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One of the first things you can do to measure instance performance efficiency is to determine the proportion of total time the database is spending working compared to the proportion of time it s merely waiting for resources. The V$SYSMETRIC view displays the system metric values for the most current time interval. The following query using the V$SYSMETRIC view reveals a database instance where waits are taking more time than the instance CPU usage time: SQL> 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 -----------------------Database Wait Time Ratio Database CPU Time Ratio SQL> VALUE -----72 28
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Once you realize that the total instance wait time ratio is much higher than the CPU time ratio, you can explore things further. Wait classes provide a quick way to figure out why the database instance is performing poorly. In the example shown in Listing 20-14, you can easily see that user I/O waits are responsible for most of the wait time. You can establish this fact by looking at the PCT_TIME column, which gives you the percentage of time attributable to each wait class. Total waits are often misleading, as you can see by looking at the NETWORK wait class. In percentage terms, network waits are only 1 percent, although total network waits constitute more than 51 percent of total waits in this instance. Listing 20-14. Determining Total Waits and Percentage Waits by Wait Class SQL> 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16* SELECT WAIT_CLASS, TOTAL_WAITS, round(100 * (TOT_WAITS / SUM_WAITS),2) PCT_TOTWAITS, ROUND((TIME_WAITED / 100),2) TOT_TIME_WAITED, round(100 * (TOT_TIME_WAITED / SUM_TIME),2) PCT_TIME FROM (select WAIT_CLASS, TOT_WAITS, TOT_TIME_WAITED FROM V$SYSTEM_WAIT_CLASS WHERE WAIT_CLASS != 'Idle'), (select sum(TOT_WAITS) SUM_WAITS, sum(TOT_TIME_WAITED) SUM_TIME from V$SYSTEM_WAIT_CLASS where WAIT_CLASS != 'Idle') ORDER BY PCT_TIME DESC;
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CH A PT ER 2 0 PERF O RMAN CE TUNI NG: TUN ING TH E I NS TA NCE
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WAIT_CLASS TOTAL_WAITS ------------- ----------User I/O 6649535191 Other 394490128 Concurrency 78768788 Network 7546925506 Application 2012092 Commit 15526036 Configuration 12898465 System I/O 53005529 Administrative 25 Scheduler 1925 10 rows selected. SQL>
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PCT_TOT_WAITS ------------45.07 2.67 .53 51.15 .01 .11 .09 .36 0 0
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TOT_TIME_WAITED PCT_TIME --------------- -------46305770.5 84.42 5375324.17 9.8 1626254.9 2.96 547128.66 1 449945.5 .82 351043.3 .64 116029.85 .21 78783.64 .14 7.6 0 .15 0
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Using V$ Tables for Wait Information
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The key dynamic performance tables for finding wait information are the V$SYSTEM_EVENT, V$SESSION_EVENT, V$SESSION_WAIT, and the V$SESSION views. The first two views show the waiting time for different events. The V$SYSTEM_EVENT view shows the total time waited for all the events for the entire system since the instance started up. The view doesn t focus on the individual sessions experiencing waits, and therefore it gives you a high-level view of waits in the system. You can use this view to find out what the top instance-wide wait events are. You can calculate the top n waits in the system by dividing the event s wait time by the total wait time for all events. The three key columns of the V$SYSTEM_EVENT view are total_waits, which gives the total number of waits; time_waited, which is the total wait time per session since the instance started; and average_wait, which is the average wait time by all sessions per event. The V$SESSION_EVENT view is similar to the V$SYSTEM_EVENT view, and it shows the total time waited per session. All the wait events for an individual session are recorded in this view for the duration of that session. By querying this view, you can find out the specific bottlenecks encountered by each session. The third dynamic view is the V$SESSION_WAIT view, which shows the current waits or justcompleted waits for sessions. The information on waits in this view changes continuously based on the types of waits that are occurring in the system. The real-time information in this view provides you with tremendous insight into what s holding up things in the database right now. The V$SESSION_WAIT view provides detailed information on the wait event, including details such as file number, latch numbers, and block number. This detailed level of information provided by the V$SESSION_WAIT view enables you to probe into the exact bottleneck that s slowing down the database. The low-level information helps you zoom in on the root cause of performance problems. The following columns from the V$SESSION_WAIT view are important for troubleshooting performance issues: EVENT: These are the different wait events described in the next section (for example, latch free and buffer busy waits). P1, P2, P3: These are the additional parameters that represent different items, depending on the particular wait event. For example, if the wait event is db file sequential read, P1 stands for the file number, P2 stands for the block number, and P3 stands for the number of blocks. If the wait is due to a latch free event, P1 stands for the latch address, P2 stands for the latch number, and P3 stands for the number of attempts for the event. WAIT_CLASS_ID: This identifies the wait class.
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