I am guessing that htop is in the wrong.

If you look in /proc/meminfo, you’ll see a MemAvailable field and a MemTotal field.

It looks to me like this is approximately (I didn’t check top’s source) what top is using: MemTotal - MemAvailable.

stackoverflow.com/…/difference-between-memfree-an…

Rik van Riel’s comments when adding MemAvailable to /proc/meminfo:

/proc/meminfo: MemAvailable: provide estimated available memory

Many load balancing and workload placing programs check /proc/meminfo to estimate how much free memory is available. They generally do this by adding up “free” and “cached”, which was fine ten years ago, but is pretty much guaranteed to be wrong today.

It is wrong because Cached includes memory that is not freeable as page cache, for example shared memory segments, tmpfs, and ramfs, and it does not include reclaimable slab memory, which can take up a large fraction of system memory on mostly idle systems with lots of files.

Currently, the amount of memory that is available for a new workload, without pushing the system into swap, can be estimated from MemFree, Active(file), Inactive(file), and SReclaimable, as well as the “low” watermarks from /proc/zoneinfo.

However, this may change in the future, and user space really should not be expected to know kernel internals to come up with an estimate for the amount of free memory.

It is more convenient to provide such an estimate in /proc/meminfo. If things change in the future, we only have to change it in one place.

Looking at the htop source:

github.com/htop-dev/htop/blob/main/MemoryMeter.c

   /* we actually want to show "used + shared + compressed" */
   double used = this->values[MEMORY_METER_USED];
   if (isPositive(this->values[MEMORY_METER_SHARED]))
      used += this->values[MEMORY_METER_SHARED];
   if (isPositive(this->values[MEMORY_METER_COMPRESSED]))
      used += this->values[MEMORY_METER_COMPRESSED];

   written = Meter_humanUnit(buffer, used, size);

It’s adding used, shared, and compressed memory, to get the amount actually tied up, but disregarding cached memory, which, based on the above comment, is problematic.

top, on the other hand, is using the kernel’s MemAvailable directly.

gitlab.com/procps-ng/procps/-/blob/…/free.c

	printf(" %11s", scale_size(MEMINFO_GET(mem_info, MEMINFO_MEM_AVAILABLE, ul_int), args.exponent, flags & FREE_SI, flags & FREE_HUMANREADABLE));