On linux, how should I calculate the amount of free memory from the information in /proc/mem?

Question

There are many fields in /proc/mem: I know that I can't just take "MemFree", because lots of memory is actually cached. So the question is, how can I calculate the amount of free memory?

Assumptions:

• The system is configured with no swap space.
• My definition of "free memory" is that malloc starts failing when this hits zero.

Answer 1

If as you say the system is configured with no swap space, then the amount of free memory can be calculating by adding the "MemFree", "Buffers" and "Cached" values from /proc/meminfo.

This is exactly what the command 'free -m' shows under 'free' in the '-/+ buffers/cache' line.

In Python, I would implement this as follows:

 with open('/proc/meminfo', 'rt') as f:
        vals = {}
        for i in f.read().splitlines():
            try:
                name, val = i.split(':')
                vals[name.strip()] = int(val.split()[0])
            except:
                pass

 memfree = vals['MemFree'] + vals['Buffers'] + vals['Cached']

This would give a value in kilobytes.

As others have said, malloc is not likely to return null ever. Linux will overallocate and when you start using a page that really cannot be found, the OOM killer will kick in.