add initial support for _mi_prim_reuse and MADV_FREE_REUSABLE on macOS (issue #1097)

include/mimalloc/internal.h

@@ -167,6 +167,7 @@ bool        _mi_os_decommit(void* addr, size_t size);
 bool        _mi_os_unprotect(void* addr, size_t size);
 bool        _mi_os_purge(void* p, size_t size);
 bool        _mi_os_purge_ex(void* p, size_t size, bool allow_reset, size_t stat_size);
+void        _mi_os_reuse(void* p, size_t size);
 mi_decl_nodiscard bool _mi_os_commit(void* p, size_t size, bool* is_zero);
 mi_decl_nodiscard bool _mi_os_commit_ex(void* addr, size_t size, bool* is_zero, size_t stat_size);
 mi_decl_nodiscard bool _mi_os_protect(void* addr, size_t size);

include/mimalloc/prim.h

@@ -63,6 +63,11 @@ int _mi_prim_decommit(void* addr, size_t size, bool* needs_recommit);
 // Returns error code or 0 on success.
 int _mi_prim_reset(void* addr, size_t size);
 
+// Reuse memory. This is called for memory that is already committed but
+// may have been reset (`_mi_prim_reset`) or decommitted (`_mi_prim_decommit`) where `needs_recommit` was false.
+// Returns error code or 0 on success. On most platforms this is a no-op.
+int _mi_prim_reuse(void* addr, size_t size);
+
 // Protect memory. Returns error code or 0 on success.
 int _mi_prim_protect(void* addr, size_t size, bool protect);
 

src/arena.c

@@ -266,12 +266,12 @@ static mi_decl_noinline void* mi_arena_try_alloc_at(mi_arena_t* arena, size_t ar
   else if (commit) {
     // commit requested, but the range may not be committed as a whole: ensure it is committed now
     memid->initially_committed = true;
+    const size_t commit_size = mi_arena_block_size(needed_bcount);      
     bool any_uncommitted;
     size_t already_committed = 0;
     _mi_bitmap_claim_across(arena->blocks_committed, arena->field_count, needed_bcount, bitmap_index, &any_uncommitted, &already_committed);
     if (any_uncommitted) {
       mi_assert_internal(already_committed < needed_bcount);
-      const size_t commit_size = mi_arena_block_size(needed_bcount);
       const size_t stat_commit_size = commit_size - mi_arena_block_size(already_committed);
       bool commit_zero = false;
       if (!_mi_os_commit_ex(p, commit_size, &commit_zero, stat_commit_size)) {
@@ -281,6 +281,10 @@ static mi_decl_noinline void* mi_arena_try_alloc_at(mi_arena_t* arena, size_t ar
         if (commit_zero) { memid->initially_zero = true; }
       }
     }
+    else {
+      // all are already committed: signal that we are reusing memory in case it was purged before
+      _mi_os_reuse( p, commit_size );
+    }
   }
   else {
     // no need to commit, but check if already fully committed

src/os.c

@@ -512,6 +512,17 @@ bool _mi_os_reset(void* addr, size_t size) {
 }
 
 
+void _mi_os_reuse( void* addr, size_t size ) {
+  // page align conservatively within the range
+  size_t csize = 0;
+  void* const start = mi_os_page_align_area_conservative(addr, size, &csize);
+  if (csize == 0) return;
+  const int err = _mi_prim_reuse(start, csize);
+  if (err != 0) {
+    _mi_warning_message("cannot reuse OS memory (error: %d (0x%x), address: %p, size: 0x%zx bytes)\n", err, err, start, csize);
+  }
+}
+
 // either resets or decommits memory, returns true if the memory needs
 // to be recommitted if it is to be re-used later on.
 bool _mi_os_purge_ex(void* p, size_t size, bool allow_reset, size_t stat_size)

src/prim/emscripten/prim.c

@@ -114,6 +114,11 @@ int _mi_prim_reset(void* addr, size_t size) {
   return 0;
 }
 
+int _mi_prim_reuse(void* addr, size_t size) {
+  MI_UNUSED(addr); MI_UNUSED(size);
+  return 0;
+}
+
 int _mi_prim_protect(void* addr, size_t size, bool protect) {
   MI_UNUSED(addr); MI_UNUSED(size); MI_UNUSED(protect);
   return 0;

src/prim/unix/prim.c

@@ -433,13 +433,27 @@ int _mi_prim_commit(void* start, size_t size, bool* is_zero) {
   return err;
 }
 
+int _mi_prim_reuse(void* start, size_t size) {
+  #if defined(__APPLE__) && defined(MADV_FREE_REUSE)
+  return unix_madvise(start, size, MADV_FREE_REUSE);
+  #endif
+  return 0;
+}
+
 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 !MI_DEBUG && MI_SECURE<=2
     *needs_recommit = false;
+    #if defined(__APPLE__) && defined(MADV_FREE_REUSABLE)
+    // decommit on macOS: use MADV_FREE_REUSABLE as it does immediate rss accounting (issue #1097)
+    err = unix_madvise(start, size, MADV_FREE_REUSABLE);
+    #else
+    // decommit: use MADV_DONTNEED as it decreases rss immediately (unlike MADV_FREE)
+    err = unix_madvise(start, size, MADV_DONTNEED);
+    #endif  
   #else
+    // note: don't use MADV_FREE_REUSABLE as the range may contain protected areas
+    err = unix_madvise(start, size, MADV_DONTNEED);
     *needs_recommit = true;
     mprotect(start, size, PROT_NONE);
   #endif
@@ -454,14 +468,21 @@ int _mi_prim_decommit(void* start, size_t size, bool* needs_recommit) {
 }
 
 int _mi_prim_reset(void* start, size_t size) {
-  // We try to use `MADV_FREE` as that is the fastest. A drawback though is that it
+  int err = 0;
+  #if defined(__APPLE__) && defined(MADV_FREE_REUSABLE)
+  // on macOS we try to use MADV_FREE_REUSABLE as it seems the fastest
+  err = unix_madvise(start, size, MADV_FREE_REUSABLE);  
+  if (err == 0) return 0;
+  // fall through
+  #endif
+
+  #if defined(MADV_FREE)
+  // Otherwise, 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
@@ -469,7 +490,7 @@ int _mi_prim_reset(void* start, size_t size) {
     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;
 }

src/prim/wasi/prim.c

@@ -149,6 +149,11 @@ int _mi_prim_reset(void* addr, size_t size) {
   return 0;
 }
 
+int _mi_prim_reuse(void* addr, size_t size) {
+  MI_UNUSED(addr); MI_UNUSED(size);
+  return 0;
+}
+
 int _mi_prim_protect(void* addr, size_t size, bool protect) {
   MI_UNUSED(addr); MI_UNUSED(size); MI_UNUSED(protect);
   return 0;

src/prim/windows/prim.c

@@ -352,6 +352,11 @@ int _mi_prim_reset(void* addr, size_t size) {
   return (p != NULL ? 0 : (int)GetLastError());
 }
 
+int _mi_prim_reuse(void* addr, size_t size) {
+  MI_UNUSED(addr); MI_UNUSED(size);
+  return 0;
+}
+
 int _mi_prim_protect(void* addr, size_t size, bool protect) {
   DWORD oldprotect = 0;
   BOOL ok = VirtualProtect(addr, size, protect ? PAGE_NOACCESS : PAGE_READWRITE, &oldprotect);