All Oracle DBAs have to know about Unix Memory Monitoring

All Oracle DBAs have to know about
Unix Memory Monitoring
2006-11-14

Copyright © 2005 TietoEnator Corporation

Example 1


Example 2


Example 3


Main questions …
 Do I have a memory shortage ?
 How many memory is utilized currently ?

 Why answers are not obvious?

Doesn't focus on particular Unix platform
Gives general idea of memory management


Content
 Concepts of Unix memory management
– Single process virtual address space & main components
– Unix swap subsystem
– Unix system memory components
– Why it is difficult to say how many memory is utilized

 Possible answers
– Do I have a memory shortage ?
– How many memory is utilized currently ?

 Oracle & Memory utilization


Single process virtual address space
“Dream world”
00000000 0x00000000
TEXT (CODE)
- 175MB - - 175MB -
0AEA8000 0x0AEA8000
TEXT Variable part
0AEA8000 0x0AEA8000
HEAP (PGA)
- 849 MB- - 849 MB-
40000000 0x40000000
Dynamically loaded
40000000 0x40000000
Libraries (CODE)
- 256 MB - - 256 MB -
DLL Variable part
50000000 0x50000000
Shared Memory
BFFE3000(1,75GB) 0xBFFE3000(1,75GB)

Stack area +
FFFFFFFF (1GB) 0xFFFFFFFF (1GB)

DEMO
pmap


Single process virtual memory
“Dream world”
 Components order and size of memory regions are dependent on used
platform and Oracle version
 Main components
– Text (constant/variable parts)
 oracle – executable in case of client process
– Heap
 PGA – Program Global Area
– Sort/Hash area
– Program variables
– Libraries (constant/variable parts)
 OS/Oracle libraries
 Example: JVM libraries in case of java call within DB
– Shared Memory
 SGA (shared pool, buffers cache, etc)
– Stack area
 Each area have strict boundaries for current installation/configuration


Single process virtual memory translation
“Dream world” “Real World”
RAM on DB Server
Random Access Memory
00000000
TEXT (CODE)
- 175MB -
0AEA8000
TEXT Variable part
0AEA8000
HEAP (PGA)
- 849 MB-
40000000 VM
Dynamically loaded Translation
40000000
Libraries (CODE)
- 256 MB -
DLL Variable part
50000000
Shared Memory
BFFE3000(1,75GB)

Stack area +
FFFFFFFF (1GB)

SWAP Space on disk FS


Single process virtual memory translation
 Single process see continues memory space

 OS translate process space to physical locations

 Some parts of virtual process space are shared across all
running processes
– OS/Oracle code
– SGA

 Some memory areas are private for each process


Single process virtual address space
One copy per process (W) One copy per server (R/O)
TEXT (CODE)

TEXT Variable part

HEAP (PGA)

Dynamically loaded
Libraries (CODE) One copy per Instance (W)
DLL Variable part
Shared Memory

Stack area


Main memory consumption areas
Process Private Area (W) Static Code (R/O)
PGA Oracle executables

Runtime size is variable Runtime size is constant

Shared Memory (W)
Oracle SGA

Runtime size is constant


RAM utilization and swap subsystem
“Real World”
RAM (size is limited)
Process Private Area (W) Static Code (R/O)
PGA Oracle executables
……..
Runtime size is variable Constant Size

Shared Memory (W)
Oracle SGA

Swap Process Swap out
Swap in

Disk

SWAP Space FS


Swap subsystem
 Allow processes to utilize more memory then available in the system
 Swap process swap out inactive parts of the memory

 On some platforms OS reserve swap space at the time RAM area is
allocated
– This is why Oracle recommends to create 2 times swap space
then RAM installed on the server
– Because of that used Swap space can’t be used as general
indicator of memory shortage

