gSystem->GetMemInfo reports wrong used RAM memory

[ferdymercury]

Describe the bug

On Linux, ROOT reports most of the RAM is full, even if it is free. Probably because it reports:

used = total - free
instead of
used = total - free - cached = total - available

See:

root/core/unix/src/TUnixSystem.cxx

Line 5153 in 4d2a865

meminfo->fMemUsed = meminfo->fMemTotal - meminfo->fMemFree;

/opt/root_bld/tutorials/gui $ root -l
root [0] MemInfo_t memInfo; gSystem->GetMemInfo(&memInfo);
root [1] cout << memInfo.fMemTotal << " " << memInfo.fMemUsed << " " << memInfo.fMemFree << endl;
15933 15381 552
root [2] .q
/opt/root_bld/tutorials/gui $ free -h
              total        used        free      shared  buff/cache   available
Mem:            15G        5.6G        658M        439M        9.3G        9.2G
Swap:          1.0G        135M        888M

Expected behavior

It should report the actual free memory, or add a new variable inside the MemInfo_t struct that specifies the 9.3 'cached' memory, which is actually free to use.

To Reproduce

MemInfo_t memInfo; gSystem->GetMemInfo(&memInfo);
cout << memInfo.fMemTotal << " " << memInfo.fMemUsed << " " << memInfo.fMemFree << endl;

or
/opt/root_bld/tutorials/gui $ root -l CPUMeter.C

Setup

1. ROOT version: 6.23/01
2. Operating system: Ubuntu 18
3. Self-built

Additional context

https://www.thegeekdiary.com/understanding-proc-meminfo-file-analyzing-memory-utilization-in-linux/

My suggestion to solve this would be to do:

  if (s.BeginsWith("MemAvailable")) {
     TPRegexp("^.+: *([^ ]+).*").Substitute(s, "$1");
     meminfo->fMemAvailable = (s.Atoi() / 1024);
  }

meminfo->fMemUsed = meminfo->fMemTotal - meminfo->fMemAvailable;

and then in https://root.cern/doc/master/structMemInfo__t.html#a62b63e5e24efd2e2b5d0f60456e239d6:

Int_t fMemAvailable; // available RAM in MB

For the SWAP it's probably not necessary.

Not sure if the same issue happens for other OS.

[pcanal]

or add a new variable inside the MemInfo_t struct that specifies the 9.3 'cached' memory

I would go for this solutions (add missing information).

This would requires update in GetLinuxMemInfo and GetDarwinMemInfo in TUnixSystem.cxx and GetWinNTMemInfo in TWinNTSystem.cxx

[ferdymercury]

or add a new variable inside the MemInfo_t struct that specifies the 9.3 'cached' memory

I would go for this solutions (add missing information).

Sounds good. And also document well what Used means in the MemInfo struct (as it is ambiguous and it does not correspond to any value in /proc/meminfo).

[pcanal]

@ferdymercury Would you be able to provide a PR?

[ferdymercury]

If it were only Linux, I would do it, but I am not familiar with OSx or WinNT...

I have put some code snippets above for the Linux version (MemAvailable), I tested it already and it seems to work.

[AdvaitDhingra]

@ferdymercury Would you be able to provide a PR?

I have attempted to fix this. I linked this issue in my PR.