Swap Space vs Shared Memory

From: Karl & Betty Schendel 
Subject: Re: swap space vs shared memory

At 11:15 PM -0400 9/3/99, Legend wrote:
>   How do swap space and shared memory do for Ingres? How is they different
>from each other? Are they referring to the same space? I know swap space is
>a temporary space for swapping in and out processes from memory, but what is
>shared memory? Is it a memory area that we assign for Ingres? Why sometimes
>the shared memory is larger than physical memory? How does the OS allocate
>the shared memory?

Swap space is used to hold pages of physical memory that are not currently
needed, so that other processes can have the physical memory for a while.

Shared memory is just part of a process address space that is explicitly
shared with other processes.  Normally all of a process address space is
private to that process.  Shared memory is not a physical memory allocation,
it's an address space allocation.  Ingres uses 3 or 4 shared memory areas:
a smallish system communicaton area, shared memory for locking/logging
including the log buffers, a cross-server event area, and possibly a shared
DMF cache. 

Karl R. Schendel, Jr.
K/B Computer Associates   schendel@kbcomputer.com
Ingres and Unix Expertise

Past President, North American Ingres Users Association
past_president@naiua.org
Ingres Q & A
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