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Old 02-06-2009, 06:38 AM   #1
user777
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Dynamic shared memory


Hi All,

How does dynamic shared memory work.
There is a note from oracle:

Quote:
On some UNIX platforms,there is no real benefit
in setting SGA_TARGET to a value smaller than SGA_MAX_SIZE. Therefore, setting SGA_MAX_SIZE on those platforms is not recommended.

Thanks in advance
 
Old 02-06-2009, 08:06 AM   #2
pixellany
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I have trouble seeing the big picture here.....

What kind of a system are you talking about?

Please provide a link for the Oracle quote---so we can see what THEY are talking about.
 
Old 02-07-2009, 06:43 AM   #3
user777
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Hi,

Please check the link from oracle, chapter -2 (2.37 page)

http://<br /> http://download.orac...102/b14231.pdf

It has been mentioned that few systems donot support them.


Thanks
 
Old 02-07-2009, 07:04 AM   #4
user777
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Oh I am sorry

 
Old 02-07-2009, 10:21 AM   #5
johnsfine
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Quote:
Originally Posted by user777 View Post
How does dynamic shared memory work.
There is a note from oracle:
What are you really looking for here?

1) You want to know about dynamic shared memory in general and just used a quote from Oracle documentation as an example?

2) You want to know how to configure Oracle and didn't understand that section.

For (2), I downloaded that link and didn't understand that section either. I think it deals with an advanced feature that you almost certainly aren't using. Even if you were on one of the OS's that support the feature, you likely wouldn't be using it.

For (1), I tried a google search. The results were surprisingly bad. I didn't find any overview of the concept nor any implementation status (what software is available, what OS's and applications support it). I couldn't even tell if there was general agreement on the meaning of the phrase.

One meaning of the phrase used in many places is that it provides the apparent features of NUMA across a less tightly coupled connection than NUMA.

A simple (AMD) example of NUMA would be a motherboard on which you have multiple CPU chips, with an integrated memory controller in each CPU chip and memory directly connected to each of those memory controllers and an interconnect (Hyper Transport) between the memory controllers. A thread running in one CPU chip can access memory connected to that CPU chip at high speed through the integrated memory controller. It can access memory connected to a different CPU chip with exactly the same code (and no OS intervention) and simply lower performance (through the Hyper Transport). The BIOS (or maybe OS) organizes the entire pool of memory into one physical address space and the OS maps pages of that physical address space into processes with some allocation bias to increase the probability that accesses will be local.

A simple "dynamic shared memory" design would look the same from the point of view of the virtual address space of a thread, but would be very different underneath. The non local pages could not be mapped because they aren't on the same motherboard nor in the same physical address space. On access to a non local location, you would have a virtual memory fault and the OS would need to move or duplicate the page from its location on another motherboard (likely across a LAN from another computer). That would be similar to bringing in a faulted page from a memory mapped file, when that file happens to be opened through a network connection to a fileserver.

Last edited by johnsfine; 02-07-2009 at 10:28 AM.
 
Old 02-16-2009, 07:18 AM   #6
user777
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Thanks john for providing points on Numa.

My question was the document mentions that few UNIX platforms that do not support dynamic shared memory, the physical memory in use by the Shared Global Area(SGA) is equal to SGA_max_size parameter...

What does that few unix platform which doesnt have some memory concept in common?

For few 32bit systems in unix the Shared memory is restricted to some size limit, trying to set sga_target doesn't benefit.
May be they are trying to tell this in different terms.Not sure.
 
Old 02-16-2009, 07:36 AM   #7
johnsfine
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I don't use Oracle, which may be the reason I haven't a clue what your last post means.

But I think you've at least answered my meta question about what you're trying to ask. You don't want to know about "dynamic shared memory". You want to know about "Configuring the SGA in Oracle".

That is a specialized enough topic that I think you would have a better chance of getting the right expert to look at your question if you put the right topic in the subject of your thread.
 
  


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