Function _mi_page_retire

3rd/mimalloc-2.0.9/src/page.c:428–457

Retire a page with no more used blocks Important to not retire too quickly though as new allocations might coming. Note: called from `mi_free` and benchmarks often trigger this due to freeing everything and then allocating again so careful when changing this.

// trigger this due to freeing everything and then
// allocating again so careful when changing this.
void _mi_page_retire(mi_page_t* page) mi_attr_noexcept {
  mi_assert_internal(page != NULL);
  mi_assert_expensive(_mi_page_is_valid(page));
  mi_assert_internal(mi_page_all_free(page));
  
  mi_page_set_has_aligned(page, false);

  // don't retire too often..
  // (or we end up retiring and re-allocating most of the time)
  // NOTE: refine this more: we should not retire if this
  // is the only page left with free blocks. It is not clear
  // how to check this efficiently though...
  // for now, we don't retire if it is the only page left of this size class.
  mi_page_queue_t* pq = mi_page_queue_of(page);
  if mi_likely(page->xblock_size <= MI_MAX_RETIRE_SIZE && !mi_page_queue_is_special(pq)) {  // not too large && not full or huge queue?
    if (pq->last==page && pq->first==page) { // the only page in the queue?
      mi_stat_counter_increase(_mi_stats_main.page_no_retire,1);
      page->retire_expire = 1 + (page->xblock_size <= MI_SMALL_OBJ_SIZE_MAX ? MI_RETIRE_CYCLES : MI_RETIRE_CYCLES/4);      
      mi_heap_t* heap = mi_page_heap(page);
      mi_assert_internal(pq >= heap->pages);
      const size_t index = pq - heap->pages;
      mi_assert_internal(index < MI_BIN_FULL && index < MI_BIN_HUGE);
      if (index < heap->page_retired_min) heap->page_retired_min = index;
      if (index > heap->page_retired_max) heap->page_retired_max = index;
      mi_assert_internal(mi_page_all_free(page));
      return; // dont't free after all
    }
  }
  _mi_page_free(page, pq, false);
}

// free retired pages: we don't need to look at the entire queues
// since we only retire pages that are at the head position in a queue.