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From: "Jonathan Lewis" <jonat...@jlcomp.demon.co.uk>
Newsgroups: comp.databases.oracle.server
References: <k7nift$rc3$2@solani.org><k6mdnUJ8sKBzgD3NnZ2dnUVZ8gWdnZ2d@bt.com> <k7pfjq$ruc$1@solani.org><10f49835-b336-4d9a-ba93-5e927761a030@q5g2000pbk.googlegroups.com><SP-dnT2X5t05uz_NnZ2dnUVZ8tadnZ2d@bt.com> <k7t91l$l4e$1@solani.org>
Subject: Re: Large pages
Date: Tue, 13 Nov 2012 17:24:32 -0000
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"Mladen Gogala" <gogala.mla...@gmail.com> wrote in message 
news:k7t91l$l4e$1@solani.org...
| On Tue, 13 Nov 2012 10:53:26 +0000, Jonathan Lewis wrote:
|
| > I can't work out from your statement exactly which bit of what I've 
said
| > is not correct. Could you please clarify.
|
| Jonathan, the part that is not correct is about every process on Unix
| having its own page table. Page tables for shared memory are also shared.
| There is one per segment, not one per process.
|
|
|
| -- 
| http://mgogala.byethost5.com



Mladen,

The reason that I referenced Christo's video and notes is that he has 
results that agree with my comment and contradict yours. From the text:
<quote:>
As discussed earlier, each process has a page table. This page table is 
private for the process and cannot be shared. (Solaris is different in this 
respect.)

In Oracle, there is usually a large shared memory segment shared amongst 
multiple processes. Each process still has a page table that is maintained. 
For example, for a 1.7 Gb SGA (the typical 32-bit limit), 445,440 x 4 Kb 
pages are needed. We would need 445,440 leaf PTE entries times 4 bytes 
each - that's about 2 Mb. Each process would need a PTE table that is 2 Mb 
in size to fully describe its 2 Gb of mappings. If you have a large number 
of processes, say 1000, then you will need 2000 Mb of RAM to manage a 1.7 
Gb SGA. Quite inefficient.

<end quote>

However,
a) he does mention Solaris and intimate shared memory - and explains that 
shared page tables are possible
b) the article is dated Dec 2007 - so things may have changed

Do you have any specific versions of Unix in mind when you state that the 
page table for a shared memory segment is automatically shared ? Is this, 
perhaps a specific default for OEL.

-- 

Regards

Jonathan Lewis
http://jonathanlewis.wordpress.com/all-postings

Author: Oracle Core (Apress 2011)
http://www.apress.com/9781430239543