From: Joerg Bruehe (joerg@sql.de)
Date: Wed Jul 03 2002 - 06:06:33 EDT
Hi admins,
on this list, there just came a summary by "Oracle administrator"
("atul" ?) containing a copy of an Oracle text entitled
"Understanding and measuring memory usage on UNIX operating systems".
While the text was interesting reading (always nice to see a good
tutorial text), it contained this quote:
| (1) Every data page in every process is "backed" by a page in
| the swap space. This size of the swap space limits the amount
| of virtual data space on the system; processes are not able to
| allocate memory if there is not enough swap space available to
| back it up, regardless of how much physical memory is available
| on the system.
Please note that there are Unix systems for which this statement
is _not_ true because they provide different paging policies,
Tru64 is one of them (AIX and Linux as well, no info about others).
I quote the Tru64 version 4 manual "System Tuning and Performance
Management", section 1.3.1.4 "Swap Space Allocation Modes":
| How swap space is allocated is determined by two modes:
| immediate mode and deferred mode. The two strategies differ
| in the point in time at which swap space is allocated.
|
| In immediate mode, swap space is allocated when modifiable
| virtual address space is created. (Note that immediate mode
| is sometimes referred to as eager mode.)
|
| In deferred mode, swap space is not allocated until the
| system needs to write a modified virtual page to swap space.
| (Note that deferred mode is sometimes referred to as
| overcommitment mode or lazy mode.)
|
| ...
|
| Immediate swap mode is used if the /sbin/swapdefault file exists.
| This file is a symbolic link to /dev/rzxx, which is the first
| defined swap device. If this file does not exist, the system
| uses deferred mode. If you change from one mode to another,
| you must reboot the system to activate the new mode.
Regards,
Joerg Bruehe
--
Joerg Bruehe, SQL Datenbanksysteme GmbH, Berlin, Germany
(speaking only for himself)
mailto: joerg@sql.de
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