mm/swap.c

    这个文件内容比较凌乱,大部分的内容在分析memory.c, filemap.c的时候都有
涉及.这里就给个大概的描述吧.以说明我们也搞过这个文件了.

    仔细阅读一下注释,freepages的含义就一目了然了:
 freepages.min :当系统的空闲页面(buddy中)少于此数量时,只有kernel有权利使
       用这些页面.
 freepages.low :当系统空闲页面低于此值时,内核开始大规模启动swap .

 freepages.high :我们努力保持系统的空闲页面不低于此值. 如果低于此值内核
        即刻开始逐渐swap out,以使内核'永不'进行较大规模的swap.
		   
/*
 * We identify three levels of free memory.  We never let free mem
 * fall below the freepages.min except for atomic allocations.  We
 * start background swapping if we fall below freepages.high free
 * pages, and we begin intensive swapping below freepages.low.
 *
 * Actual initialization is done in mm/page_alloc.c or 
 * arch/sparc(64)/mm/init.c.
 */
freepages_t freepages = {
	0,	/* freepages.min */ /*When the number of free pages in the system
		reaches this number, only the kernel can allocate more memory. */

	0,	/* freepages.low *//*If the number of free pages gets below this
		point, the kernel starts swapping aggressively.*/

	0	/* freepages.high *//*The kernel tries to keep up to this amount of
		memory free; if memory comes below this point,
		the kernel gently starts swapping in the hopes
		that it never has to do real aggressive swapping.*/
};

  关于alloc page的详细情况,见filemap.c的分析.

  下面是page_cluster和memory_pressure:  
/* How many pages do we try to swap or page in/out together? */
int page_cluster; /*每个cluster拥有的页面个数，2^page_cluster*/
/*
    page_cluster: 用于filemap, swap in   的预读簇
    SWAPFILE_CLUSTER: swap 设备分配swap entry(swap page)使用的簇
*/

  
/*
 * This variable contains the amount of page steals the system
 * is doing, averaged over a minute. We use this to determine how
 * many inactive pages we should have.
 *
 * In reclaim_page and __alloc_pages: memory_pressure++
 * In __free_pages_ok: memory_pressure--
 * In recalculate_vm_stats the value is decayed (once a second)
 */
int memory_pressure;

   也请看filemap.c的相关分析.
 
   
   
   然后是 nr_async_pages: 
/* We track the number of pages currently being asynchronously swapped
 *   out, so that we don't try to swap TOO many pages out at once 
 */
 /* rw_swap_page_base: inc nr_async_pages    end_buffer_io_async:dec this one*/
 /*异步io 状态就是提交读/写后不等待页面get unlocked*/
 atomic_t nr_async_pages = ATOMIC_INIT(0); /*所有在异步io状态的页面总数*/



   buffer_mem, page_cache 几乎被放弃了,2.6的实现较为利落.不再分析.
   
   
   
  在pager_daemon中只有pager_daemon.swap_cluster 还有意义,其他值都没有地方
使用.	
    pager_daemon.swap_cluster; /*最多预读的cluster个数*/
    面分析的page_cluster,是每个cluster的页面数量,这里的swap_cluster是一次
最多容许swap的cluster数量.

  
   剩下的函数实现操作page->age,以及在lru cache内移动page.filemap.c的分析中
解释了lru cache的组成.请见相关分析.
   这里申明一下PG_referenced的具体含义:
   
 /* -- user page 在try_to_swap_out 通过遍历进程页
  *  目录,操作page->age. 而对于内核使用的页
  *  面,比如buffer,inode等,try_to_swap_out无法确定
  *  是否应该老化此页面. 解决的办法就是
  *  通过PG_referenced bit,内核访问的时候置此位
  *  相当于hit.
  *  mm.h 的注释如下
  * For choosing which pages to swap out, inode pages carry a
  * PG_referenced bit, which is set any time the system accesses
  * that page through the (inode,offset) hash table.
  */
   
   剩余的函数逻辑都是清楚的,理解lru,page cache,swap cache后不难. 不再列出.