struct arena_bin_s {
	/*
	 * All operations on runcur, runs, and stats require that lock be
	 * locked.  Run allocation/deallocation are protected by the arena lock,
	 * which may be acquired while holding one or more bin locks, but not
	 * vise versa.
	 */
	malloc_mutex_t	lock;

	/*
	 * Current run being used to service allocations of this bin's size
	 * class.
	 */
	arena_run_t	*runcur;

	/*
	 * Tree of non-full runs.  This tree is used when looking for an
	 * existing run when runcur is no longer usable.  We choose the
	 * non-full run that is lowest in memory; this policy tends to keep
	 * objects packed well, and it can also help reduce the number of
	 * almost-empty chunks.
	 */
        // 红黑树 non-full runs，按照地址排序
	arena_run_tree_t runs;

	/* Bin statistics. */
	malloc_bin_stats_t stats;
};