4GB swap allocated, but top reports 2GB

[mb2015]

[Apologies if this is the wrong forum; I wasn't sure if it was specific to the BBB.]

On my BeagleBone Black rev. C, I am booting into FreeBSD 11.2-PRERELEASE on an SD card.

I have allocated the eMMC's Linux partition for encrypted swap, i.e. in /etc/fstab there is this:

/dev/mmcsd1s2.eli       none    swap    sw      0       0

I can confirm with gpart list mmcsd1 that the partition is well over 2 GB:

2. Name: mmcsd1s2
   Mediasize: 3723493376 (3.5G)

swapinfo reports the full amount is allocated:

Device          1K-blocks     Used    Avail Capacity
/dev/mmcsd1s2.eli   3636224    25896  3610328     1%

Yet when I run top, it shows the swap space is exactly 2 GB (1971 MiB):

Swap: 1971M Total, 25M Used, 1945M Free, 1% Inuse

Why is top not saying something more like 3551 MB Total?

 

[Phishfry]

I am using the eMMC for the OS and its defiantly 4GB on the BBB.

Have you cruised the FreeBSD mailing list. There have been some pertinent discussions about swap.
Basically gist is you can have too much swap. There was a discussion on freebsd-arm.
That was a discussion about interleaved swap(swap using several different devices) and it was acting weird(stalling).
I had never heard of interleaved swap but there was a lot of information in the thread about swap in general.
Sometimes the mailing-lists are hard to follow but there is some good info there. Spread over 2 months and a topic change.
https://lists.freebsd.org/pipermail/freebsd-arm/2018-June/017998.html
https://lists.freebsd.org/pipermail/freebsd-arm/2018-July/018167.html

Mark Millard
As I remember, the "increase kern.maxswzone" only applies if one has
previously decreased it: the modern default is the maximum recommended
as I remember (allowing for half the theoretical maximum swap). If
I remember right the code overrode attempts to set more than the
default, only setting less (when requested).

The discussions might have been on here too, but more in regards to swap oversize problems and ZFS.
In alot that I have read you can have too much swap.

 

[mb2015]

Thanks for pointing to that mailing list thread. Indeed, it is a challenging read. Apparently I'm not the only one whose system likes to sometimes lock up when swap is used. But the issue I am asking about is just that top is either misreporting the actual amount of swap, or maybe the system is really only using 2 GB. (Or maybe I'm misunderstanding what I am seeing.) The topic of this strange top output is not covered in the discussion.

If there is also a problem in that I have too much swap allocated, I don't mind reducing it; I've never seemed to need more than a few hundred MB. I just don't want to mess with gpart any more than I have to.