26 May SQL University – Parallelism Week
I may be in the middle of a career makeover, but that does not excuse me from missing out on SQL University. This week we are going to talk about parallelism, an aspect of SQL Server that many DBAs have only a cursory amount of information.
The first thing you need to do is to understand what parallelism is and why it happens. Adam Machanic (blog | twitter) has already provided you with a great background on this and I won’t bother trying to explain things in any more detail than he already has. But in my role as teacher/coach I have found the need to be able to explain things in various ways to make certain that everyone has an understanding. And at this level you need awareness of the topic so that if someone says the word ‘parallelism’ you don’t think they are referring to some aspect of literature.
Consider the story of the Manhattan Project. Inside of that project you had someone put together a plan, the plan was to be executed in parallel, and someone had to coordinate the output into a defined deliverable as an end result. Parallelism works in much the same way: you submit a query, something decides that the query is best run in parallel, and at the end the streams are gathered to produce an output that may or may not set off a chain reaction that will destroy the Earth.
Once you have awareness of parallelism the next step is to recognize it when the opportunity arises. Chances are the first time you will see evidence of a parallel query is inside of an execution plan, and you will be able to distinguish it by a special icon as follows:
(You can find a full list of graphical execution plan icons at http://technet.microsoft.com/en-us/library/ms175913.aspx)
This special icon can show up on various pieces of your execution plan, you will need to be mindful of this when examining your plans.
Another way to find out about query parallelism is to use wait stats. You can see a list of wait types and their definitions over at http://technet.microsoft.com/en-us/library/ms179984.aspx. The specific wait type that you primarily want to look for is named CXPACKET, but there are other wait types that are related to parallel processing as well such as ASYNC_DISKPOOL_LOCK, CXROWSET_SYNC, EXCHANGE, EXECSYNC, and the ever popular QUERY_EXECUTION_INDEX_SORT_EVENT_OPEN (yeah, I had never heard of that one before today). The point here is that there is more than one way for you to be aware that statements are being run in parallel.
The real question you now face is whether or not parallelism is a bad thing for you or not. In others words, why should you even care about this topic? At the end of the day all anyone ever cares about is performance, and if performance is degraded as a result of parallelism then there are two options for you to use.
The first option that you should be aware of involves the use of sp_configure:
sp_configure 'show advanced options', 1; GO RECONFIGURE WITH OVERRIDE; GO sp_configure 'max degree of parallelism', 8; GO RECONFIGURE WITH OVERRIDE; GO
Note that this setting is for the instance as a whole. Also, keep in mind this information before you start playing around. In the past I have often set this configuration to be equal to the number of physical processors minus one. The reason behind this was that I always wanted to leave one CPU available to service other requests should a query decide to go parallel and consume all available CPUs at the same time. By limiting this at the instance level I can ensure a certain amount of stability for that instance.
Another option is to use the MAXDOP query hint. This query hint will override the sp_configure setting, so it would be possible for you to allow for more CPUs to run in parallel for your query and leave the server at a lower amount, or vice-versa. The hint goes at the end and looks something like this:
SELECT blah FROM blahblah OPTION (MAXDOP 1)
If you want to be at this level then you are going to be designing queries to either run in parallel or serially. What’s that? Oh yes, the optimizer is going to make the decision for you at run time, sure. But you already know that you can change the behavior by altering some configurations, right?
However, at this level, you will find yourself not settling for those configuration changes in order to force the engine to do what you want. Instead you will dive deep into the query to see if you can understand why the optimizer is deciding that a parallel plan is the best choice. In some cases this could be the result of poor indexing, or perhaps some fragmentation, or a host of other reasons as to why a parallel plan was chosen as the least expensive and ultimately selected as the one to use.
To be a master of parallelism you will need to start viewing queries with that level of discern.