swap space vs shared memory

swap space vs shared memory

Post by Legen » Sat, 04 Sep 1999 04:00:00



Hello all,

    Actually, I have a UNIX question rather than an Ingres question but this
question came up when I was setting up shared memory for Ingres.

   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?

    Your comments are apprecaited.

Kind regards,

Benny Pei

 
 
 

swap space vs shared memory

Post by Karl & Betty Schend » Sun, 05 Sep 1999 04:00:00



Quote:>   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.

Ingres and Unix Expertise

Past President, North American Ingres Users Association


 
 
 

1. HP "memory" swap space

Sort of off-topic, but since it affects available memory which affects a
good tune, and there are plenty of HP masters out there in cdi-land:

What the h*ll is this memory swap space on HP-UX which is apparently enabled
by default? What drugs were these kernel engineers on when they decided it
would be a good idea to page out the virtual memory to other memory? Who can
justify this nonsense?

As the man page says:

2.   If all file system paging areas are completely allocated and the
     request is still not satisfied, the system will try to use memory
     paging as described on the memory line above.  (Memory paging is
     controlled by the tunable parameter swapmem_on, which defaults to
     1 (on).

--
Space Corps Directive #592
In an emergency situation involving two or more
officers of equal rank, seniority will be granted to
whichver officer can program a VCR.
    -- Red Dwarf
..  ... .--. .. -  --- -.  --- .-. .- -.-. .-.. .

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