Figure 5-5. Oracle processes call the getrusage system call to determine their resource consumption details. This DBWR operating system trace duration was 300 seconds, which means the database writer background process called getrusage about once every 6 seconds. By the way, notice the DBWR does make some read calls.

Oracle does, in fact, update the v$sess_time_model view with the database writer resource consumption about every 6 seconds. I tested this by simply repeatedly running the first query shown in Figure 5-6. The background cpu time statistic stayed constant for 6 seconds, and then I saw the value incremented. Notice the database writer CPU time is classified as background cpu time, not DB CPU time. Oracle server process time is placed into the DB CPU bucket.

Also notice the statistic value in the second query in Figure 5-6. This is the CPU time consumed (instance statistic CPU used by this session) by the database writer since the instance has started. But it shows zero seconds of CPU consumed! This demonstrates that for the database writer, v$sesstat does not contain background process CPU consumption. Therefore, if you want to classify background process CPU time, you must either gather from the /proc virtual file system or simply query from v$sess_time_model or v$sys_time_model.