Do You Have a Wait Problem?

It’s easy to find out the percentage of time a database has spent waiting for resources instead of actually executing. Issue the following query to find out the relative percentages of wait times and actual CPU processing in the database:

select metric_name,value from v$sysmetric where metric_name in ('Database CPU Time Ratio','Database Wait Time Ratio') and intsize_csec = (select max(intsize_csec) from v$sysmetric);

METRIC_NAME VALUE

————————————------------ -----------

Database Wait Time Ratio 11.371689

Database CPU Time Ratio 87.831890

SQL>

If the query shows a very high value for the Database Wait Time Ratio, or if the Database Wait Time Ratio is much greater than the Database CPU Time Ratio, the database is spending more time waiting than processing and you must dig deeper into the Oracle wait events to identify the specific wait events causing this.

Find Detailed Information:

You can use the following Oracle views to find out detailed information of what a wait event is actually waiting for and how long it has waited for each resource. 

• V$SESSION: This view shows the event currently being waited for as well as the event last waited for in each session.
• V$SESSION_WAIT: This view lists either the event currently being waited for or the event last waited on for each session. It also shows the wait state and the wait time.
• V$SESSION_WAIT_HISTORY: This view shows the last ten wait events for each current session.
• V$SESSION_EVENT: This view shows the cumulative history of events waited on for each session. The data in this view is available only so long as a session is active.
• V$SYSTEM_EVENT: This view shows each wait event and the time the entire instance waited for that event since you started the instance.
• V$SYSTEM_WAIT_CLASS: This view shows wait event statistics by wait classes.

How It Works

Your goal in tuning performance is to minimize the total response time. If the Database Wait Time Ratio  is high, your response time will also be high due to waits or bottlenecks in your system. On the other hand, high values for the Database CPU Time Ratio indicate a well-running database, with few waits or bottlenecks.

The Database CPU Time Ratio is calculated by dividing the total CPU used by the database by the Oracle time model statistic DB time.

Oracle uses time model statistics to measure the time spent in the database by the type of operation. Database time, or DB time, is the most important time model statistic—it represents the total time spent in database calls, and serves as a measure of total instance workload. DB time is computed by adding the CPU time and wait time of all sessions (excluding the waits for idle events).

An AWR report shows the total DB time for the instance (in the section titled “Time Model System Stats”) during the period covered by the AWR snapshots. If the time model statistic DB CPU consumes most of the DB time for the instance, it shows the database was actively processing most of the time. DB time tuning, or understanding how the database is spending its time, is fundamental to understanding performance. The total time spent by foreground sessions making database calls consists of I/O time, CPU time, and time spent waiting for non-idle events. Your DB time will increase as the system load increases—

that is, as more users log on and larger queries are executed, the greater the system load. However, even in the absence of an increase in system load, DB time can increase, due to deterioration either in I/O or application performance. As application performance degrades, wait time will increase and consequently DB time (that is, response time) will increase.

DB time is captured by internal instrumentation, ASH, AWR, and ADDM, and you can find detailed performance information by querying various views or through Enterprise Manager.

The V$SESSION_WAIT view shows more detailed information than the V$SESSION_EVENT and the V$SYSTEM_EVENT views. While both the V$SESSION_EVENT and the V$SESSION_WAIT views show that there are waits such as the event db file scattered read, for example, only the V$SESSION_WAIT view shows the file number (P1), the block number read  (P2), and the number of blocks read (P3). The columns P1 and P2 from this view help you identify the segments involved in the wait event that is currently occurring.

The Automatic Workload Repository (AWR) queries the V$SYSTEM_EVENT view for its wait event–related analysis.

You can first query the V$SYSTEM_EVENT view to rank the top wait events by total and average time

waited for that event.

In addition to providing information about blocking and blocked users and the current wait events, the V$SESSION view also shows the objects that are causing the problem, by providing the file number and block number for the object.