Lesson 5: Correlating Performance and Monitoring Data in Visual Studio .NET

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Lesson 5: Correlating Performance and Monitoring Data
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All this processing means that memory has to be allocated at least five times. If there is memory pressure on the system, the thread has to wait for memory to be allocated each time it is required, resulting in five trips to the waiting queue along with five trips up the runnable queue. The same goes for processor, disk I/O, memory, locks, and so on. Each of these resource allocations adds time to the overall duration of a query. Thus, identifying anything causing a bottleneck increases overall performance. Writing queries so that they access the minimum amount of data and use the minimum amount of resources also means better performance. Minimizing all these factors requires correlating many pieces of data together into a single cohesive picture of the processing state within a SQL Server instance.
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Correlating System Monitor Data with SQL Server Profiler Traces
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The most common use of Profiler is to gather traces related to long-running queries. Although Profiler enables you to capture long-running queries, it does not provide the context to explain why queries might be running long. Consider that you have configured Profiler to capture queries that are taking longer than three seconds to execute. After capturing several dozen queries that meet the criteria, each one is executed against a test system that mimics production in both hardware and database size. Each of the queries completes in 30 milliseconds or less. Why would these queries take longer than three seconds to complete in production To find the answer, you can take advantage of a new capability in SQL Server 2005 to provide context to a trace being captured in Profiler by correlating the trace to a System Monitor counter log. The Profiler trace must include the Start Time as one of the data columns to allow events to be correlated. After a trace has been stopped and is no longer capturing events, you can correlate it to a counter log by using the File, Import Performance Data menu. After selecting the counter log and the counters to correlate, you see a consolidated screen like the one shown in Figure 15-21. Using the context provided by the counter logs, you can evaluate further information with respect to the previous trace for long-running queries. You might determine, for example, that every time a query takes more than three seconds to execute, the processor utilization is at 100 percent. So instead of trying to tune the queries, you would investigate the cause of high CPU utilization to improve query performance.
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Monitoring and Troubleshooting SQL Server Performance
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Figure 15-21 Correlating a System Monitor counter log with a Profiler trace
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Correlating DMVs/DMFs with SQL Server Profiler Traces
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Continuing with the earlier trace for queries executing longer than three seconds, you determine that each query has a less-than-optimal query plan. Instead of using indexes, each of the queries is performing a table scan. However, all the appropriate indexes have been created the query optimizer is simply not using them.
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Don t try to outguess the optimizer
In this case, some developers and DBAs would begin trying to rewrite the queries or, even worse, adding query hints to force the query optimizer to use the indexes. Keep in mind, however, that the optimizer is an extremely intelligent piece of software that is constantly sampling data distributions and making adjustments so that it can process queries with the fewest resources possible. It is extraordinarily rare that anyone is going to outguess the optimizer and force it down a more optimal path than the one it has chosen.
By combining the data from the sys.dm_db_index_physical_stats DMV, you might find that the optimizer is not selecting the indexes that are expected because they have become heavily fragmented. Simply rebuilding the indexes to eliminate the fragmentation would cause the optimizer to begin selecting the expected indexes, immediately improving query performance without anyone ever having to change the code or the database structure.
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