Change Swap and Memory Priority.

hack3rcon · 04-26-2015, 09:18 AM

Hello.
How can I change Swap Priority and tell Linux to use Swap more than RAM?

Thank you.

T3RM1NVT0R · 04-26-2015, 09:31 AM

Interesting question, why would you want to do that? I have never seen a parameter which can provide precedence to swap over RAM. You can set priorities between different swap devices / files but prioritizing swap over RAM, never heard of it.

It is not possible as far as I am aware. Even if somehow it is possible it will definitely a blow to performance, you can't compare RAM performance with swap.

michaelk · 04-26-2015, 10:00 AM

You can set swappiness=100 which may suit your purposes.

http://web.archive.org/web/201403280...vm-swappiness/

JeremyBoden · 04-26-2015, 10:04 AM

You can change the "swappiness" parameter to swap more aggressively.

Your RAM would not necessarily show more free storage - but if RAM were needed
there would be no delay to swap out memory as it would have already been swapped out.

Doing this is usually a sub-optimal strategy as Linux tries to use as much unused memory as possible for extra buffer space.

I would leave it alone.

Rezney · 04-26-2015, 10:13 AM

No idea about this but if you would like to affect swapping on your system you can play with vm.swappiness but you probably already know this.

Cheers

T3RM1NVT0R · 04-26-2015, 10:39 AM

Quote:

Originally Posted by michaelk

You can set swappiness=100 which may suit your purposes.

http://web.archive.org/web/201403280...vm-swappiness/

I am not sure if that will serve what OP is looking for. By default if swappiness is set to low it will make the kernel to drop cached pages for freeing up the memory instead of swapping. If swappiness is set to higher value instead of dropping cached pages kernel will use swap. Setting swap to 100 will not give precedence to swap over RAM.

T3RM1NVT0R · 04-26-2015, 10:55 AM

Just did a small test with swappiness set to 100 (rebooted the VM after putting it in sysctl) and the output does not shows system giving precedence to swap over RAM.

Command for testing load was taken from stackexchange (did little change), here is the link: http://unix.stackexchange.com/questi...he-free-memory

Code:

[root@rhel6-test ~]# free -m
             total       used       free     shared    buffers     cached
Mem:          1500        327       1173          0         13        153
-/+ buffers/cache:        159       1340
Swap:         3007          0       3007
[root@rhel6-test ~]# cat /proc/sys/vm/swappiness
100
[root@rhel6-test ~]# cat /etc/sysctl.conf | grep swappiness
vm.swappiness = 100
[root@rhel6-test ~]# stress --vm-bytes $(awk '/MemFree/{printf "d\n", $2 * 1.2;}' < /proc/meminfo)k --vm-keep -m 1 &
[1] 2477
[root@rhel6-test ~]# stress: info: [2477] dispatching hogs: 0 cpu, 0 io, 1 vm, 0 hdd

[root@rhel6-test ~]# free -m

             total       used       free     shared    buffers     cached
Mem:          1500       1450         49          0          0         21
-/+ buffers/cache:       1429         71
Swap:         3007        169       2838
[root@rhel6-test ~]#
[root@rhel6-test ~]# free -m
             total       used       free     shared    buffers     cached
Mem:          1500       1428         72          0          0         14
-/+ buffers/cache:       1413         86
Swap:         3007        388       2619
[root@rhel6-test ~]# free -m
             total       used       free     shared    buffers     cached
Mem:          1500       1427         72          0          0         13
-/+ buffers/cache:       1414         86
Swap:         3007        482       2525
[root@rhel6-test ~]# free -m
             total       used       free     shared    buffers     cached
Mem:          1500       1444         56          0          0         12
-/+ buffers/cache:       1431         68
Swap:         3007        581       2426
[root@rhel6-test ~]# free -m
             total       used       free     shared    buffers     cached
Mem:          1500       1435         65          0          0          9
-/+ buffers/cache:       1425         74
Swap:         3007        696       2311
[root@rhel6-test ~]# free -m
             total       used       free     shared    buffers     cached
Mem:          1500       1450         49          0          0          8
-/+ buffers/cache:       1442         58
Swap:         3007        884       2123
[root@rhel6-test ~]#

michaelk · 04-26-2015, 12:38 PM

Thanks for posting the data.

T3RM1NVT0R · 04-26-2015, 12:42 PM

You're welcome!

Pearlseattle · 04-26-2015, 04:38 PM

Hi
Maybe using cgroups as mentioned here?

hack3rcon · 04-26-2015, 11:55 PM

I guess that in the new version Swap file, Speed between Swap partition and Swap file solved and both are same. If we use Some HDD like SSD can Swap speed same RAM speed?

T3RM1NVT0R · 04-27-2015, 12:42 AM

No, RAM will still be significantly faster than swap on SSD. You might want to have a look at this article where they had similar discussion: http://superuser.com/questions/61786...d-space-as-ram

It is not only about the speed but your SSD will wear out faster due to continuous read / write / erase / re-write.