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Date:      Sun, 18 Aug 2024 02:08:17 -0700
From:      Yuri <yuri@FreeBSD.org>
To:        Freebsd hackers list <freebsd-hackers@FreeBSD.org>
Subject:   Can free swap size be determined from the sysctl information ?
Message-ID:  <6076fe34-eb23-47cc-8bd2-6000ddf0a091@FreeBSD.org>
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It seems that it should be possible to find free swap size from the 
sysctl information, but it isn't obvious how to do this.


Here are all *swap* entries in the sysctl information:
$ sysctl -a | grep swap
0 MD md98 68719441408 512 u 98 s 512 f 0 fs 0 l 68719441408 t vnode file 
/disk-large1/swap-64GB label
z0xfffff8004d862400 [shape=box,label="SWAP\nswap\nr#2"];
      <name>swap</name>
            <file>/disk-large1/swap-64GB</file>
vm.swap_enabled: 1
vm.domain.0.stats.unswappable: 0
vm.swap_idle_threshold2: 10
vm.swap_idle_threshold1: 2
vm.swap_idle_enabled: 0
vm.disable_swapspace_pageouts: 0
vm.stats.vm.v_swappgsout: 433264
vm.stats.vm.v_swappgsin: 94457
vm.stats.vm.v_swapout: 65078
vm.stats.vm.v_swapin: 11121
vm.stats.swap.free_completed: 65205
vm.stats.swap.free_deferred: 86103
vm.nswapdev: 1
vm.swap_fragmentation:
vm.swap_async_max: 4
vm.swap_maxpages: 48890752
vm.swap_total: 68719439872
vm.swap_reserved: 16267402461184


top(1) shows 775M of free swap out of 64G total.
top(1) gets swap information from kvm_getswapinfo(3).


Motivation: one Golang-based port needs free swap size information, and 
I am trying to figure out how to get this information in Golang.

Golang has the "sysctl" module.



Thanks,

Yuri


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    <font face="monospace">It seems that it should be possible to find
      free swap size from the sysctl information, but it isn't obvious
      how to do this.</font><br>
    <font face="monospace"><br>
    </font><br>
    <font face="monospace">Here are all *swap* entries in the sysctl
      information:</font><br>
    <font face="monospace"><span
        style="color:#000000;background-color:#ffffff;">$ sysctl -a |
        grep swap
      </span><br>
      0 MD md98 68719441408 512 u 98 s 512 f 0 fs 0 l 68719441408 t
      vnode file /disk-large1/swap-64GB label  <br>
      z0xfffff8004d862400 [shape=box,label="SWAP\nswap\nr#2"];
      <br>
           &lt;name&gt;swap&lt;/name&gt;
      <br>
                 &lt;file&gt;/disk-large1/swap-64GB&lt;/file&gt;
      <br>
      vm.swap_enabled: 1
      <br>
      vm.domain.0.stats.unswappable: 0
      <br>
      vm.swap_idle_threshold2: 10
      <br>
      vm.swap_idle_threshold1: 2
      <br>
      vm.swap_idle_enabled: 0
      <br>
      vm.disable_swapspace_pageouts: 0
      <br>
      vm.stats.vm.v_swappgsout: 433264
      <br>
      vm.stats.vm.v_swappgsin: 94457
      <br>
      vm.stats.vm.v_swapout: 65078
      <br>
      vm.stats.vm.v_swapin: 11121
      <br>
      vm.stats.swap.free_completed: 65205
      <br>
      vm.stats.swap.free_deferred: 86103
      <br>
      vm.nswapdev: 1
      <br>
      vm.swap_fragmentation:  <br>
      vm.swap_async_max: 4
      <br>
      vm.swap_maxpages: 48890752
      <br>
      vm.swap_total: 68719439872
      <br>
      vm.swap_reserved: 16267402461184<br>
    </font>
    <font face="monospace"><br>
    </font><br>
    <font face="monospace">top(1) shows <span
        style="color:#000000;background-color:#ffffff;">775M of free
        swap out of </span><span
        style="color:#000000;background-color:#ffffff;">64G total.</span></font><br>
    <font face="monospace">top(1) gets swap information from <span
        style="color:#000000;background-color:#ffffff;">kvm_getswapinfo(3).</span></font><br>
    <p><br>
    </p>
    <p><font face="monospace">Motivation: one Golang-based port needs
        free swap size information, and I am trying to figure out how to
        get this information in Golang.</font></p>
    <p><font face="monospace">Golang has the "sysctl" module.<br>
      </font></p>
    <p><font face="monospace"><br>
      </font></p>
    <p><font face="monospace"><br>
      </font></p>
    <p><font face="monospace">Thanks,</font></p>
    <p><font face="monospace">Yuri</font></p>
    <p><font face="monospace"><br>
      </font></p>
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