merge from dev (support for mi_prim_reuse, #1097)

include/mimalloc/internal.h

@@ -188,6 +188,7 @@ size_t        _mi_os_virtual_address_bits(void);
 
 bool          _mi_os_reset(void* addr, size_t size);
 bool          _mi_os_decommit(void* addr, size_t 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

@@ -216,12 +216,13 @@ static mi_decl_noinline void* mi_arena_try_alloc_at(
       }
     }
     else {
-      // already fully commited.
+      // already fully committed.
+      _mi_os_reuse(p, mi_size_of_slices(slice_count));
       // if the OS has overcommit, and this is the first time we access these pages, then
       // count the commit now (as at arena reserve we didn't count those commits as these are on-demand)
       if (_mi_os_has_overcommit() && touched_slices > 0) {
         mi_subproc_stat_increase( arena->subproc, committed, mi_size_of_slices(touched_slices));
-      }
+      }      
     }
     // tool support
     if (memid->initially_zero) {

src/os.c

@@ -532,6 +532,17 @@ 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, mi_commit_fun_t* commit_fun, void* commit_fun_arg)

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

@@ -434,13 +434,27 @@ 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
@@ -455,14 +469,21 @@ 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
@@ -470,7 +491,7 @@ 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

@@ -376,6 +376,11 @@ 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);