record max_clear bit

src/arena.c

@@ -476,23 +476,30 @@ void* _mi_arena_alloc(size_t size, bool commit, bool allow_large, mi_arena_id_t
   Arena page allocation
 ----------------------------------------------------------- */
 
-static bool mi_arena_claim_abandoned(size_t slice_index, void* arg1, void* arg2, bool* keep_abandoned) {
+static bool mi_arena_try_claim_abandoned(size_t slice_index, void* arg1, void* arg2, bool* keep_abandoned) {
   // found an abandoned page of the right size
   mi_arena_t* const   arena   = (mi_arena_t*)arg1;
   mi_subproc_t* const subproc = (mi_subproc_t*)arg2;
   mi_page_t* const    page    = (mi_page_t*)mi_arena_slice_start(arena, slice_index);
   // can we claim ownership?
   if (!mi_page_try_claim_ownership(page)) {
+    // there was a concurrent free .. 
+    // we need to keep it in the abandoned map as the free will call `mi_arena_page_unabandon`,
+    // and wait for readers (us!) to finish. This is why it is very important to set the abandoned
+    // bit again (or otherwise the unabandon will never stop waiting).
     *keep_abandoned = true;
     return false;
   }
   if (subproc != page->subproc) {
-    // wrong sub-process.. we need to unown again, and perhaps not keep it abandoned
+    // wrong sub-process.. we need to unown again
+    // (an unown might free the page, and depending on that we can keep it in the abandoned map or not)
+    // note: a minor wrinkle: the page will still be mapped but the abandoned map entry is (temporarily) clear at this point.
+    //       so we cannot check in `mi_arena_free` for this invariant to hold.
     const bool freed = _mi_page_unown(page);
     *keep_abandoned = !freed;
     return false;
   }
-  // yes, we can reclaim it
+  // yes, we can reclaim it, keep the abandaned map entry clear
   *keep_abandoned = false;
   return true;
 }
@@ -515,7 +522,7 @@ static mi_page_t* mi_arena_page_try_find_abandoned(size_t slice_count, size_t bl
     size_t slice_index;
     mi_bitmap_t* const bitmap = arena->pages_abandoned[bin];
 
-    if (mi_bitmap_try_find_and_claim(bitmap, tseq, &slice_index, &mi_arena_claim_abandoned, arena, subproc)) {
+    if (mi_bitmap_try_find_and_claim(bitmap, tseq, &slice_index, &mi_arena_try_claim_abandoned, arena, subproc)) {
       // found an abandoned page of the right size
       // and claimed ownership.
       mi_page_t* page = (mi_page_t*)mi_arena_slice_start(arena, slice_index);
@@ -703,6 +710,9 @@ void _mi_arena_page_free(mi_page_t* page) {
     mi_assert_internal(mi_bitmap_is_setN(arena->slices_committed, slice_index, slice_count));
     mi_assert_internal(mi_bitmap_is_clearN(arena->slices_purge, slice_index, slice_count));
     mi_assert_internal(mi_bitmap_is_clearN(arena->pages_abandoned[bin], slice_index, 1));
+    // note: we cannot check for `!mi_page_is_abandoned_and_mapped` since that may
+    // be (temporarily) not true if the free happens while trying to reclaim
+    // see `mi_arana_try_claim_abandoned`
   }
   #endif
 
@@ -1087,10 +1097,11 @@ int mi_reserve_os_memory_ex(size_t size, bool commit, bool allow_large, bool exc
     return ENOMEM;
   }
   _mi_verbose_message("reserved %zu KiB memory%s\n", _mi_divide_up(size, 1024), is_large ? " (in large os pages)" : "");
+  // mi_debug_show_arenas(true, true, false);
+
   return 0;
 }
 
-
 // Manage a range of regular OS memory
 bool mi_manage_os_memory(void* start, size_t size, bool is_committed, bool is_large, bool is_zero, int numa_node) mi_attr_noexcept {
   return mi_manage_os_memory_ex(start, size, is_committed, is_large, is_zero, numa_node, false /* exclusive? */, NULL);
@@ -1121,13 +1132,22 @@ static size_t mi_debug_show_bitmap(const char* prefix, const char* header, size_
   size_t bit_set_count = 0;
   for (size_t i = 0; i < mi_bitmap_chunk_count(bitmap) && bit_count < slice_count; i++) {
     char buf[MI_BCHUNK_BITS + 64]; _mi_memzero(buf, sizeof(buf));
+    size_t k = 0;
     mi_bchunk_t* chunk = &bitmap->chunks[i];
-    for (size_t j = 0, k = 0; j < MI_BCHUNK_FIELDS; j++) {
+    
+    if (i<10)  { buf[k++] = ' '; }
+    if (i<100) { itoa((int)i, buf+k, 10); k += (i < 10 ? 1 : 2); }
+    buf[k++] = ' ';
+
+    for (size_t j = 0; j < MI_BCHUNK_FIELDS; j++) {
       if (j > 0 && (j % 4) == 0) {
         buf[k++] = '\n';
         _mi_memcpy(buf+k, prefix, strlen(prefix)); k += strlen(prefix);
         buf[k++] = ' ';
         buf[k++] = ' ';
+        buf[k++] = ' ';
+        buf[k++] = ' ';
+        buf[k++] = ' ';
       }
       if (bit_count < slice_count) {
         mi_bfield_t bfield = chunk->bfields[j];

