example showing how MADV_FREE does not incur in RSS reduction right away

madvise-sample.c

#include <signal.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdio.h>
#include <sys/mman.h>
#include <unistd.h>

bool cont = true;
static const ptrdiff_t len = 1 << 25; // 32 MB

void
handle_signal(int sig)
{
        if (sig != SIGINT) {
                return;
        }

        cont ^= true;
}

void
wait()
{
        cont = false;
        signal(SIGINT, handle_signal);

        printf("CTRL+C to continue\n");

        while (!cont) {
                sleep(1);
        }
}

int
main(void)
{
        char *start, *end;
        void* pb;

        pb = sbrk(0);
        if (pb == (void*)-1) {
                perror("sbrk");
                return 1;
        }

        start = (char*)pb;
        end = start + len;

        // "allocate" mem by increasing the program break
        //
        if (!~brk(end)) {
                perror("brk");
                return 1;
        }

        wait();

        // "touch" the memory so that we get it really utilized - at this point,
        // we should see the faults taking place, and both RSS and active anon
        // going up
        //
        for (; start < end;) {
                *(start++) = 123;
        }

        wait();

        // let the kernel know that we don't really need half of the memory we
        // allocated anymore - while this will not change RSS, it'll definitely
        // change active and inactive.
        //
        if (!~madvise(pb + (len >> 1), (len >> 1), MADV_FREE)) {
                perror("madvise");
                return 1;
        }

        wait();

        return 0;
}

pause-1.diff

diff --git a/tmp/1 b/tmp/2
index 3e93f6b..f37efef 100644
--- a/tmp/1
+++ b/tmp/2
@@ -1,33 +1,33 @@
 cache 0
-rss 278528
+rss 33800192
 rss_huge 0
 shmem 0
 mapped_file 0
 dirty 0
 writeback 0
-pgpgin 61545
+pgpgin 69729
 pgpgout 61524
-pgfault 75009
+pgfault 83193
 pgmajfault 0
 inactive_anon 0
-active_anon 352256
+active_anon 33906688
 inactive_file 0
 active_file 0
 unevictable 0
 hierarchical_memory_limit 9223372036854771712
 total_cache 0
-total_rss 278528
+total_rss 33800192
 total_rss_huge 0
 total_shmem 0
 total_mapped_file 0
 total_dirty 0
 total_writeback 0
-total_pgpgin 61545
+total_pgpgin 69729
 total_pgpgout 61524
-total_pgfault 75009
+total_pgfault 83193
 total_pgmajfault 0
 total_inactive_anon 0
-total_active_anon 352256
+total_active_anon 33906688
 total_inactive_file 0
 total_active_file 0
 total_unevictable 0

pause-2.diff

diff --git a/tmp/2 b/tmp/3
index f37efef..cc81ef9 100644
--- a/tmp/2
+++ b/tmp/3
@@ -10,8 +10,8 @@ pgpgout 61524
 pgfault 83193
 pgmajfault 0
 inactive_anon 0
-active_anon 33906688
-inactive_file 0
+active_anon 17129472
+inactive_file 16777216
 active_file 0
 unevictable 0
 hierarchical_memory_limit 9223372036854771712
@@ -27,7 +27,7 @@ total_pgpgout 61524
 total_pgfault 83193
 total_pgmajfault 0
 total_inactive_anon 0
-total_active_anon 33906688
-total_inactive_file 0
+total_active_anon 17129472
+total_inactive_file 16777216
 total_active_file 0
 total_unevictable 0

USAGE

1. compile it
  gcc -O2 -static -o sample ./madvise-sample.c
    
2. run it in a terminal that has the current proc in a cgroup
  mkdir /sys/fs/cgroup/memory/test
  echo $$ > /sys/fs/cgroup/memory/test/cgroup.procs
  ./sample
  
3. in another terminal, observe `memory.stat` for that cgroup
  cat /sys/fs/cgroup/memory/test/memory.stat
  
4. use CTRL+C to make `sample` advance - see `memory.stat` as you do it.