Usable memory estimate is way off in Linux

[marcosfrm]

Once system has enough cached memory, getUsableMemory() is way off.

SystemInfo::instance().getUsableMemory() / 1024 returns:

$ ./a.out
1729184

While:

$ grep '^MemAvailable:' /proc/meminfo
MemAvailable:    2665180 kB

(this machine does not have swap)

MemAvailable was added in kernel 3.14:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0ae398fc54ea69ff85ec700722c9da773

free show its value in "available" column.

Adding it to the sysinfo structure was attempted, without success:

https://lore.kernel.org/linux-mm/1641483250-18839-1-git-send-email-quic_pintu@quicinc.com/T/

Hence parsing /proc/meminfo is the only choice:

https://bugs.openjdk.org/browse/JDK-8305770
openjdk/jdk@b6bcbc0

For older kernels without MemAvailable, a not so wrong estimate is MemFree + Cached + Buffers - Shmem:

rfjakob/earlyoom#12

but cannot be done only with sysinfo() information, because it lacks cached memory metric.

Using cbang-git b3f10ca.