added new command line option heap_size_hint for greedy GC

[rssdev10]

The changes add a new command-line option --heap_size_hint for triggering aggressive garbage collection.

The following code might be used for checking that option.

using Dates

const mem_size = 0x1_000_000

counter = let
    counter = 0
    start_t = now()

    while (now() - start_t) / Millisecond(1000) < 15
        counter += 1
        arr = Vector{Int32}(undef, mem_size)
        arr[rand(1:mem_size)] = rand(Int32, 1)[begin]
        # sleep(1)
    end
    counter
end

@show counter, mem_size

Memory consumption might be estimated with % gtime --verbose usr/bin/julia memory_test.jl :

(counter, mem_size) = (348403, 0x01000000)
	Command being timed: "usr/bin/julia memory_test.jl"
	User time (seconds): 20.86
...
	Maximum resident set size (kbytes): 440704
...

Use time for Linux and gtime for MacOS.

And, memory consumption after adding the hint
% gtime --verbose usr/bin/julia --heap-size-hint=100m memory_test.jl:

(counter, mem_size) = (436, 0x01000000)
	Command being timed: "usr/bin/julia --heap-size-hint=100m memory_test.jl"
	User time (seconds): 15.24
...
	Maximum resident set size (kbytes): 267024
...

[chflood]

Looks good

[vtjnash]

Why are these in multiples of 500x or 1.5x?

[oscardssmith]

those are just examples. It arguably should be written specified in megabytes (e.g. 500M) or gigabytes (e.g. 1.5G)

[rssdev10]

Why are these in multiples of 500x or 1.5x?

long double value is used for the numeric part. With 1e-7 restriction as a minimal value. So, even 0.000123G is working.

[MilesCranmer]

Is there an environment variable for this? Would be useful for GitHub actions

[sefffal]

I second this: I would like to set a heap size hint via environment variable for a Julia library embedded in Python.

[MilesCranmer]

^Trying to add this here: #55631