src/bitmap.c

@@ -708,17 +708,25 @@ static bool mi_bchunk_find_and_try_clearN_(mi_bchunk_t* chunk, size_t n, size_t*
   const size_t field_count = _mi_divide_up(n, MI_BFIELD_BITS);  // we need this many fields
   for (size_t i = 0; i <= MI_BCHUNK_FIELDS - field_count; i++)
   {
-    // first pre-scan for a range of fields that are all set
+    // first pre-scan for a range of fields that are all set (up to the last one)
     bool allset = true;
     size_t j = 0;
+    size_t m = n;
     do {
       mi_assert_internal(i + j < MI_BCHUNK_FIELDS);
       mi_bfield_t b = mi_atomic_load_relaxed(&chunk->bfields[i+j]);
-      if (~b != 0) {
+      size_t idx;
+      if (mi_bfield_find_least_bit(~b,&idx)) {
+        if (m > idx) {
           allset = false;
           i += j;  // no need to look again at the previous fields
           break;
         }
+      }
+      else {
+        // all bits in b were set
+        m -= MI_BFIELD_BITS;  // note: can underflow
+      }
     } while (++j < field_count);
     
     // if all set, we can try to atomically clear them
@@ -796,6 +804,11 @@ static bool mi_bitmap_chunkmap_try_clear(mi_bitmap_t* bitmap, size_t chunk_idx)
     mi_bchunk_set(&bitmap->chunkmap, chunk_idx);
     return false;
   }
+  // record the max clear 
+  size_t oldmax = mi_atomic_load_relaxed(&bitmap->chunk_max_clear);
+  do {
+    if mi_likely(chunk_idx <= oldmax) break;
+  } while (!mi_atomic_cas_weak_acq_rel(&bitmap->chunk_max_clear, &oldmax, chunk_idx));
   return true;
 }
 
@@ -853,6 +866,7 @@ void mi_bitmap_unsafe_setN(mi_bitmap_t* bitmap, size_t idx, size_t n) {
       if ((chunk_idx % MI_BFIELD_BITS) == 0 && (chunk_idx + MI_BFIELD_BITS <= end_chunk)) {
         // optimize: we can set a full bfield in the chunkmap
         mi_atomic_store_relaxed( &bitmap->chunkmap.bfields[chunk_idx/MI_BFIELD_BITS], mi_bfield_all_set());
+        mi_bitmap_chunkmap_set(bitmap, chunk_idx + MI_BFIELD_BITS - 1);  // track the max set
         chunk_idx += MI_BFIELD_BITS;
       }
       else {
@@ -1032,20 +1046,24 @@ bool mi_bitmap_is_xsetN(mi_xset_t set, mi_bitmap_t* bitmap, size_t idx, size_t n
   { \
   /* start chunk index -- todo: can depend on the tseq to decrease contention between threads */ \
   MI_UNUSED(tseq); \
-  const size_t chunk_start = 0; /* tseq % (1 + mi_bitmap_find_hi_chunk(bitmap)); */ \
+  const size_t chunk_max = mi_atomic_load_acquire(&bitmap->chunk_max_clear);  /* mi_bitmap_chunk_count(bitmap) */ \
+  const size_t chunk_start = 0; /* (chunk_max <= 1 ? 0 : (tseq % chunk_max)); */      /* space out threads */ \
   const size_t chunkmap_max_bfield = _mi_divide_up( mi_bitmap_chunk_count(bitmap), MI_BCHUNK_BITS ); \
   const size_t chunkmap_start = chunk_start / MI_BFIELD_BITS; \
   const size_t chunkmap_start_idx = chunk_start % MI_BFIELD_BITS; \
   /* for each chunkmap entry `i` */ \
   for (size_t _i = 0; _i < chunkmap_max_bfield; _i++) { \
     size_t i = (_i + chunkmap_start); \
-    if (i >= chunkmap_max_bfield) { i -= chunkmap_max_bfield; } /* adjust for the start position */ \
-    \
+    if (i >= chunkmap_max_bfield) { \
+      i -= chunkmap_max_bfield; /* adjust for the start position */ \
+    } \
     const size_t chunk_idx0 = i*MI_BFIELD_BITS; \
     mi_bfield_t cmap = mi_atomic_load_relaxed(&bitmap->chunkmap.bfields[i]); \
     size_t      cmap_idx_shift = 0;   /* shift through the cmap */ \
-    if (_i == 0) { cmap = mi_bfield_rotate_right(cmap, chunkmap_start_idx); cmap_idx_shift = chunkmap_start_idx; }   /* rotate right for the start position (on the first iteration) */ \
-    \
+    if (_i == 0) { \
+      cmap = mi_bfield_rotate_right(cmap, chunkmap_start_idx); /* rotate right for the start position (on the first iteration) */ \
+      cmap_idx_shift = chunkmap_start_idx; \
+    } \
     size_t cmap_idx; \
     while (mi_bfield_find_least_bit(cmap, &cmap_idx)) { \
       /* set the chunk idx */ \
@@ -1065,6 +1083,7 @@ bool mi_bitmap_is_xsetN(mi_xset_t set, mi_bitmap_t* bitmap, size_t idx, size_t n
 
