In one of my previous post i showed that it was not possible to draw a complete latch call graph for a specific process based only on dedicated function call such as ksl_get_shared_latch,kslgetl and kslfre. The reason for this is that UltraFast latch can be acquired without the need for a dedicated function call (ex: inside kcbgtcr function).
So how to track them ?
The purpose of this blog post is to show how we can troubleshoot contention on a specific latch using a systemtap script. This post is highly inspired by the “latchprof” script developed by Tanel Poder and his systematic approach for latch contention troubleshooting (For more info please check latch-contention-troubleshooting .)
This is what we are going to achieve :
Tested in : oracle 11.2.0.4/OEL6/UEK4
stap -v monitor_latch.stp “latch_address” “latch#” “refresh_time”
This script show a breakdown of latch holder by pid/session id/sql_hash for “cache buffers chains” latch with address “0x000000009F69FF60”
When troubleshooting a performance problem or investigating oracle internal using dynamic tracing tools like systemtap,it’s often useful to have the session address at hand. In fact, having the session address we can access many useful information such as : wait_event,p1 and p2 value,sql_id,and many other fields as stored in X$KSUSE (underlying table to V$SESSION). Luca Canali have already done a great work ,he identified that when the function “kskthewt” is called at the end of a wait event the register R13 (tested with Oracle 11.2.0.4 on RHEL6.5 and with Oracle 12.1.0.2 on OEL7 respectively) is pointing to the session addr with some offset and he manged also to determine the offset of the different column of X$KSUSE using X$KQFCO and X$KQFTA as in here.
The question is : Can we determine the session address without probing any function call ?
One way to answer this question is to determine how the value stored in the register R13 was set in the function “kskthewt”. Time to disassemble !
NOTE : This post contain no disassembly code of the oracle executable just the finding !
For basic info on reverse engineering please take look at my previous post.
In one of my previous post entitled Latch acquisition/release call-graph : Dynamic tracing tools in action i have assumed that the function “ksl_get_shared_latch” (in version 11.2.0.4/12.1.0.2) took only 5 arguments :
As an exercise to my previous post Reverse engineering : What we need to know as a DBA ? i decided to take a deeper look
“Reverse engineering, also called back engineering, is the processes of extracting knowledge or design information from anything man-made and re-producing it or re-producing anything based on the extracted information” Ref wikipedia
There are many purpose for reverse engineering such discovering software bug,security auditing, removal of copy protection,improving documentation shortcomings,learning purposes,etc. But why is this important to us ?
Hatem Mahmoud's blog 