 Intensive swap out-s is good indicator of memory shortage


Resident size of the process (RSS)
00000000
TEXT (CODE)
- 175MB -
0AEA8000 SWAP
TEXT Variable part
0AEA8000 or
HEAP (PGA)
- 849 MB-
40000000 DISK
RAM
Dynamically loaded
40000000
Libraries (CODE)
- 256 MB - or
DLL Variable part
50000000
NOT
Shared Memory
BFFE3000(1,75GB) ALLOCATED
Stack area
FFFFFFFF (1GB)


RSS Part
00000000
TEXT (CODE)
- 175MB -
0AEA8000
TEXT Variable part
0AEA8000
HEAP (PGA)
- 849 MB-
40000000
Dynamically loaded
40000000
Libraries (CODE)
- 256 MB -
DLL Variable part
50000000
Shared Memory
BFFE3000(1,75GB)

Stack area
FFFFFFFF (1GB)

VSIZE Part


How many memory utilized? [OS view]
 OS Tools reflect different information on different platforms

 In general ps, top and other utilities
– doesn’t reflect Size of Process Private area correctly
– Include shared parts of the process

 We can’t get figure how many memory utilized by adding
all RSS or VSIZE values

 pmap like utility can be used (on single process basis)


Unix system memory components
Server dedicated to run Oracle Instance

RAM
Process Private Area (W) Static
PGA Code
…….. (R/O)
Kernel Runtime size is variable
Oracle
& exe File System
System Shared Memory (W) Constant Buffers
Processes Oracle SGA Size


How many memory is free? [OS View]
 Can we rely on free memory figure from top and others tools?

 The file system cache uses all available free memory to buffer files
on the file system

 On most systems, the amount of free memory is almost zero

 file system cache used memory freed first of all if there are memory
shortage

 Free memory figure doesn’t represent available memory in the system


Do I have a memory shortage ?
 Free memory indicator
– Because of file system cache almost useless
– If free memory is close to 0 don’t worry

 Swapping activities
– One of the best OS level indicators
– Swap out (so), Page out (po), Scan-rate (sr)


How many memory is utilized currently ?
 Constant memory areas
– Kernel & OS Processes
– Static Code (executables & libraries)
– Shared Memory (SGA)
 Variable memory areas
– File System Buffers (freed on demand)
– Process Private Area (PGA)
RAM
Process Private Area (W) Static
PGA Code
…….. (R/O)
Kernel Runtime size is variable
Oracle
& exe File System
System Shared Memory (W) Constant Buffers
Processes Oracle SGA Size


How to see used PGA size?
 Easily can be retrieved from database

 Tuned using DB parameters
– pga_aggregate_target
– sort_area_size/hash_area_size
– SGA


Oracle & Memory utilization
 In general Oracle doesn't return used memory back to OS
 Shared server configuration
– UGA located in shared pool PGA is NOT !
– Single call runtime variables located in PGA
 Background processes
– Job processes
– Parallel processes
 SGA_MAX_SIZE
– Oracle Reserves memory straight after instance startup
 Oracle on 32bit platform
– Can’t address more then 4GB memory
– By default 1.7GB big SGA allowed, increase up to 2.7 GB (Note:200266.1)
– Very Large Memory (VLM) up to 64GB (Note:260152.1)
 Heapdump could be used to troubleshoot memory related problems
– oradebug dump heapdump 5 (Note:174555.1)


Conclusion
 Do I have a memory shortage ?
– Free memory is not a good metric
– Swap size not always good to consider
– Swap outs is one reliable indicator

 How many memory is utilized currently ?
– Difficult to see from Unix side
– Variable part could be retrieved from Oracle
– There some problems to consider


Thank you for your attention …
Q&A


Tools to calculate memory usage [OS View]
 ipcs : Shared memory information
ipcs -m
 pmap : Memory map for a particular process
pmap <pid> [cat /proc/<pid>/maps]
 ps : Memory information for list of the processes
ps -u oraenv01 -o pid,vsize,rss,cmdtop
 top : General memory utilization information
top [cat /proc/meminfo]
 vmstat, sar : Swap activities indicators


All Oracle DBAs have to know about Unix Memory Monitoring

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