 // Find a sequence of `n` bits in the bitmap with all bits set, and atomically unset all.
 // Returns true on success, and in that case sets the index: `0 <= *pidx <= MI_BITMAP_MAX_BITS-n`.
+// (Used to find fresh free slices.)
 mi_decl_nodiscard bool mi_bitmap_try_find_and_clearN(mi_bitmap_t* bitmap, size_t n, size_t tseq, size_t* pidx)
 {
   mi_bitmap_forall_chunks(bitmap, tseq, epoch, chunk_idx)
@@ -1087,6 +1106,8 @@ mi_decl_nodiscard bool mi_bitmap_try_find_and_clearN(mi_bitmap_t* bitmap, size_t
 }
 
 
+// Find a set bit in the bitmap and try to atomically clear it and claim it.
+// (Used to find pages in the pages_abandoned bitmaps.)
 mi_decl_nodiscard bool mi_bitmap_try_find_and_claim(mi_bitmap_t* bitmap, size_t tseq, size_t* pidx,
                                                     mi_claim_fun_t* claim, void* arg1, void* arg2)
 {

src/bitmap.h

@@ -92,7 +92,8 @@ typedef mi_bchunk_t mi_bchunkmap_t;
 // An atomic bitmap
 typedef mi_decl_align(MI_BCHUNK_SIZE) struct mi_bitmap_s {
   _Atomic(size_t)  chunk_count;      // total count of chunks (0 < N <= MI_BCHUNKMAP_BITS)
-  size_t           _padding[MI_BCHUNK_SIZE/MI_SIZE_SIZE - 1];    // suppress warning on msvc
+  _Atomic(size_t)  chunk_max_clear;  // max chunk index that was once cleared 
+  size_t           _padding[MI_BCHUNK_SIZE/MI_SIZE_SIZE - 2];    // suppress warning on msvc
   mi_bchunkmap_t   chunkmap;
   mi_bchunk_t      chunks[MI_BITMAP_DEFAULT_CHUNK_COUNT];        // usually dynamic MI_BITMAP_MAX_CHUNK_COUNT
 } mi_bitmap_t;

src/page-map.c

@@ -13,7 +13,10 @@ mi_decl_cache_align uint8_t* _mi_page_map = NULL;
 static bool        mi_page_map_all_committed = false;
 static size_t      mi_page_map_entries_per_commit_bit = MI_ARENA_SLICE_SIZE;
 static mi_memid_t  mi_page_map_memid;
-static mi_bitmap_t mi_page_map_commit = { MI_BITMAP_DEFAULT_CHUNK_COUNT, { 0 }, { 0 }, { { 0 } } };
+
+// (note: we need to initialize statically or otherwise C++ may run a default constructors after process initialization)
+static mi_bitmap_t mi_page_map_commit = { MI_ATOMIC_VAR_INIT(MI_BITMAP_DEFAULT_CHUNK_COUNT), MI_ATOMIC_VAR_INIT(0),
+                                          { 0 }, { {MI_ATOMIC_VAR_INIT(0)} }, {{{ MI_ATOMIC_VAR_INIT(0) }}} };
 
 bool _mi_page_map_init(void) {
   size_t vbits = _mi_os_virtual_address_bits();