c - malloc 如何在多线程环境中工作？

Question

典型的 malloc(对于 x86-64 平台和 Linux 操作系统)是在开始时天真地锁定一个互斥锁并在完成后释放它，还是在一个更聪明的方式锁定一个互斥锁？更精细的级别，从而减少锁争用？如果确实是第二种方式，它是怎么做到的？

Answer 1

glibc 2.15 操作多个分配arenas。每个竞技场都有自己的锁。当一个线程需要分配内存时，malloc()选择一个arena，锁定它，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(为简洁而编辑):

  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;

这个分配器叫做 ptmalloc .它基于 earlier work由 Doug Lea 编写，由 Wolfram Gloger 维护。