direct sga access without sql
DESCRIPTION
TRANSCRIPT
Oaktable Jonathan Lewis and ORACLE_TRACE Oracle_Trace crashes my Database I start the SGA attach by searching every offset Anjo Kolk says James Morle wrote a program using
x$ksmmem I show James my first draft using x$ksmmem James is baffled by why I'm hard coding offsets James says the offsets are in some X$ table I search, turn up a mail by Jonathan Lewis on
x$kqfco Goldmine – all the offsets Thanks Mogens Nogard! Thanks to TomKyte's Decimal to Hex
http://oraperf.sourceforge.net
Direct Oracle SGA Memory Access
Reading data directly from Oracle’s shared memory segment using C code
Monday, April 10, 2023
SGA on UNIX
oracle oracle sqlplussqlplus
DBWRDBWR
CKPTCKPT
LGWRLGWR
ARCHARCH
PMONPMON
SMONSMON SSnnnnnnPPnnnnnn
SGAShared PoolShared Pool Database Buffer CacheDatabase Buffer Cache
Redo LogRedo LogBufferBuffer
DDnnnnnn
Machine Memory
SGA on NT
Shared PoolShared Pool Database Buffer CacheDatabase Buffer CacheRedo LogRedo Log
BufferBuffer
SMONSMON
PMONPMON
DBWRDBWRLGWRLGWR
SSnnnnnn DDnnnnnn PPnnnnnn CKPTCKPT
ARCHARCH oracle oracle
sqlplussqlplusProcess Space
Machine Memory
What is the SGA
Memory CacheOften Used Data Rapid AccessShareable Concurrently Access
SGA 4 main regions
Fixed information– Users info– Database statistics– X$dual– etc
Data block cache SQL cache ( library cache/shared pool) Redo log buffer
How is the SGA info Used?
Automatically– data blocks cached– Log buffer– Sql cache– Updates of system and user statistics
User Queries– User info v$session– System info v$parameter– Performance statistics v$sysstat, v$latch, v$system_event– Buffer cache headers, x$bh
Why Direct Access with C?
Reading Hidden Information– Sort info on version 7 – OPS locking info version 8 – Contents of data blocks (only the headers or visible in X$)
Access while Database is Hung High Speed Access
– Sampling User Waits, catch ephemeral data– Scan LRU chain in X$bh– Statistically approximate statistics
SQL statistics per user
Low overhead
Database Slow or Hung
Often happens at the largest sites when cutting edge support is expected.
Shared Pool errors ORA 4031 Archiver or Log file Switch Hangs Hang Bugs Library Cache Latch contention ORA-00379: no free buffers available in
buffer pool DEFAULT
Statistical Sampling
By Rapidly Sampling SQL statistics and the users who have the statistics open, one can see how much work a particular user does with a particular SQL statement
Low Overhead
Marketing Appeal Clients are sensitive about their
production databases Heisenberg uncertainty affect – less
overhead less affect monitoring has on performance which we are monitoring
SGA made visible through x$tables
Most of the SGA is not visible X$KSMMEM Exception, Raw Dump of SGA Information Externalized through X$ tables Useful or Necessary information is Externalized Externalized publicly through V$ Tables
Machine Memory
0x80000000
SGASGA
Graphic SGA
SGA0x80000000
Buffer Cache
Buffer Cache
Fixed Area
Shared Pool
Log Buffer
Fixed Area
0x85251EF4
X$KSUSECST- user waitsSGA0x80000000
X$KSUSECST
170 Records2328 bytes
Row 10x85251EF4 Row 2Row 3 …
X$KSUSECST Record
One Record in X$KSUSECST
2328 bytes
1276
X$KSUSECST Fields
1276 1278 1280 1284 1288
Seq # Event # p3p2p1
Externalization of C structs: X$ tables
If Structure foo was externalized in a X$
