Re: Large pages

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Tue, 13 Nov 2012 10:53:26 -0000
Message-ID: <SP-dnT2X5t05uz_NnZ2dnUVZ8tadnZ2d_at_bt.com>

|"Noons" <wizofoz2k_at_gmail.com> wrote in message
news:10f49835-b336-4d9a-ba93-5e927761a030_at_q5g2000pbk.googlegroups.com...
|On Nov 12, 11:24 am, Mladen Gogala <gogala.mla..._at_gmail.com> wrote:
|
|> > In Unix (unless you use intimate shared memory) you get one map per
|> > process - so an Oracle system with 1,000 processes would end up using
as
|> > much memory for maps of the SGA as it would on the SGA itself if it
were
|> > using standard pages. In Windows Oracle runs as a single process with
|
|Actually, that is not correct. The problem has to do with the memory
|map for ALL physical memory, the number of virtual memory processes
|using it is irrelevant.

Noons,

I can't work out from your statement exactly which bit of what I've said is not correct. Could you please clarify.

The "memory map" certainly maps logical addresses to physical memory addresses - easy enough to see (if you know the answer) when you look at some of the x$ objects and see wildly different addresses for objects - for example, Oracle maps the PGA for a process into an entirely different set of (logical) addresses from the SGA - to the extent that the (logical) gap between memory addresses can be far larger than the physical memory available on the machine.

But (depending on O/S and choice of O/S parameters the page size used in the mapping may vary, each process may have it's own map, or all processes that attach to the same physical memory may share a single map of the memory (e.g. Solaris Intimate Shared Memory). I wrote a note about this a coupld of years ago highlighting a surprise side effect (http://jonathanlewis.wordpress.com/2010/06/23/memory/ ); more significantly Christo Kutrovsky (of Pythian) posted a video of a presentation demonstrating the various effects (and how to measure them) when configuring Linux.
(http://www.pythian.com/news/741/pythian-goodies-free-memory-swap-oracle-and-everything/  )

-- 


Regards

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

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

Received on Tue Nov 13 2012 - 11:53:26 CET