Question

Tempfs relationship to pga and sga

Asked by: jhucofsa

I have a new Oracle 11 database using amm.  I do not understand the relationship between
the sga and pga usage and the  tempfs used.


MEMORY_MAX_TARGET  =       1264M
MEMORY_TARGET =       1264M

Output of v$memory_dynamic_components is
                                                                                   
COMPONENT                   CURRENT_SIZE           MIN_SIZE               MAX_SIZE  
-------------------------------------------------- ---------------------- -----------
shared pool                 503316480              234881024              503316480  
large pool                      16777216               16777216               16777216  
java pool                        16777216               16777216               16777216  
SGA Target                  872415232              687865856              872415232  
DEFAULT bcache         318767104              301989888              402653184  
PGA Target                   452984832              452984832              637534208  

df -  k                                                                              
tmpfs                  1310720    850672    460048  65% /dev/shm

Why is tmpfs only using 850672 which matches only the SGA?

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Asked On
2009-11-02 at 13:09:47ID24865201
Tags

SGA

,

PGA. Tempfs

Topic

Oracle Database

Participating Experts
2
Points
500
Comments
5

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Answers

 

by: mrjoltcolaPosted on 2009-11-02 at 13:29:24ID: 25723947

SGA is shared memory and stores "shared things" like buffer cache, SQL parse plans, etc.
By default, PGA is not stored in shared memory. It is heap and stack area for each server process. Things like sorts happen in PGA, among others.

It might help to take a few hours to read through the Oracle Concepts Guide.

http://download.oracle.com/docs/cd/B19306_01/server.102/b14220/toc.htm

 

by: franckpachotPosted on 2009-11-03 at 13:51:58ID: 25734199

Hi,
All oracle processes requires some memory.
Some objects must be shared among several processes, so they go to SGA (memory that can be addressed by all instance processes)
exemple: data blocks must be shared to manage concurrency on data -> the is buffer cache in SGA
Some objects are used only by one process, so they go to PGA (the process private memory)
exemple: direct reads (without concurrency) or query result sorting -> the session data in UGA is in PGA (if not shared because of shared server configuration)
Some objects are bigger then the available memory, so they are processed in chunks that are swapped to tempfiles.
example: query result sorting -> the workarea to sort or to hash join is written to tempfiles once sorted, in order to sort another chunks, and at the end, they are merged
Regards,
Franck.

 

by: jhucofsaPosted on 2009-11-04 at 09:14:19ID: 25741546

Thanks for the info  but I am still having problems understanding the tmpfs .

My understanding is :
I set MEMORY_MAX_TARGET  =   1264M.  That is how much memory oracle has to  allocate as it needs between the SGA and  the PGA  and   then within the SGA  it can reallocate as  needed shared pool, buffer cache etc.  

Why do I need to set  tempfs to 1264m ?  Is this because it needs to be as large as  SGA can potentially get?  and If I allocate 1264m to tempfs ( and SGA gets all of it)  does the pga grab the memory left over on the server potentially taking away for the OS?.

Thanks,
Rose

 

by: mrjoltcolaPosted on 2009-11-04 at 09:19:59ID: 25741614

All server memory takes away from the OS, regardless of whether it is PGA/SGA or shared/unshared.
/dev/shm is just memory, but it is mapped for shared usage.

If a server has 8GB RAM, and you allocate 1GB SGA, then that 1GB is shared RAM. 7GB left over.

PGA is just standard program memory.

The important thing is that ALL memory should be in RAM, so don't go over your available RAM or it starts swapping to disk, not good.

Yes, any memory usage takes away from the OS, but that is not Oracle specific.

 

by: franckpachotPosted on 2009-11-04 at 13:39:39ID: 25744240

Hi,

It seems that I didn't get your question.

>> Why is tmpfs only using 850672 which matches only the SGA?

Yes. And to be more precise, it is used only for the buffer cache that is part of the SGA
I suppose the find the 850672 in the 'Database Buffers' part when issuing 'show sga' in sqlplus.

>> Why do I need to set  tempfs to 1264m ?
When you are using 'extended buffer cache' feature, that uses /dev/shm instead of direct use of memory, then you must size /dev/shm so that your buffer cache fits in.  So here you need only 850672 because with 'extended buffer cache' the buffer cache cannot be resized dynamically.

But that feature is used on 32-bits systems where you need to use more than 4GB of memory. Maybe you don't need to use that feature.
(the feature is set by USE_INDIRECT_DATA_BUFFERS=TRUE)

Hope that answered your questions.
Regards,
Franck.



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