Ttyjfyk

swappiness

This control is used to define how aggressive the kernel will swap
memory pages. Higher values will increase aggressiveness, lower values
decrease the amount of swap. A value of 0 instructs the kernel not to
initiate swap until the amount of free and file-backed pages is less
than the high water mark in a zone.

-- linux-5.0/Documentation/sysctl/vm.txt

MemAvailable

An estimate of how much memory is available for starting new
applications, without swapping. Calculated from MemFree,
SReclaimable, the size of the file LRU lists, and the low
watermarks in each zone.
The estimate takes into account that the system needs some
page cache to function well, and that not all reclaimable
slab will be reclaimable, due to items being in use. The
impact of those factors will vary from system to system.

-- linux-5.0/Documentation/filesystems/proc.txt (file /proc/meminfo).

What are the current values of the low and high watermarks on my system? And what value are they relative to, e.g. if I want to express them as a percentage?

The watermarks are the low and high values in /proc/zoneinfo, shown in units of pages (4096 bytes on x86).

On my 8GB system, most of the pages are split between the DMA32 zone and the Normal zone. (And everything belongs to Node 0, because it is not a NUMA system).

# cat /proc/zoneinfo
Node 0, zone DMA
...
 pages free 3961
 min 33
 low 41
 high 49
 spanned 4095
 present 3996
 managed 3961
...
Node 0, zone DMA32
 pages free 139960
 min 7184
 low 8980
 high 10776
 spanned 1044480
 present 888973
 managed 866327
...
Node 0, zone Normal
 pages free 33907
 min 31449
 low 33868
 high 36287
 spanned 1173504
 present 1173504
 managed 1140349
...

I expect the watermarks are a proportion of managed. Very broadly speaking, the watermarks on my system are somewhere between 1% and 3%. Or if you are thinking in terms of the "high-watermark" metaphor, they are between 97% and 99% :-).

(The Linux low watermark does NOT correspond to a metaphorical "low watermark"; the low and high values are both thresholds in the same direction, they are just used for slightly different purposes).