Use a more clever formula for available_mem

[gbaraldi]

We might want to use a different combination of values from vm_info64, I chose this because that's what Stats uses but I'm not too inclined to it or against it.
@staticfloat

Fixes JuliaLang/julia#48473

[staticfloat]

What is bnoordhuis?

[maleadt]

Ben Noordhuis, the libuv maintainer

[staticfloat]

I don't think speculative_count should be in used; since the kernel counts it as part of free: https://github.com/apple/darwin-xnu/blob/2ff845c2e033bd0ff64b5b6aa6063a1f8f65aa32/osfmk/kern/host.c#L815

I think there's an easier way to figure out "available" memory:

uint64_t available = (info.free_count + info.inactive_count) * sysconf(_SC_PAGESIZE);

You could arguably include purgeable_count into that. This completely ignores the external page count (which I believe represents pages which have been swapped to disk), and I think that.... is probably the right thing to do? If macOS pages things out to make room for us, that's fine, and we can continue to use more physical memory.

[gbaraldi]

I'm not sure if external is swapped pages, or pages that are on disk by default, but the os is caching on RAM.

[gbaraldi]

But your code seems to give very similar results while looking cleaner.

[vustef]

In case anyone needs to calculate this in Julia, here's a hacked up snippet:

mutable struct VmStatistics64
	free_count::UInt32
	active_count::UInt32
	inactive_count::UInt32
	wire_count::UInt32
	zero_fill_count::UInt64
	reactivations::UInt64
	pageins::UInt64
	pageouts::UInt64
	faults::UInt64
	cow_faults::UInt64
	lookups::UInt64
	hits::UInt64
	purges::UInt64
	purgeable_count::UInt32
	
	speculative_count::UInt32

	decompressions::UInt64
	compressions::UInt64
	swapins::UInt64
	swapouts::UInt64
	compressor_page_count::UInt32
	throttled_count::UInt32
	external_page_count::UInt32
	internal_page_count::UInt32
	total_uncompressed_pages_in_compressor::UInt64

	VmStatistics64() = new(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0)
end

vms = VmStatistics64()

mach_host_self = @ccall mach_host_self()::UInt32


count = UInt32(sizeof(vms) / sizeof(Int32))
ref_count = Ref(count)

@ccall host_statistics64(mach_host_self::UInt32, 4::Int64, pointer_from_objref(vms)::Ptr{Int64}, ref_count::Ref{UInt32})::Int64

# @show vms
# # values should correspond to:
# @info read(`vm_stat`, String)

page_size = Int(@ccall sysconf(29::UInt32)::UInt32)

used = vms.active_count + vms.inactive_count + vms.compressor_page_count +vms.speculative_count + vms.wire_count
not_used = vms.purgeable_count + vms.external_page_count
available = Sys.total_memory() - (used - not_used) * page_size
@show available / 2^20