SQL> describe x$fooColumn Name Type------------------------------ --------ADDR RAW(8)INDX NUMBERID NUMBERB NUMBER
SGA is One Large C Struct
struct foo{
int id; int A; int B; int C;};
struct foo foo[N];
Struct C code
#include <stdio.h>
#include <fcntl.h>
#define N 20
/* structure definition: */
struct foo
{
int id;
int a;
int b;
int c;
};/* end structure definition */
Struct Recordmain(){
struct foo foo[20];
int fptr;
/* zero out memory of struct */
memset(foo,0,sizeof(foo));
foo[0].id=1; /* row 0 */
foo[0].a=12;
foo[0].b=13;
foo[0].c=13;
Struct Write to File
foo[1].id=2; /* row 1 */
foo[1].a=22;
foo[1].b=23;
foo[1].c=24;/* write to file, simulate SGA */
if ((fptr = open("foo.out",O_WRONLY | O_CREAT,0777)) < 0 )
return -1;
write(fptr,foo,sizeof(foo)); return 0;
}
Simulate SGA with a File
write(fp,foo,sizeof(foo));
Simulate SGA with a File
Memory address Increasing
0 16
32
48
64
80
AID B IDC A …
Row 0 Row 1
0 4 8 12
16
20
bitsbytes
0 4 8 C 10
14
hex bytes0 4 1
014
20
24
oct bytes
Struct File Contents
$ ./foo
$ ls -l foo.out
-rw-r--r-- joe dba 320 Feb 10 19:41 foo.out
int = 32 bits
Int = 4 bytes
20 entries * 4 int * 4 bytes/int = 320 bytes
od – octal dump
$ od -l foo.out
0000000 1 12 13 13
0000020 2 22 23 24
0000040 0 0 0 0
*
0000500
Struct File Contents
Address is in HexColumn 2 is the IDColumn 3 is field AColumn 4 is field BColumn 5 is field C
X$ tables ?
Ok, x$foo =~ foo[20]How do I get a list of x$ tables?Where is each X$ located?V$Fixed_Tables
V$Fixed_Table – list of X$ tables
SQL> desc v$fixed_table;
Name Null? Type
----------------------------------------- -------- -----------------
NAME VARCHAR2(30)
OBJECT_ID NUMBER
TYPE VARCHAR2(5)
TABLE_NUM NUMBER
Graphic: X$ Addresses
0x8????????
X$????
SGA0x80000000
V$Fixed_Table
spool addr.sqlselect 'select 'addr, ||''''||name||''''||' from ' || name ||'
where rownum < 2;' from v$fixed_table where name like 'X%'/spool [email protected]
Example: finding the address
select a.addr ,'X$KSUSE'
from X$KSUSE
where rownum < 2 ;
X$ layout6802B244 X$KSLEMAP
6802B7EC X$KSLEI
6820B758 X$KSURU
6820B758 X$KSUSE - v$session
6820B758 X$KSUSECST – v$session_wait
6820B758 X$KSUSESTA – v$session_stat
6820B758 X$KSUSIO
6826FBD0 X$KSMDD
6831EA0C X$KSRCHDL
What's in these X$ views
V$ views are documented V$ views are based often on X$ tables The map from v$ to X$ is described in :
V$Fixed_View_Definition
V$Fixed_View_Definition
SQL> desc V$Fixed_View_Definition Name Type ----------------------------------- -------------- VIEW_NAME VARCHAR2(30) VIEW_DEFINITION
VARCHAR2(4000)
Definition of V$Session_Wait
SQL> select VIEW_DEFINITION from V$FIXED_VIEW_DEFINITION where view_name='GV$SESSION_WAIT';VIEW_DEFINITION-----------------------------------------------------------------------select s.inst_id,s.indx,s.ksussseq,e.kslednam, e.ksledp1,s.ksussp1,s.ksussp1r,e.ksledp2, s.ksussp2,s.ksussp2r,e.ksledp3,s.ksussp3,s.ksussp3r, decode(s.ksusstim,0,0,-1,-1,-2,-2, decode(round(s.ksusstim/10000),0,-1,round(s.ksusstim/10000))), s.ksusewtm, decode(s.ksusstim, 0, 'WAITING', -2, 'WAITED UNKNOWN TIME', -1, 'WAITED SHORT TIME', 'WAITED KNOWN TIME') from x$ksusecst s, x$ksled e where bitand(s.ksspaflg,1)!=0 and bitand(s.ksuseflg,1)!=0 and s.ksussseq!=0 and s.ksussopc=e.indx
The Fields in X$ tables
OK, I've picked an X$ I've got the starting address Now, how do I get the fields?
