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Use a more clever formula for available_mem #34

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Use a more clever formula for available_mem #34

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15 changes: 13 additions & 2 deletions src/unix/darwin.c
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,7 @@
#include <mach-o/dyld.h> /* _NSGetExecutablePath */
#include <sys/resource.h>
#include <sys/sysctl.h>
#include <sys/types.h>
#include <unistd.h> /* sysconf */

static uv_once_t once = UV_ONCE_INIT;
Expand Down Expand Up @@ -130,9 +131,19 @@ uint64_t uv_get_constrained_memory(void) {
return 0; /* Memory constraints are unknown. */
}


uint64_t uv_get_available_memory(void) {
return uv_get_free_memory();
vm_statistics64_data_t info;
mach_msg_type_number_t count = sizeof(info) / sizeof(integer_t);

if (host_statistics64(mach_host_self(), HOST_VM_INFO64,
(host_info64_t)&info, &count) != KERN_SUCCESS) {
return UV_EINVAL; /* FIXME(bnoordhuis) Translate error. */
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What is bnoordhuis?

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Ben Noordhuis, the libuv maintainer

}
// Based on https://github.com/exelban/stats/blob/6c991de101957065f579a550ac2ae358d733d0c0/Modules/RAM/readers.swift#L47-L57
uint64_t used = info.active_count + info.inactive_count + info.compressor_page_count +info.speculative_count + info.wire_count;
uint64_t not_used = info.purgeable_count + info.external_page_count;
uint64_t available = uv_get_total_memory() - (used - not_used) * sysconf(_SC_PAGESIZE);
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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.

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

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

return available;
}


Expand Down