【Linux 内核 内存管理】分区伙伴分配器 ⑦ ( z-＞watermark[WMARK_MIN] 最低水位计算 | min_free_kbytes 初始化 )

文章目录

• ​​一、z->watermark[WMARK_MIN] 最低水位计算​​
• ​​二、min_free_kbytes 初始化过程​​
• ​​三、最低水位计算过程​​
• ​​四、nr_free_buffer_pages 函数原型​​
• ​​五、nr_free_zone_pages 函数原型​​

一、z->watermark[WMARK_MIN] 最低水位计算

在 内存区域 的水位控制机制 中 , 在 内存区域 ​

​zone​

​​ 结构体中的 ​

​watermark​

​ 成员 表示 " 页分配器 " 使用的 区域水线 ;

struct zone {
  /* Read-mostly fields */

  /* zone watermarks, access with *_wmark_pages(zone) macros */
  unsigned long watermark[NR_WMARK];
}      

源码路径 : linux-4.12\include\linux\mmzone.h#255

其中涉及到的 ​

​NR_WMARK​

​​ 值为 , 定义在如下枚举数据结构中 ,

enum zone_watermarks {
  WMARK_MIN,
  WMARK_LOW,
  WMARK_HIGH,
  NR_WMARK
};

#define min_wmark_pages(z) (z->watermark[WMARK_MIN])
#define low_wmark_pages(z) (z->watermark[WMARK_LOW])
#define high_wmark_pages(z) (z->watermark[WMARK_HIGH])      

源码路径 : linux-4.12\include\linux\mmzone.h#255

其中 ​

​min_wmark_pages(z)​

​​ 宏定义对应的 ​

​z->watermark[WMARK_MIN]​

​​ 需要通过 ​

​min_free_kbytes​

​ 值进行计算 ,

​

​min_free_kbytes​

​ 表示 " 系统保留的最低空闲内存数 " ,

二、min_free_kbytes 初始化过程

​

​min_free_kbytes​

​​ 值在 ​

​init_per_zone_wmark_min​

​ 函数中进行初始化 ,

​

​lowmem_kbytes ​

​ 用于统计 低内存 中超过 高水位 物理页的个数 ;

​

​lowmem_kbytes ​

​ 是 高水位线 之上的内存 ;

内存区域 的 整体 物理页个数 是 zone 结构体中的 ​

​managed_pages ​

​ 成员 ;

下图是整个内存区域的图 ;

lowmem_kbytes = nr_free_buffer_pages() * (PAGE_SIZE >> 10);
  new_min_free_kbytes = int_sqrt(lowmem_kbytes * 16);      

​

​min_free_kbytes ​

​​ 值的范围是 ~ ;

if (new_min_free_kbytes > user_min_free_kbytes) {
    min_free_kbytes = new_min_free_kbytes;
    if (min_free_kbytes < 128)
      min_free_kbytes = 128;
    if (min_free_kbytes > 65536)
      min_free_kbytes = 65536;
  }      

​

​init_per_zone_wmark_min​

​ 函数源码如下 :

/*
 * Initialise min_free_kbytes.
 *
 * For small machines we want it small (128k min).  For large machines
 * we want it large (64MB max).  But it is not linear, because network
 * bandwidth does not increase linearly with machine size.  We use
 *
 *  min_free_kbytes = 4 * sqrt(lowmem_kbytes), for better accuracy:
 *  min_free_kbytes = sqrt(lowmem_kbytes * 16)
 *
 * which yields
 *
 * 16MB:  512k
 * 32MB:  724k
 * 64MB:  1024k
 * 128MB: 1448k
 * 256MB: 2048k
 * 512MB: 2896k
 * 1024MB:  4096k
 * 2048MB:  5792k
 * 4096MB:  8192k
 * 8192MB:  11584k
 * 16384MB: 16384k
 */
int __meminit init_per_zone_wmark_min(void)
{
  unsigned long lowmem_kbytes;
  int new_min_free_kbytes;

  lowmem_kbytes = nr_free_buffer_pages() * (PAGE_SIZE >> 10);
  new_min_free_kbytes = int_sqrt(lowmem_kbytes * 16);

  if (new_min_free_kbytes > user_min_free_kbytes) {
    min_free_kbytes = new_min_free_kbytes;
    if (min_free_kbytes < 128)
      min_free_kbytes = 128;
    if (min_free_kbytes > 65536)
      min_free_kbytes = 65536;
  } else {
    pr_warn("min_free_kbytes is not updated to %d because user defined value %d is preferred\n",
        new_min_free_kbytes, user_min_free_kbytes);
  }
  setup_per_zone_wmarks();
  refresh_zone_stat_thresholds();
  setup_per_zone_lowmem_reserve();

#ifdef CONFIG_NUMA
  setup_min_unmapped_ratio();
  setup_min_slab_ratio();
#endif

  return 0;
}      

源码路径 : linux-4.12\mm\page_alloc.c#6976

三、最低水位计算过程

先调用 linux-4.12\mm\page_alloc.c#4331 位置的 ​

​nr_free_buffer_pages​

​ 函数 ,

/**
 * nr_free_buffer_pages - count number of pages beyond high watermark
 *
 * nr_free_buffer_pages() counts the number of pages which are beyond the high
 * watermark within ZONE_DMA and ZONE_NORMAL.
 */
unsigned long nr_free_buffer_pages(void)      

在该函数中调用了 linux-4.12\mm\page_alloc.c#4305 位置的 ​

​nr_free_zone_pages​

​ 函数 ;

/**
 * nr_free_zone_pages - count number of pages beyond high watermark
 * @offset: The zone index of the highest zone
 *
 * nr_free_zone_pages() counts the number of counts pages which are beyond the
 * high watermark within all zones at or below a given zone index.  For each
 * zone, the number of pages is calculated as:
 *
 *     nr_free_zone_pages = managed_pages - high_pages
 */
static unsigned long nr_free_zone_pages(int offset)      

四、nr_free_buffer_pages 函数原型

/**
 * nr_free_buffer_pages - count number of pages beyond high watermark
 *
 * nr_free_buffer_pages() counts the number of pages which are beyond the high
 * watermark within ZONE_DMA and ZONE_NORMAL.
 */
unsigned long nr_free_buffer_pages(void)
{
  return nr_free_zone_pages(gfp_zone(GFP_USER));
}      

源码路径 : linux-4.12\mm\page_alloc.c#4331

五、nr_free_zone_pages 函数原型

/**
 * nr_free_zone_pages - count number of pages beyond high watermark
 * @offset: The zone index of the highest zone
 *
 * nr_free_zone_pages() counts the number of counts pages which are beyond the
 * high watermark within all zones at or below a given zone index.  For each
 * zone, the number of pages is calculated as:
 *
 *     nr_free_zone_pages = managed_pages - high_pages
 */
static unsigned long nr_free_zone_pages(int offset)
{
  struct zoneref *z;
  struct zone *zone;

  /* Just pick one node, since fallback list is circular */
  unsigned long sum = 0;

  struct zonelist *zonelist = node_zonelist(numa_node_id(), GFP_KERNEL);

  for_each_zone_zonelist(zone, z, zonelist, offset) {
    unsigned long size = zone->managed_pages;
    unsigned long high = high_wmark_pages(zone);
    if (size > high)
      sum += size - high;
  }

  return sum;
}