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POMinimizing deadlocks with purposely contrived + highly concurrent transactions?
 

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  1. POMinimizing deadlocks with purposely contrived + highly concurrent transactions?
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    <p>I'm currently working on benchmarking different isolation levels in SQL Server 2008 -- but right now I'm stuck on what seems to be a trivial deadlocking problem, but I can't seem to figure it out. Hopefully someone here can offer advice (I'm a novice to SQL)</p> <p>I currently have two types of transactions (to demonstrate dirty reads, but that's irrelevant):</p> <p>Transaction Type A: Select all rows from Table A.</p> <p>Transaction Type B: Set value 'cost' = 0 in all rows in Table A, then rollback immediately.</p> <p>I currently run a threadpool of 1000 threads and 10,000 transactions, where each thread randomly chooses between executing Transaction Type A and Transaction Type B. However, I'm getting a ton of deadlocks even with forced row locking.</p> <p>I assume that the deadlocks are occurring because of the row ordering of locks being acquired -- that is, if both Type A and Type B 'scan' table A in the same ordering, e.g. from top to bottom, such deadlocks cannot occur. However, I'm having trouble figuring out how to get SQL Server to maintain row ordering during SELECT and UPDATE statements. </p> <p>Any tips? First time poster to stackoverflow, so please be gentle :-)</p> <p>EDIT: The isolation level is purposely set to READ_COMMITTED to show that it eliminates dirty reads (and it does). Deadlocks only occur on any level equal to or higher than READ_COMMITTED; obviously no deadlocks occur on READ_UNCOMMITTED.</p> <p>EDIT 2: These transactions are being run on a fresh instance of AdventureWorks LT on SQL Server 2008R2.</p>
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    1. USakwok
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      1. POMinimizing deadlocks with purposely contrived + highly concurrent transactions?
      1. USMichael Fredrickson
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    1. COTo clarify, my understanding is this: A deadlock can occur if XACT A selects rows in an increasing fashion, and XACT B updates rows in a decreasing fashion, e.g.: XACT A acquires lock on row 1; XACT A acquires lock on row 2, relinquishes lock on row 1; XACT B acquires lock on row 3, wants to access row 2, but is blocked; XACT A wants to access row 3, but is blocked. However, if XACT A and XACT B goes through rows in the same order, obviously such deadlocks cannot occur. How do I enforce this?
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      1. USakwok
    2. COCan you provide table structure (including indexes) and the deadlock graph from SQL Profiler?
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      1. POMinimizing deadlocks with purposely contrived + highly concurrent transactions?
      1. USMartin Smith
 

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