Apple friendly way to madvice on commit/decommit: MADV_FREE_REUSABLE/MADV_FREE_REUSE

src/prim/unix/prim.c

@@ -430,13 +430,29 @@ int _mi_prim_commit(void* start, size_t size, bool* is_zero) {
     err = errno;
     unix_mprotect_hint(err);
   }
+
+  #if defined(__APPLE__)
+    // MADV_FREE_REUSABLE is paired with MADV_FREE_REUSE for accounting
+    // if this memory was not marked as MADV_FREE_REUSABLE, this call is noop
+    madvise(start, size, MADV_FREE_REUSE);
+  #endif
   return err;
 }
 
 int _mi_prim_decommit(void* start, size_t size, bool* needs_recommit) {
-  int err = 0;
-  // decommit: use MADV_DONTNEED as it decreases rss immediately (unlike MADV_FREE)
-  err = unix_madvise(start, size, MADV_DONTNEED);
+  #if defined(__APPLE__)
+    // On Apple platforms there is MADV_FREE_REUSABLE -- it marks memory as RESUABLE to reduce memory pressure.
+    int err = unix_madvise(start, size, MADV_FREE_REUSABLE);
+    if (err != 0) // if MADV_FREE_REUSABLE fails - fallback to MADV_DONTNEED
+    {
+      // use MADV_DONTNEED as it decreases rss immediately (unlike MADV_FREE)
+      err = unix_madvise(start, size, MADV_DONTNEED);
+    }
+  #else
+    // use MADV_DONTNEED as it decreases rss immediately (unlike MADV_FREE)
+    int err = unix_madvise(start, size, MADV_DONTNEED);
+  #endif
+
   #if !MI_DEBUG && MI_SECURE<=2
     *needs_recommit = false;
   #else
@@ -454,22 +470,30 @@ int _mi_prim_decommit(void* start, size_t size, bool* needs_recommit) {
 }
 
 int _mi_prim_reset(void* start, size_t size) {
+  int err = 0;
+  #if defined(__APPLE__)
+    // On Apple platforms there is MADV_FREE_REUSABLE -- it marks memory as RESUABLE to reduce memory pressure.
+    err = unix_madvise(start, size, MADV_FREE_REUSABLE);
+    if (err == 0)
+      return 0;
+    // if this fails - fallback to MADV_FREE/MADV_DONTNEED
+  #endif
+
   // We try to use `MADV_FREE` as that is the fastest. A drawback though is that it
   // will not reduce the `rss` stats in tools like `top` even though the memory is available
   // to other processes. With the default `MIMALLOC_PURGE_DECOMMITS=1` we ensure that by
   // default `MADV_DONTNEED` is used though.
   #if defined(MADV_FREE)
-  static _Atomic(size_t) advice = MI_ATOMIC_VAR_INIT(MADV_FREE);
-  int oadvice = (int)mi_atomic_load_relaxed(&advice);
-  int err;
-  while ((err = unix_madvise(start, size, oadvice)) != 0 && errno == EAGAIN) { errno = 0;  };
-  if (err != 0 && errno == EINVAL && oadvice == MADV_FREE) {
-    // if MADV_FREE is not supported, fall back to MADV_DONTNEED from now on
-    mi_atomic_store_release(&advice, (size_t)MADV_DONTNEED);
-    err = unix_madvise(start, size, MADV_DONTNEED);
-  }
+    static _Atomic(size_t) advice = MI_ATOMIC_VAR_INIT(MADV_FREE);
+    int oadvice = (int)mi_atomic_load_relaxed(&advice);
+    while ((err = unix_madvise(start, size, oadvice)) != 0 && errno == EAGAIN) { errno = 0;  };
+    if (err != 0 && errno == EINVAL && oadvice == MADV_FREE) {
+      // if MADV_FREE is not supported, fall back to MADV_DONTNEED from now on
+      mi_atomic_store_release(&advice, (size_t)MADV_DONTNEED);
+      err = unix_madvise(start, size, MADV_DONTNEED);
+    }
   #else
-  int err = unix_madvise(start, size, MADV_DONTNEED);
+    err = unix_madvise(start, size, MADV_DONTNEED);
   #endif
   return err;
 }