Is low memory warning based on a fixed number of on the actual sketch?

Question

I have the following output of my sketch for the Arduino Mega so far:

Sketch uses 25020 bytes (9%) of program storage space. Maximum is 253952 bytes.
Global variables use 6919 bytes (84%) of dynamic memory, leaving 1273 bytes for
local variables. Maximum is 8192 bytes.

Low memory available, stability problems may occur.

I think 1273 bytes is not so less for most sketches, although mine is quite big (several thousand of lines, and I'm about half).

I was wondering if the IDE is so smart to statically analyze my code to see what my dynamic storage is (about 600 bytes) and/or calculates some expected stack size (several hundred bytes at least).

Or is it just a (very) safe fixed or percentage-based value?

Answer 1

I just did some tests and found the answer.

char a[6135];

void setup() 
{
  a[0] = 0;
}

void loop() 
{
}

This sketch does not give a warning:

Sketch uses 660 bytes (0%) of program storage space. Maximum is 253952 bytes.
Global variables use 6144 bytes (75%) of dynamic memory, leaving 2048 bytes for local variables. Maximum is 8192 bytes.

If I increase the array with 1, I get an error:

Sketch uses 660 bytes (0%) of program storage space. Maximum is 253952 bytes.
Global variables use 6145 bytes (75%) of dynamic memory, leaving 2047 bytes for local variables. Maximum is 8192 bytes.
Low memory available, stability problems may occur.

Conclusion: below 2 KB (2048 bytes) of free space, the warning is shown.

I did the same test for the Uno, and there the warning is shown when less than 1536 bytes are free.

Both percentages are 75%, so the end conclusion is that the value is based on 75% of memory.