X$KQFTA
Kernel Query Fixed_view Table INDX use to find column information KQFTANAM X$ table names
X$KQFCO
Kernel Query Fixed_view Column KQFCOTAB Join with X$KQFTA.INDX KQFCONAM Column name KQFCOOFF Offset from beginning of the
row KQFCOSIZ Columns size in bytes
X$KSUSECST Fields
1276 1278 1280 1284 1288
Seq # Event # p3p2p1
2 2 4 4 4 BYTES
Address
SGA Contents in Resume
Fixed SGA
Buffer Cache
Redo Buffer
Library Cache
Memory address Increasing0x8000000
In resume:
Oracle takes the C structure defining the SGA and maps it onto a shared memory segment
Oracle provides access to some of the SGA contents via X$ tables
**** Procedure *****
1. Choose a V$ view
2. Find base X$ Tables for v$ view
3. Map X$ fields to V$ fields
4. Get address of X$ table in SGA
5. Get the size of each record in X$ table
6. Get the number of records in X$ table
7. Get offsets for each desired field in X$ table
8. Get the base address of SGA
1) V$SESSION_WAIT Example
List of all users waiting Detailed information on the waits Data is ephemeral Useful in Bottleneck diagnostics High sampling rate candidate Event 10046 captures this info
Good table for SGA sampling
V$SESSION_WAIT DescriptionSQL> desc v$session_wait
Name Type
----------------------------------------- --------------------------
SID ,NUMBER
SEQ# ,NUMBER
EVENT ,VARCHAR2(64)
P1TEXT ,VARCHAR2(64)
P1 ,NUMBER
P1RAW ,RAW(4)
P2TEXT ,VARCHAR2(64)
P2 ,NUMBER
P2RAW ,RAW(4)
P3TEXT ,VARCHAR2(64)
P3 ,NUMBER
P3RAW ,RAW(4)
WAIT_TIME ,NUMBER
SECONDS_IN_WAIT ,NUMBER
STATE ,VARCHAR2(19)
)
V$SESSION_WAIT Short
SQL> desc v$session_wait
Name Type
---------------------------- -------------
SID NUMBER
SEQ# NUMBER
EVENT VARCHAR2(64)
P1 NUMBER
P2 NUMBER
P3 NUMBER)
V$FIXED_VIEW_DEFINITION
Gives mappings of V$ views to X$ tables
SQL> select
VIEW_DEFINITION
from
V$FIXED_VIEW_DEFINITION
where
view_name='V$SESSION_WAIT‘;
V$SESSION_WAIT View DefinitionVIEW_DEFINITION---------------------------------------------------------------------select s.inst_id, s.indx, s.ksussseq, e.kslednam,e.ksledp1,s.ksussp1,s.ksussp1r,e.ksledp2,s.ksussp2,s.ksussp2r,e.ksledp3,s.ksussp3,s.ksussp3r,round(s.ksusstim / 10000), s.ksusewtm, decode(s.ksusstim, 0, 'WAITING', -2, 'WAITED UNKNOWN TIME', -1, 'WAITED SHORT TIME', 'WAITED KNOWN TIME') from x$ksusecst s, x$ksled e where bitand(s.ksspaflg,1)!=0 and bitand(s.ksuseflg,1)!=0 and s.ksussseq!=0 and s.ksussopc=e.indx
View Definition ShortVIEW_DEFINITION---------------------------------------------------------------------select
s.indx,s.ksussseq,e.kslednam,s.ksussp1,s.ksussp2,s.ksussp3
from x$ksusecst s, x$ksled e
where s.ksussopc=e.indx
2) V$SESSION_WAIT Based on X$KSUSECT
VIEW_DEFINITION----------------------------------------------------select
indx,ksussseq,ksussopc,ksussp1,ksussp2,ksussp3
from x$ksusecst
Equivalent SQL Statements
select indx,ksussseq,ksussopc,ksussp1,ksussp2,ksussp3
from x$ksusecst
select sid seq# event p1 p2 p3 from v$session_wait )
