12-13-2018
Hi all,
I am sorry to bring up this thread again.
Recently, i have same heavy swap usage against the physical swap space despite having physical free ram.
I read up this old thread >
Out of swap but RAM available >> and my understanding is that memory reservation will eat up swap-space if there isn't enough virtual swap for reservation ?
But again, read these directly from Solaris (
Doc ID 1010585.1) in Oracle metalink somehow indicate that memory reservation against physical memory or swap doesn't actually take up the physical space.
Quote:
When a process calls malloc()/sbrk() only virtual swap is reserved.
Reservation is done against the physical disk swap first.
If that is exhausted or not configured then reservation is done against physical memory. If both are exhausted then malloc() fails
As discussed, when measuring virtual swap available for reservation, consider monitoring vmstat ("swap" column) or swap -s (the "available" value).
Free memory as reported by vmstat ("free" column) or swap usage as reported by swap -l ("free" column) is unrelated with virtual swap available for reservation.
When allocating swap reservation from memory, there is no memory deducted and vmstat continues to show same amount of "free" memory.
Similarly, when a swap reservation is made from physical disk-based swap, swap -l will continue to show the same amount of "free" swap.
Free memory is only deducted due to a page fault and free swap is deducted during a memory shortage, when data needs to be migrated from the physical memory to physical disk swap to maintain a sufficient supply of free memory.
So does memory reservation actually take up swap space or not ?
Regards,
Noob
Last edited by javanoob; 12-13-2018 at 02:50 AM..
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LEARN ABOUT REDHAT
swapoff
SWAPON(8) Linux Programmer's Manual SWAPON(8)
NAME
swapon, swapoff - enable/disable devices and files for paging and swapping
SYNOPSIS
/sbin/swapon [-h -V]
/sbin/swapon -a [-v] [-e]
/sbin/swapon [-v] [-p priority] specialfile ...
/sbin/swapon [-s]
/sbin/swapoff [-h -V]
/sbin/swapoff -a
/sbin/swapoff specialfile ...
DESCRIPTION
Swapon is used to specify devices on which paging and swapping are to take place. Calls to swapon normally occur in the system multi-user
initialization file /etc/rc making all swap devices available, so that the paging and swapping activity is interleaved across several
devices and files.
Normally, the first form is used:
-h Provide help
-V Display version
-s Display swap usage summary by device. Equivalent to "cat /proc/swaps". Not available before Linux 2.1.25.
-a All devices marked as ``swap'' swap devices in /etc/fstab are made available. Devices that are already running as swap are silently
skipped.
-e When -a is used with swapon, -e makes swapon silently skip devices that do not exist.
-p priority
Specify priority for swapon. This option is only available if swapon was compiled under and is used under a 1.3.2 or later kernel.
priority is a value between 0 and 32767. See swapon(2) for a full description of swap priorities. Add pri=value to the option field
of /etc/fstab for use with swapon -a.
Swapoff disables swapping on the specified devices and files. When the -a flag is given, swapping is disabled on all known swap devices
and files (as found in /proc/swaps or /etc/fstab).
NOTE
You should not use swapon on a file with holes. Swap over NFS may not work.
SEE ALSO
swapon(2), swapoff(2), fstab(5), init(8), mkswap(8), rc(8), mount(8)
FILES
/dev/hd?? standard paging devices
/dev/sd?? standard (SCSI) paging devices
/etc/fstab ascii filesystem description table
HISTORY
The swapon command appeared in 4.0BSD.
Linux 1.x 25 September 1995 SWAPON(8)