malloc如何在多线程环境中工作？

Z时代
2024-01-10
分类：问答

典型的malloc（对于x86-64平台和Linux

OS）是在开始时幼稚地锁定互斥锁并在完成后将其释放，还是以更巧妙的方式将互斥锁锁定在更精细的级别，从而减少了锁争用？如果确实采用第二种方法，那么该如何做？

回答：

glibc 2.15经营多个分配场所

。每个竞技场都有自己的锁。当线程需要分配内存时，malloc()选择一个竞技场，将其锁定，然后从中分配内存。

选择竞技场的机制有些复杂，旨在减少锁争用：

/* arena_get() acquires an arena and locks the corresponding mutex. First, try the one last locked successfully by this thread. (This is the common case and handled with a macro for speed.) Then, loop once over the circularly linked list of arenas. If no arena is readily available, create a new one. In this latter case, `size' is just a hint as to how much memory will be required immediately in the new arena. */

考虑到这一点，malloc()基本上看起来像这样（为简便起见编辑）：

  mstate ar_ptr;
  void *victim;
  arena_lookup(ar_ptr);
  arena_lock(ar_ptr, bytes);
  if(!ar_ptr)
    return 0;
  victim = _int_malloc(ar_ptr, bytes);
  if(!victim) {
    /* Maybe the failure is due to running out of mmapped areas. */
    if(ar_ptr != &main_arena) {
      (void)mutex_unlock(&ar_ptr->mutex);
      ar_ptr = &main_arena;
      (void)mutex_lock(&ar_ptr->mutex);
      victim = _int_malloc(ar_ptr, bytes);
      (void)mutex_unlock(&ar_ptr->mutex);
    } else {
      /* ... or sbrk() has failed and there is still a chance to mmap() */
      ar_ptr = arena_get2(ar_ptr->next ? ar_ptr : 0, bytes);
      (void)mutex_unlock(&main_arena.mutex);
      if(ar_ptr) {
        victim = _int_malloc(ar_ptr, bytes);
        (void)mutex_unlock(&ar_ptr->mutex);
      }
    }
  } else
    (void)mutex_unlock(&ar_ptr->mutex);
  return victim;