vb.net 2d barcode generator Excessive compilation or recompilation Inefficient query plan Intra-query parallelism in C#

Drawer EAN-13 in C# Excessive compilation or recompilation Inefficient query plan Intra-query parallelism

Excessive compilation or recompilation Inefficient query plan Intra-query parallelism
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Part VII
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Performance Tuning and Troubleshooting
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Otherwise, you will need to determine the cause of the high processor utilization of your SQL Server instance by determining the root cause. Excessive Recompilations To determine whether excessive recompilations are the cause of your high processor utilization, examine the following metrics:
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SQL Server: SQL Statistics object in System Monitor. Examine the Batch Requests/ sec, SQL Compilations/sec, and SQL Re-Compilations/sec counters. A high ratio of SQL Re-Compilations/sec to Batch Requests/sec would indicate excessive recompilations. The SP:Recompile and SQL:StmtRecompile events in SQL Trace. The optimizations and elapsed time counter values in the sys.dm_exec_query_optimizer_info DMV. The elapsed time counter is the average elapsed time (in seconds) per optimization of an individual query). The plan_generation_num, which returns the number of times the plan has been recompiled and the execution_count columns of the sys.dm_exec_query_stats DMV. To determine the query, you can use the sql_handle value to query the sys.dm_exec_sql_text(sql_handle) DMV, as demonstrated in 31.
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If you have determined that your SQL Server instance is experiencing excessive recompilations, examine the T-SQL batches that are the cause. There are a number of techniques that your developers can use to reduce the excessive recompilation that you might be experiencing. It might also be caused by a problem with statistics or poor indexes. Consider running the Database Engine Tuning Advisor, discussed in 30, to see what it recommends. More Info For more information on recompilation issues, read the Batch Compilation, Recompilation, and Plan Caching Issues in SQL Server 2005 white paper available at http://www.microsoft.com/technet/prodtechnol/sql/2005/ recomp.mspx Inefficient Query Plans Another potential cause of excessive processor utilization is a high number of computeintensive query plans being generated. Use the sys.dm_exec_query_stats DMV, together with the sys.dm_exec_sql_text(sql_handle) DMV as shown below, to find these processor intensive queries by looking for queries that have consumed the most CPU resources (total_worker_time column).
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29
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Database System Tuning
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SELECT * FROM sys.dm_exec_query_stats AS QueryStats CROSS APPLY sys.dm_exec_query_plan(QueryStats.plan_handle) ORDER BY total_worker_time DESC ;
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Another approach is to look for compute-intensive operators such as Hash Matches and Sorts in the sys.dm_exec_cached_plans DMV. This can be done by running the following query and filtering for either '%Hash Match%' or '%Sort%' on the query_plan column:
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SELECT * FROM sys.dm_exec_cached_plans AS CachedPlans CROSS APPLY sys.dm_exec_query_plan(CachedPlans.plan_handle) ;
As with excessive recompilations, you will need to identify the poorly executing queries and present your findings to your developers. Again, the cause might be related to poor statistics or inappropriate indexes, so consider running the Database Engine Tuning Advisor. Otherwise, your developers will have to examine their queries and the indexing strategy of the database. They might have to use query hints to override the optimizer, but this should be considered a last resort. Intra-Query Parallelism Queries that are executed using parallel execution plans can be expensive and can be the cause of your high processor utilization. Use the following techniques to identify whether your SQL Server instance is running a large number of parallel queries running:
Look for where the CPU value is greater than the duration value in the RPC:Completed event class using SQL Trace. Look for cached execution plans thathave the Parallelism operator indicating they will potentially run in parallel depending on the activity on your SQL Server instance:
SELECT * FROM sys.dm_exec_cached_plans ASCachedPlans CROSS APPLY sys.dm_exec_query_plan(CachedPlans.plan_handle) AS QueryPlan CROSS APPLY sys.dm_exec_sql_text(CachedPlans.plan_handle) AS SQLText WHERE CachedPlans.cacheobjtype = 'Compiled Plan' AND QueryPlan.query_plan.value('declare namespace ns="http://schemas.microsoft.com/sqlserver/2004/07/showplan"; max(//ns:RelOp/@Parallel)', 'float') > 0 ;
Part VII
Performance Tuning and Troubleshooting
The sys.dm_exec_requests, sys.dm_os_tasks, sys.dm_exec_sessions, sys.dm_exec_sql_text and sys.dm_exec_cached_plan DMVs can be queried, as shown below to, to determine whether any currently executing queries are running in parallel. For queries running in parallel, you will see multiple rows for the session_id and request_id columns of the sys.dm_os_tasks DMV. You can retrieve the Transact-SQL code via the sys.dm_exec_sql_text DMV and the execution plan from the sys.dm_exec_cached_plan through their respective handles.
SELECT Requests.session_id, Requests.request_id, MAX(ISNULL(exec_context_id, 0)) AS number_of_workers, Requests.sql_handle, Requests.statement_start_offset, Requests.statement_end_offset, Requests.plan_handle FROM sys.dm_exec_requests AS Requests JOIN sys.dm_os_tasks AS Tasks ON Requests.session_id = Tasks.session_id JOIN sys.dm_exec_sessions AS Sessions ON Requests.session_id = Sessions.session_id WHERE Sessions.is_user_process = 0x1 GROUP BY Requests.session_id, Requests.request_id, Requests.sql_handle, Requests.plan_handle, Requests.statement_start_offset, Requests.statement_end_offset HAVING MAX(ISNULL(exec_context_id, 0)) > 0;
Where total_worker_time column value is greater than the total_elapsed_time column value for the sys.dm_exec_query_stats DMV as shown below. Not all parallel queries will exhibit this behavior.
SELECT * FROM sys.dm_exec_query_stats AS QueryStats CROSS APPLY sys.dm_exec_sql_text(QueryStats.plan_handle) AS SQLText WHERE total_worker_time > total_elapsed_time ;
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