Does DB Time include wait on cpu queue time?

DB TIME1=DB CPU2+ Foreground NO-Idle wait time 1：Amount of elapsed time (in microseconds) spent performing Database user-level calls. This does not include the elapsed time spent on instance background processes such as PMON. 2:Amount of CPU time (in microseconds) spent on database user-level calls. This does not include the CPU time spent on instance background processes such as PMON. 10g中引入了基于时间统计信息的调优模型，其核心或者说最值得我们关心的大概是DB TIME和AASC 2项指标。DB TIME是Oracle在数据库层对用户级别(不包括后台进程的CPU和非空闲等待时间)各类操作耗时的一个总计，它直接反映了数据库前台的"工时"。DB TIME是否包含了前台进程在CPU队列上的等待时间呢，换而言之我们的命题是"Does DB TIME equal to DB CPU+Foreground NO-Idle wait time + DB CPU ON QUEUE ?" 让我们来看看以下测试：

[maclean@rh2 ~]$ cat /proc/cpuinfo |grep processor|wc -l
2
/*这是一台双核的pc服务器*/

打开session A:
begin
/* first one busy on cpu */
loop
null;
end loop;
end;
/*死循环调用NULL，会尝试独占一个逻辑CPU,没有其他等待事件*/

打开SESSION B:
begin
/* second one busy on cpu */
loop
null;
end loop;
end;

打开SESSION C:
begin
/* third one busy on cpu */
loop
null;
end loop;
end;

打开SESSION D:
SQL> exec dbms_workload_repository.create_snapshot;
PL/SQL procedure successfully completed.

/*手动收集AWR 快照*/

/*等待几分钟，以便收集信息*/

SQL> exec dbms_workload_repository.create_snapshot;
PL/SQL procedure successfully completed.
/*再次手动收集快照*/

我们来看看AWR报告中的Time Model Statistics: DB TIME为344s,而DB CPU为226s。以上匿名PL/SQL块仅引起CPU争用(不会产生其他前台等待事件)，而这里DB TIME要远大于DB CPU，可见DB TIME=DB CPU+Foreground wait time+Cpu on queue;  我们也可以观察以下OEM界面上的AAS图形: ASH报告也正确反映了这一事实，虽然它把cpu time和wait on cpu queue归并做了Cpu+wait for CPU: 可以看到CPU ON QUEUE有时被计为Wait time部分。当以上3个回话同时执行CPU敏感的匿名块过程时，其瞬时的Average Session Count为3，而该pc服务器上只有2个逻辑CPU，可以认为实例在该短期内存出现严重阻塞，表现到OS层也就是短期内持续队列较高。

[maclean@rh2 ~]$ vmstat 2
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 3  0      0 752180  93912 2127440    0    0   194   168  536  302  6  1 90  3  0
 3  0      0 752056  93912 2127508    0    0     0     0 1012  661 100  0  0  0  0
 2  0      0 751436  93928 2127548    0    0     0   932 1131  750 99  1  0  0  0
 2  0      0 751436  93928 2127596    0    0     0    16 1012  644 99  0  0  0  0
 2  0      0 751444  93928 2127596    0    0     0     0 1011  695 100  0  0  0  0