Force free() to return malloc memory back to OS

The only reliable and portable way to have the OS reclaim memory is to exit the process and restart it again, restoring any state you need to continue.

Of course, writing your own malloc/free implementation using brk/sbrk according to your needs is the other option.

With glibc malloc try to call malloc_trim function. It is not well documented and there were changes inside it at around 2007 (glibc 2.9) - https://stackoverflow.com/a/42281428.

Since 2007 this function will: Iterate over all malloc memory arenas (used in multithreaded applications) doing trim and fastbin consolidation; and release all aligned (4KB) pages fully freed.

https://sourceware.org/git/?p=glibc.git;a=commit;f=malloc/malloc.c;h=68631c8eb92ff38d9da1ae34f6aa048539b199cc

Ulrich Drepper Sun, 16 Dec 2007 22:53:08 +0000 (22:53 +0000)

• malloc/malloc.c (public_mTRIm): Iterate over all arenas and call mTRIm for all of them.

(mTRIm): Additionally iterate over all free blocks and use madvise to free memory for all those blocks which contain at least one memory page.

https://sourceware.org/git/?p=glibc.git;a=blobdiff;f=malloc/malloc.c;h=c54c203cbf1f024e72493546221305b4fd5729b7;hp=1e716089a2b976d120c304ad75dd95c63737ad75;hb=68631c8eb92ff38d9da1ae34f6aa048539b199cc;hpb=52386be756e113f20502f181d780aecc38cbb66a

+  malloc_consolidate (av);
...
+  for (int i = 1; i < NBINS; ++i)
...
+        for (mchunkptr p = last (bin); p != bin; p = p->bk)
+         {
...
+               /* See whether the chunk contains at least one unused page.  */
+               char *paligned_mem = (char *) (((uintptr_t) p
+                                               + sizeof (struct malloc_chunk)
+                                               + psm1) & ~psm1);
...
+               /* This is the size we could potentially free.  */
+               size -= paligned_mem - (char *) p;
+
+               if (size > psm1)
+                 {
...
+                   madvise (paligned_mem, size & ~psm1, MADV_DONTNEED);

So, calling malloc_trim will release almost all freed memory back to the OS. Only pages containing still not freed data will be kept; OS may unmap or not unmap physical page when madvised with MADV_DONTNEED and linux usually does unmap. madvised pages are still count to VSIZE (total virtual memory size of the process), but usually help to reduce RSS (amount of physical memory used by process).

Alternatively, you can try to switch into alternative malloc library: tcmalloc (gperftools / google-perftools) or jemalloc (facebook), both of them have aggressive rules of returning freed memory back to OS (with madvise MADV_DONTNEED or even MADV_FREE).