一个用户提交了一个awr报告，发现有大量的PX Deq Credit: send blkd事件:
Top 5 Timed Events
Event | Waits | Time(s) | Avg Wait(ms) | % Total Call Time | Wait Class |
CPU time |
| 3,023 |
| 43.0 |
|
db file sequential read | 320,800 | 1,205 | 4 | 17.1 | User I/O |
PX Deq Credit: send blkd | 452 | 998 | 2,207 | 14.2 | Other |
library cache lock | 19,034 | 425 | 22 | 6.0 | Concurrency |
latch: library cache | 3,023 | 366 | 121 | 5.2 | Concurrency |
Oracle对这个事件的描述：
This wait event indicates that we are doing a non-blocking test to see if any channel has returned a message. We should see that although there is a high number of these events, the time accumulated to it should be zero (or really low).
这个事件按Oracle的说法为空闲事件，不用太去关注，多发生在并行DML时。但在客户的系统中，并没有使PDML;在TOP SQL中，找到有对gv$session的查询语句，一条sql执行了872秒：
Elapsed Time (s) | CPU Time (s) | Executions | Elap per Exec (s) | % Total DB Time | SQL Id | SQL Module | SQL Text |
872 | 0 | 1 | 871.68 | 12.40 | PL/SQL Developer | select * from gv$session |
在metalink上找到了多篇gv$session相关问题的文章;看来以后运行gv$session要慎重呀…
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