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#1 2016-05-18 16:02:31

yochaigal
Member
From: Boston
Registered: 2011-02-06
Posts: 202

Swap space isn't being used [SOLVED]

Hello. I've recently noticed that my swap space isn't being used:

free -h
                    total        used        free          shared     buff/cache   available
Mem:           7.8G        4.0G       1.8G        126M        2.0G            3.3G
Swap:          7.8G        0B           7.8G

What's the big idea? Here's my setup:

fdisk -l /dev/sda
Disk /dev/sda: 232.9 GiB, 250059350016 bytes, 488397168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x5d60a6ae

Device     Boot    Start           End              Sectors       Size      Id  Type
/dev/sda1  *        2048           206847         204800       100M   83  Linux
/dev/sda2           206848       472066047   471859200  225G   83  Linux
/dev/sda3           72066048  488396799   16330752     7.8G    82  Linux swap / Solaris

I've got a 250GB SSD with a LUKS partition for the entire install, and a swap at the end (not encrypted - yet). I do not use LVM.

My fstab:

# /dev/sda1
UUID=89a2ebbd-a775-45fd-a9ea-8a9d41f1111c    /boot         ext4          rw,relatime,stripe=4,data=ordered    0 2

# /dev/sda2
UUID=ace99b22-8998-415e-bd84-e9417fc54f21    /             ext4          defaults,noatime,discard             0 1

# /dev/sda3 swap
UUID=a23212f8-69a3-4cd4-8bf4-d6bf2bf6c0a9    swap    swap     defaults,noatime,discard         0 0


Any help would be appreciated. Thanks!

Last edited by yochaigal (2016-05-18 20:14:02)

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#2 2016-05-18 16:05:20

frostschutz
Member
Registered: 2013-11-15
Posts: 1,409

Re: Swap space isn't being used [SOLVED]

What's your /proc/sys/vm/swappiness set to?

You have enough free memory so strictly speaking no need for swap (yet).

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#3 2016-05-18 16:06:43

yochaigal
Member
From: Boston
Registered: 2011-02-06
Posts: 202

Re: Swap space isn't being used [SOLVED]

Ah right forgot to say that. It's set to 60.

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#4 2016-05-18 16:29:49

graysky
Wiki Maintainer
From: :wq
Registered: 2008-12-01
Posts: 10,595
Website

Re: Swap space isn't being used [SOLVED]

Under Windows, the swap is aggressively used whereas under Linux, it is used as needed.  As frost points out, you likely don't need it.


CPU-optimized Linux-ck packages @ Repo-ck  • AUR packagesZsh and other configs

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#5 2016-05-18 17:25:25

frostschutz
Member
Registered: 2013-11-15
Posts: 1,409

Re: Swap space isn't being used [SOLVED]

Do you want it to be used more pro-actively or do you just want to test whether it would work if actually needed?

You could temporarily set swappiness to 100 and see if something happens. You could use more memory (for example by installing memtester and letting it test your entire free memory [3.3G in your above output]) and then open another browser or whatever; even with low swappiness surely something would start swapping at that point.

Basically I'd not be worried about it, unless you have errors showing up in dmesg, everything seems to be normal.

Last edited by frostschutz (2016-05-18 17:26:20)

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#6 2016-05-18 17:50:42

yochaigal
Member
From: Boston
Registered: 2011-02-06
Posts: 202

Re: Swap space isn't being used [SOLVED]

I'm just confused as to why NONE is being used - my there other Arch systems all use it a fair amount.

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#7 2016-05-18 17:56:26

alex.theoto
Member
From: Athens Greece
Registered: 2014-11-30
Posts: 307

Re: Swap space isn't being used [SOLVED]

None because you have 8g memory dim...
Open many applications to stress it and you'll see that it will use some swap

Last edited by alex.theoto (2016-05-18 17:57:18)

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#8 2016-05-18 18:05:57

Magnus2552
Member
Registered: 2016-04-07
Posts: 63

Re: Swap space isn't being used [SOLVED]

But why should swap space be used as long as there is even only a byte free RAM left? Swap is so much slower, so why should Linux prefer hard disk memory over actual RAM at a point where you have enough RAM?

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#9 2016-05-18 18:07:40

yochaigal
Member
From: Boston
Registered: 2011-02-06
Posts: 202

Re: Swap space isn't being used [SOLVED]

I'm just curious as to why it is always zero - regardless of what I'm doing. I do max out RAM occasionally, if I had setup my swap wrong or something I wouldn't be surprised.

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#10 2016-05-18 18:12:12

alex.theoto
Member
From: Athens Greece
Registered: 2014-11-30
Posts: 307

Re: Swap space isn't being used [SOLVED]

Swap is for pc's hibernation state.
Also, it is very useful on systems with low memory.

In my example, I have 2gb memory and 5gb swap:

free -h
              total        used        free      shared  buff/cache   available
Mem:           2.0G        590M        512M        2.1M        901M        1.2G
Swap:          4.7G          0B        4.7G 

Last edited by alex.theoto (2016-05-18 18:28:19)

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#11 2016-05-18 20:12:00

Vain
Member
Registered: 2008-10-19
Posts: 179
Website

Re: Swap space isn't being used [SOLVED]

Well, if you really want to see your swap being used, just save this as "memfill.c", compile it ("gcc -o memfill memfill.c"), and run it:

#include <stdlib.h>
#include <strings.h>
#include <unistd.h>

int
main()
{
    char *p;
    size_t s = 100 * 1024 * 1024;

    for (;;)
    {
        p = malloc(s);
        bzero(p, s);
        sleep(1);
    }

    /* unreached */
}

Watch memory consumption in something like htop. At some point, your swap will be used. smile

As others have pointed out, if your swap was not being used before, it simply means there was no need to.

(IMHO, if you do not intend to use hibernation, just turn off swap space altogether. 8GB is more than enough memory for common tasks. And, if you really do need more memory than 8GB, you should try to buy more RAM. Swap is so utterly slow, I try to avoid it at all costs.)

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#12 2016-05-18 20:13:48

yochaigal
Member
From: Boston
Registered: 2011-02-06
Posts: 202

Re: Swap space isn't being used [SOLVED]

This is exactly what I did (a friend sent me a stackexchange article suggesting exactly this).

It seems to be being used (only 1M so far). So hurray! I'll mark this as solved.

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