft’s new guidance for maximum degree of parallelism:

I’ve heard some folks claim that keeping all parallel workers on a single hard NUMA nodes can be important for query performance. I’ve even seen some queries experience reduced performance when thread 0 is on a different hard NUMA node than parallel worker threads. I haven’t heard of anything about the importance of keeping all of a query’s worker threads on a single soft-NUMA node. It doesn’t really make sense to say that query performance will be improved if all worker threads are on the same soft-NUMA node. Soft-NUMA is a configuration setting. Suppose I have a 24 core hard NUMA node and my goal is to get all of a parallel query’s worker threads on a single soft-NUMA node. To accomplish that goal the best strategy is to disable auto soft-NUMA because that will give me a NUMA node size of 24 as opposed to 8. So disabling auto soft-NUMA will increase query performance?

Joe takes a careful look at the documentation and brings up some really good questions.

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