Note: x$ksusecst. Ksussopc is the event #
x$ksled.kslednam is a list of the event names where
x$ksled.indx = x$ksusecst. ksussopc
3) V$ to X$ Field Mapping
4) Get base SGA address for X$ table
Find the location of X$KSUSECST in the SGA
SQL> select addr from x$ksusecst where rownum < 2
ADDR
--------
85251EF4
5) Find the Size of Each Record
SQL> select
((to_dec(e.addr)-to_dec(s.addr))) row_size
from
(select addr from x$ksusecst where rownum < 2) s,
(select max(addr) addr from x$ksusecst where rownum < 3) e ;
ROW_SIZE
----------------
2328
6) Find the Number of Records in the structure
SQL> select count(*) from x$ksusecst ;
COUNT(*)
--------------
170
Get Offsets for Each Desired Field in X$ table
SQL> select c.kqfconam field_name, c.kqfcooff offset, c.kqfcosiz szfrom x$kqfco c, x$kqfta twhere t.indx = c.kqfcotab and t.kqftanam='X$KSUSECST'order by offset;
X$KQFTA - X$ Tables Names
List of X$ tables
INDX use to find column information
KQFTANAM X$ table names
To get Column information join with X$KQFCO
X$KQFTA.INDX = X$KQFCO.KQFCOTAB
X$KQFCO – X$ Table Columns
List of all the columns in X$ Tables
KQFCOTAB Join with X$KQFTA.INDX KQFCONAM Column name KQFCOOFF Offset from beginning of the
row KQFCOSIZ Columns size in bytes
Field OffsetsFIELD_NAME OFFSET SZ------------------------------ ---------- ----------ADDR 0 4INDX 0 4KSUSEWTM 0 4INST_ID 0 4KSSPAFLG 1 1KSUSSSEQ 1276 2KSUSSOPC 1278 2KSUSSP1 1280 4KSUSSP1R 1280 4KSUSSP2 1284 4KSUSSP2R 1284 4KSUSSP3 1288 4KSUSSP3R 1288 4KSUSSTIM 1292 4KSUSENUM 1300 2KSUSEFLG 1308 4
What are all the fields at OFFSET 0?
These are all calculated values and not stored explicitly in the SGA.
ADDR memory address
INDX record number, like rownum
INST_ID database instance ID
KSUSEWTM calculated field
Unexposed Fields
What happens between OFFSET 1 and 1276?
• Unexposed Fields• Sometimes exposed elsewhere, in our case
• V$SESSION• V$SESSTAT
Fields at Same Address
Why do some fields start at the same address?KSUSSP1KSUSSP1R
Are at the same addressEquivalent of
V$SESSION_WAIT.P1 V$SESSION_WAIT.P1RAW
These are the same data, just exposed asHexDecimal
7) Offsets of Fields
8) Get Base SGA Address
SQL> select addr from x$ksmmem where rownum < 2
ADDR
--------------
80000000
Results X$KSUSECST
Machine Memory
0x80000000
SGASGA
Fixed Area
0x85251EF4
X$KSUSECST- user waitsSGA0x80000000
X$KSUSECST
170 Records2328 bytes
Row 10x85251EF4 Row 2Row 3 …
X$KSUSECST Record
One Record in X$KSUSECST
2328 bytes
1276
X$KSUSECST Fields
1276 1278 1280 1284 1288
Seq # Event # p3p2p1
Attaching to the SGA
UNIX System Call “shmat”
To attach to shared memory Unix as a system call
void *shmat( int shmid,
const void *shmaddr,
int shmflg );
ID and Address arguments to “shmat”
The arguments are:
shmid – shared memory identifier specified shmaddr – starting address of the shared memory shmflg - flags
The argument shmflg can be set to SHM_RDONLY . To avoid any possible data corruption the SGA should only be attached read only.
