Reading SQL deadlock graph

Question

Can someone please help me to read/understand this deadlock graph?

I don't understand why process 75 is requesting a lock on an object that he has a lock on already?

Chuipek

can you send "show innoDB status" response it will be more informative. It should contain last deadlock – Whatever

@Whatever - This is SQL Server. jaques What version? Looks like a paralellism issue. – Copperas

@Martin Smith. SQL Server 2008 – Chuipek

@jaques - If you post your relevant table structures, including indexes, and the XML version of the deadlock graph into your question you may well be able to get a more specific answer. – Copperas

@Martin. This is a commercial product and I don't feel comfortable exposing the internals – Chuipek

Answer 1

According to a blog article that I've found the existance of an "Exchange Event" indicates that the source of your problem may be parallelism in your query.

Today's Annoyingly-Unwieldy Term: "Intra-Query Parallel Thread Deadlocks"

The above article goes into much more detail, however the punchline is:

Workaround #1: Add an index or improve the query to eliminate the need for parallelism. In most cases, the use of parallelism in a query indicates that you have a very large scan, sort, or join that isn't supported by proper indexes. If you tune the query, you will often find that you end up with a much quicker and more efficient plan that doesn't use parallelism, and therefore isn't subject to this type of problem. Of course, in some queries (DSS/OLAP-type queries, in particular) it may be difficult to eliminate all large scans.

Workaround #2: Force single-threaded execution with an "OPTION (MAXDOP 1)" query hint at the end of the query. If you can't modify the query, you can apply the hint to any query with a plan guide.

You might want to try this to see if there is any improvement.

Starryeyed

Thanks, I managed to identify and improve a slow running query and no locks since then. Not sure how to prevent this in the future? And why does SQL server try to be smart if there is a possibility that it might itself into a deadlock situation? (E.G Why use parallelism?) – Chuipek

The referenced link is focused on "pure" exchange event intra-query deadlock where there is just one SPID (thus "intra-query"). The questions's deadlock graph includes an exchange event plus a more traditional deadlock involving 2 queries (another SPID). I have read that isolated deadlocking exchange events can fix themselves (as they may be due to internal bugs) so the focus should be on the other shared objects--page locks in this case. – Wormwood