Tales From A Lazy Fat DBA

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What is a short stack dump ?

Posted by FatDBA on May 23, 2022

Hi Everyone,

Short stacks are a rich source of information which can help you to do analyzing and deconstructing a process which is part of a problem in Oracle databases, and the good part is you don’t have to rely on any OS utilities like gdb, pstack etc. to extract stacks or traces. Its pretty simple and quick to generate the short stacks, you have to pass on the OSPID of the process, and issue SHORT_STACK option for ORADEBUG, and will generate a sequence of all functions involved and their current position in a calls stack. You can also use errorstack for the same, but short_stacks are fast and point-to-point!

Now one can ask – What is a ‘call stack’ ?
A call stack is a list of names of methods called at run time from the beginning of a program until the execution of the current statement. A call stack is mainly intended to keep track of the point to which each active subroutine should return control when it finishes executing. Call stack acts as a tool to debug an application when the method to be traced can be called in more than one context. This forms a better alternative than adding tracing code to all methods that call the given method.

Let me run it on a real time system where we are facing issues where due to adaptive switching between Log write methods LGWR was causing excessive ‘log file sync‘ Waits

[oracle@oracleontario ~]$ ps -ef|grep lg
oracle     4402      1  0 00:25 ?        00:00:00 ora_lgwr_dixitdb
oracle     4719   3201  0 00:25 pts/1    00:00:00 grep --color=auto lg

-- Here in the example I am generating short stacks for LGWR process
SQL> oradebug setospid 4402
Oracle pid: 19, Unix process pid: 4402, image: oracle@oracleontario.ontadomain (LGWR)
SQL> oradebug short_stack

-- After a gap of ~ 10-15 Mins
-- Let me generate the short stack once again just to see if there is any difference.
SQL> oradebug short_stack
ksedsts()+380<-ksdxfstk()+52<-ksdxcb()+3524<-sspuser()+140<-__sighndlr()+12<-call_user_handler()+992 <-sigacthandler()+104
<-_syscall6()+32<-sskgpwwait()+236<-ksliwat()+1752<-kslwait()+240<-ksarcv()+212 <-ksbabs()+764<-ksbrdp()+1616<-opirip()+1680

So, as you’ve seen in above code, the clear change is there in the LGWR short stacks, we did few log switches, modified a big table in the database followed by multiple commits. If you carefully look at the stack, it was function ksedsts() where it was at the time we generated the stack, and function ksdxfstk () called it, followed by ksdxcb(), sspuser() and immediate change after functions called by LGWR internal call stacks. They matched one of the known BUG which gave us a hint that we resolved after modified a undocumented parameter.

So, it can help us locating the buggy functions called by any user session or background processes and is very useful in case when you are battling with a strange looking problem in your Oracle database.

Few of the functions that I am able to recall are given below.
semtimedop() is one of the function for semaphore operations also known as semop and is used to perform atomically an array of semaphore operations on the set of semaphores associated with the semaphore identifier specified by semid.
‘ksdxcb’ is a function that’s called usually when the command oradebug is being used.
sighandler is the programming signal handler and is used to locate an exception. When a signal is sent to a process, various register and stack operations occur that make it look as though the leaf PC at the time of the signal is the return address for a call to a system function, sigacthandler(). sigacthandler() calls the user-specified signal handler just as any function would call another.
The sigaltstack() function allows a thread to define and examine the state of an alternate stack area on which signals are processed.

opidrv() is ORACLE Program Interface DRiVer (IGNORE)
opiodr is ORACLE code request driver – route the current request
main() is the standard executable entry point
ksedst() is the KSE dump the call stack
skgmstack() is the call specified function with extra STACK space
rpidrv() or the RPI is theRecursive Program Interface DRiVer
pfrrun() or PSDEVN is the PL/SQL Interpreter Main Instruction Loop
kkxexe() or KKX is to execute plsql

Hope It Helped!
Prashant Dixit


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