The arguments shmid and shmaddr need to be set to Oracle’s SGA id and address.
Finding Oracle SGA’s ID and Address
Use ORADEBUG to find the SGA id
SQL> oradebug setmypid
Statement processed.
SQL> oradebug ipc
Information written to trace file.
Finding Trace File
SQL> show parameters user_dumpNAME VALUE
----------------------- --------------------------------
user_dump_dest /u02/app/oracle/admin/V901/udump
SQL> exit
$ cd /u02/app/oracle/admin/V901/udump
$ ls -ltr | tail -1
-rw-r----- usupport dba Aug 24 18:01 v901_ora_23179.trc
Finding SHMID in Trace File
$ vi v901_ora_23179.trc
…
Total size 004456c Minimum Subarea size 00000000
Area Subarea Shmid Stable Addr Actual Addr
0 0 34401 0080000000 0080000000
…
Attaching to the SGA
Shmid 34401Shmaddr0x80000000Shmflg SHM_RDONLY The SGA attach call in C would be: Shmat(34401, 0x80000000, SHM_RDONLY); This call needs to be executed as a UNIX user who has
read permission to the Oracle SGA
C Code Headers
#include <stdio.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <errno.h>
#include "event.h"
event.h is for translating the event #s into event names
Events.h
Spool events.h
select 'char event[][100]={' from dual;
select '"'||name||'",' from v$event_name;
select ' "" };' from dual;
spool off
Define Base Addresses and Sizes
/* SGA BASE ADDRESS */
#define SGA_BASE 0x80000000
/* START ADDR of KSUSECST(V$SESSION_WAIT) */
#define KSUSECST_ADDR 0x85251EF4
/* NUMBER of ROWS/RECORDS in KSUSECST */
#define SESSIONS 150
/* SIZE in BYTES of a ROW in KSUSECST */
#define RECORD_SZ 2328
Define Offsets to Fields
#define KSUSSSEQ 1276 /* sequence # */
#define KSUSSOPC 1278 /* event # */
#define KSUSSP1R 1280 /* p1 */
#define KSUSSP2R 1284 /* p2 */
#define KSUSSP3R 1288 /* p3 */
Set Up Variables
main(argc, argv) int argc; char **argv; { void *addr; int shmid; int shmaddr; void *current_addr; long p1r, p2r, p3r; unsigned int i, seq, tim, flg, evn;
Attach to SGA
/* ATTACH TO SGA */ shmid=atoi(argv[1]); shmaddr=SGA_BASE; if (
(void *)shmat(shmid,(void *)shmaddr,SHM_RDONLY)
== (void *)-1 ) { printf("shmat: error attatching to SGA\n"); exit(); }
Set Up Sampling Loop
/* LOOP OVER ALL SESSIONS until CANCEL */
while (1) {
/* set current address to beginning of Table */
current_addr=(void *)KSUSECST_ADDR;
sleep(1);
printf("^[[H ^[[J"); /* clear screen */
/* print page heading */
printf("%4s %8s %-20.20s %10s %10s %10s \n",
"sid", "seq", "wait","p1","p2","p3");
Loop over all Sessions
for ( i=0; i < SESSIONS ; i++ ) { seq=*(unsigned short *)((int)current_addr+KSUSSSEQ); evn=*(short *) ((int)current_addr+KSUSSOPC); p1r=*(long *) ((int)current_addr+KSUSSP1R); p2r=*(long *) ((int)current_addr+KSUSSP2R); p3r=*(long *) ((int)current_addr+KSUSSP3R); if ( evn != 0 ) { printf("%4d %8u %-20.20s %10X %10X %10X \n", i, seq, event[evn] ,p1r, p2r,p3r ); } current_addr=(void *)((int)current_addr+RECORD_SZ); } } }
Output
$ sga_read_session_wait 34401
sid seq wait p1 p2 p3
0 40582 pmon timer 12C 0 0
1 40452 rdbms ipc message 12C 0 0
