Top 10 waits

SQL Server Waits Statistics provides you with a different perspective on performance problems in SQL Server, seeing the progress of interacting processes in terms of the time they must spend waiting for the resources they need. Every time a request is forced to wait, SQL Server records the length of the wait (wait_time), and the cause of the wait (wait_type), the latter generally indicating the resource on which the request was waiting. As just one example among very many, a request to read a page may experience the PAGEIOLATCH_SH wait type, which is a wait that occurs when SQL Server needs to read a data page from disk into the buffer cache and can't immediately obtain a latch on the buffer page.

SQL Monitor collects wait statistics from the sys.dm_os_wait_stats dynamic management view to provide details of causes of server waits on the SQL Server instance, over time. It presents this data in the Top 10 Waits section, which comprises:

  • Waits history graph – frequency or duration (ms) of wait (/s), for each wait type detected over the period of analysis (determined by the focus window on the activity graph).
  • Top 10 waits table – where you can see the ten significant wait types, how long SQL Server requests spent waiting for each wait type, and which queries were affected (instance overview only). You can click on any wait type to see a detailed description of the wait type, an explanation of possible possible causes with troubleshooting advice, and details of any queries affected.

To learn more about using wait statistics as a performance diagnostic tool, we strongly recommend the following white paper (PDF format), produced as a collaboration between Redgate and SQL Server Performance Tuning Using Wait Statistics.

See the video demo of how to use the Top 10 Waits table at Redgate University.

Waits history graph

The waits history graph plots the number of times, or length of time (ms), a thread had to wait (per second), for each wait type detected, over the period. By default it shows Wait time (ms) as the y-axis, but when you click on one of the four columns, in the Top 10 Waits table below, it repopulates the table with fresh data, ordered by that column, and the wait history graph y-axis switches to show that column.

Top 10 waits table

The Top 10 waits table reveals the primary reasons why requests were being forced to wait, and for which resources. The data is read from the SQL Server DMV sys.dm_os_wait_stats.

Like the Top 10 Queries table, the Top 10 Waits table shows data for the period defined in the focus window of the Activity graph.

The table shows:

  • Wait type – the name of the wait from the list of 600+ described in sys.dm_os_wait_stats (MSDN). 
  • Wait description – a short definition, so you know at a glance what SQL Server is waiting for.  
  • Waiting tasks – the total number of wait events.
  • Wait time (ms) – the total time (in milliseconds) spent waiting for resource to become available. By default, waits are listed in descending order according to this metric.
  • Avg. wait time – the total time (in milliseconds) spent waiting for resource, divided by the number of waiting tasks.
  • Signal wait time (ms) – the total time (in milliseconds) spent waiting in a runnable state once SQL Server has signaled that resource is available. 

To update the table manually, click on a column header containing numeric data. This generates a new table of queries based on that column; it doesn't just change the sort order of the waits you can see currently.

When you select a wait type:

Viewing affected queries

Clicking on any wait type reveals a detail description, advice on how to investigate possible causes, and a list of queries affected by this type of wait, accompanied by query details, plans and query execution statistics, as described for Top 10 Queries.

The table contents is almost identical to the Top 10 Queries table; the only thing missing is the Database column. For more details, see: Top 10 Queries table.

Click on a query to show the query statement, the latest query plan handle, and a list of other waits that had the greatest impact on the query during execution. See: Viewing query details.

Viewing wait documentation

Click on a wait type to show a detailed description of the selected wait below the Top Waits table:

For the most common waits, you can see:

  • Details of why this wait occurred.
  • Tips on how to start investigating, including links to expert articles, as well as MSDN and TechNet resources.

Next step

SQL Server and host machine metrics

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