MADV_FREE vs MADV_DONTNEED: how the memory is accounted

madvise.py

#!/usr/bin/env python3

# systemd-run --user --unit test-madvise ./test.py
# journalctl -u -r

import mmap
import resource
import os

def get_cgroup_id():
    with open("/proc/self/cgroup") as f:
        content = f.read().strip()
        path = content.split(':')[-1]
        return path

def get_cgroup_mem(cgroup_id):
    with open('/sys/fs/cgroup/' + cgroup_id + '/memory.stat') as f:
        content = f.read().strip()
        content = content.split('\n')
        content = filter(lambda line: 'active_anon' in line, content)
        content = map(lambda line: line.split(' '), content)
        content = list(content)
        ret = {}
        for key, value in content:
            ret[key] = int(value)
        return ret

def get_rss():
    with open('/proc/self/statm') as f:
        rss = f.read().strip().split(' ')[1]
        return int(rss)

def print_mem(prefix, cgroup_id):
    cgroup_mem = get_cgroup_mem(cgroup_id)
    rss = get_rss()
    # metrics are in bytes
    print('{:30}: rss {:10}, cgroup mem: inactive_anon: {}, active_anon: {}'.format(
        prefix,
        rss*4096,
        cgroup_mem['inactive_anon'],
        cgroup_mem['active_anon']))

def main():
    cgroup_id = get_cgroup_id()
    print('cgroup id: ' + cgroup_id)

    print_mem('before mmap', cgroup_id)

    vma = mmap.mmap(-1, 1<<30, mmap.MAP_PRIVATE)
    print_mem('after mmap', cgroup_id)
    vma.write(b'0' * int(1<<30))
    print_mem('after mmap fill', cgroup_id)
    vma.madvise(mmap.MADV_FREE)
    print_mem('after MADV_FREE', cgroup_id)
    vma.madvise(mmap.MADV_DONTNEED)
    print_mem('after MADV_DONTNEED', cgroup_id)
    vma.close()
    print_mem('after munmap', cgroup_id)

if __name__ == "__main__":
    main()

$ systemd-run --user --unit test-madvise ./test.py
$ journalctl -u test-madvise --user
Dec 16 14:47:45 local systemd[2171]: Started /tmp/madvise/./test.py.
Dec 16 14:47:45 local test.py[19339]: cgroup id: /user.slice/user-1000.slice/user@1000.service/app.slice/test-madvise.service
Dec 16 14:47:45 local test.py[19339]: before mmap                   : rss    9437184, cgroup mem: inactive_anon: 4096, active_anon: 4100096
Dec 16 14:47:45 local test.py[19339]: after mmap                    : rss    9437184, cgroup mem: inactive_anon: 4096, active_anon: 4100096
Dec 16 14:47:45 local test.py[19339]: after mmap fill               : rss 1083637760, cgroup mem: inactive_anon: 4096, active_anon: 1078915072
Dec 16 14:47:45 local test.py[19339]: after MADV_FREE               : rss 1083637760, cgroup mem: inactive_anon: 4096, active_anon: 6230016
Dec 16 14:47:45 local test.py[19339]: after MADV_DONTNEED           : rss    9895936, cgroup mem: inactive_anon: 4096, active_anon: 4124672
Dec 16 14:47:45 local test.py[19339]: after munmap                  : rss    9895936, cgroup mem: inactive_anon: 4096, active_anon: 4128768