2 43248 rdbms ipc message 12C 0 0
3 24706 rdbms ipc message 12C 0 0
4 736 smon timer 12C 0 0
5 88 rdbms ipc message 2BF20 0 0
8 178 SQL*Net message from 6265710 1 0
Pitfalls
Byte Swapping 32 bit vs 64 bit Multiple Shared Memory Segments Segmented Memory Addresses are "unsigned int" Misaligned Access
Little Endian vs Big Endian
Is low byte values first or high byte values first ? a byte is 8 bits
– 00000000-11111111 bits,0 – 31 dec, 0x0 - 0xFF hex Big Endian is "normal" , highest bit first In ascii, the word "byte" is stored as
– b = 62, y = 79, t = 74, e = 65 echo 'byte' | od -x
– b y t e– 62 79 74 65
Little Endian, ie byte swapped (Linux, OSF, Sequent, ? )
– y b e t – 79 62 65 74
Byte Swap Example
Short = 2 bytes ie 16 bitsGoal, get the flag in the "second" byte
#ifdef __linux uflg=*(short *)
((int)sga_address)>>8;#else uflg=*(short *)((int)sga_address);#endif
Byte Swap
Big Endian:00 00 00 00 00 00 00 01Little Endian00 00 00 01 00 00 00 00Solution, push the value over 8 places, to
the right, ie >>8
64 bit vs 32 bit
SQL> desc x$ksmmem Name Type
------------------------------------- --------- ADDR RAW(4) INDX NUMBER INST_ID NUMBER KSMMMVAL RAW(4)-> 32 bitRaw(8) -> 64 bit
Segmented Memory
x$ksuse – can be dis-contiguous
Work around:select 'int users[]={' from dual;select '0x'||addr||',' from x$ksuse;select '0x0};' from dual;
Misaligned Access
Some platforms seg fault when addressing misaligned bytes, need to read in even bytes or units of 4 bytes depending on platform
1 2 3 4 5 6 7 8
x$ksusecst Record: What's Missing?
One Record in X$KSUSECST
2328 bytes
1276
??? ???
Select Addr from X$? where Rownum< 2;
6802B244 X$KSLEMAP
6802B7EC X$KSLEI
6820B758 X$KSURU
6820B758 X$KSUSE – v$session
6820B758 X$KSUSECST – v$session_wait
6820B758 X$KSUSESTA – v$sesstat
6820B758 X$KSUSIO
6826FBD0 X$KSMDD
6831EA0C X$KSRCHDL
x$ksuse Record Contains x$ksusecst
One Record in X$Ksusecst
2328 bytes
1276
v$session_wait v$sessionv$session v$sesstat
x$ksusecstx$ksusesta
x$ksuse
236
Getting v$sesstat addresses
select '#define '|| upper(translate(s.name,' :-()/*''','________'))||' '|| to_char(c.kqfcooff + STATISTIC# * 4 )from x$kqfco c, x$kqfta t, v$statname swhere t.indx = c.kqfcotab and ( t.kqftanam='X$KSUSESTA' ) and
c.kqfconam='KSUSESTV' and kqfcooff > 0order by c.kqfcooff/
User Drilldown Query: 4 joins select w.sid sid, w.seq# seq, w.event event, w.p1raw p1, w.p2raw p2, w.p3raw p3, w.SECONDS_IN_WAIT ctime, s.sql_hash_value sqlhash, s.prev_hash_value psqlhash, st.value cpu from v$session s, v$sesstat st, v$statname sn, v$session_wait w where w.sid = s.sid and st.sid = s.sid and st.statistic# = sn.statistic# and
sn.name = 'CPU used when call started' andw.event != 'SQL*Net message from client'
order by w.sid;
Other Fun Stuff
The next example is output from an SGA program that follows the LRU of the Buffer Cache
The program demonstrates the • insertion point of LRU• cold end of LRU• hot end of the LRU• Full Table Scan Insertion Point
LRU HOT
LRU COLD