Linux free命令前后变化和available内存

时间:2021-12-09 08:57:24


老版的free命令长成这个样子:

Linux free命令前后变化和available内存


较新版本的Ubuntun、CentOS等中的free命令是这个样子的:

Linux free命令前后变化和available内存


主要的变化是buff/cache被合并为一列,并且增加了available这一列


available这一列的具体含义可以参见内核的这个commit:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0ae398fc54ea69ff85ec700722c9da773


天冷手冻,不详细解释和翻译了,具体直接看英文:


Linux free命令前后变化和available内存

涉及到的代码改动如下:

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt

index 22d89aa3..8533f5f 100644

--- a/Documentation/filesystems/proc.txt

+++ b/Documentation/filesystems/proc.txt

@@ -767,6 +767,7 @@ The "Locked" indicates whether the mapping is locked in memory or not.

 

 MemTotal:     16344972 kB

 MemFree:      13634064 kB

+MemAvailable: 14836172 kB

 Buffers:          3656 kB

 Cached:        1195708 kB

 SwapCached:          0 kB

@@ -799,6 +800,14 @@ AnonHugePages:   49152 kB

     MemTotal: Total usable ram (i.e. physical ram minus a few reserved

               bits and the kernel binary code)

      MemFree: The sum of LowFree+HighFree

+MemAvailable: An estimate of how much memory is available for starting new

+              applications, without swapping. Calculated from MemFree,

+              SReclaimable, the size of the file LRU lists, and the low

+              watermarks in each zone.

+              The estimate takes into account that the system needs some

+              page cache to function well, and that not all reclaimable

+              slab will be reclaimable, due to items being in use. The

+              impact of those factors will vary from system to system.

      Buffers: Relatively temporary storage for raw disk blocks

               shouldn't get tremendously large (20MB or so)

       Cached: in-memory cache for files read from the disk (the

diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c

index a77d2b2..24270ec 100644

--- a/fs/proc/meminfo.c

+++ b/fs/proc/meminfo.c

@@ -26,7 +26,11 @@ static int meminfo_proc_show(struct seq_file *m, void *v)

 unsigned long committed;

 struct vmalloc_info vmi;

 long cached;

+long available;

+unsigned long pagecache;

+unsigned long wmark_low = 0;

 unsigned long pages[NR_LRU_LISTS];

+struct zone *zone;

 int lru;

 

 /*

@@ -47,12 +51,44 @@ static int meminfo_proc_show(struct seq_file *m, void *v)

 for (lru = LRU_BASE; lru < NR_LRU_LISTS; lru++)

 pages[lru] = global_page_state(NR_LRU_BASE + lru);

 

+for_each_zone(zone)

+wmark_low += zone->watermark[WMARK_LOW];

+

+/*

+ * Estimate the amount of memory available for userspace allocations,

+ * without causing swapping.

+ *

+ * Free memory cannot be taken below the low watermark, before the

+ * system starts swapping.

+ */

+available = i.freeram - wmark_low;

+

+/*

+ * Not all the page cache can be freed, otherwise the system will

+ * start swapping. Assume at least half of the page cache, or the

+ * low watermark worth of cache, needs to stay.

+ */

+pagecache = pages[LRU_ACTIVE_FILE] + pages[LRU_INACTIVE_FILE];

+pagecache -= min(pagecache / 2, wmark_low);

+available += pagecache;

+

+/*

+ * Part of the reclaimable swap consists of items that are in use,

+ * and cannot be freed. Cap this estimate at the low watermark.

+ */

+available += global_page_state(NR_SLAB_RECLAIMABLE) -

+     min(global_page_state(NR_SLAB_RECLAIMABLE) / 2, wmark_low);

+

+if (available < 0)

+available = 0;

+

 /*

  * Tagged format, for easy grepping and expansion.

  */

 seq_printf(m,

 "MemTotal:       %8lu kB\n"

 "MemFree:        %8lu kB\n"

+"MemAvailable:   %8lu kB\n"

 "Buffers:        %8lu kB\n"

 "Cached:         %8lu kB\n"

 "SwapCached:     %8lu kB\n"

@@ -105,6 +141,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)

 ,

 K(i.totalram),

 K(i.freeram),

+K(available),

 K(i.bufferram),

 K(cached),

 K(total_swapcache_pages()),


另外,关于free命令里面的cached、buffers的区别,下面一幅图可以表达:

Linux free命令前后变化和available内存


Linux free命令前后变化和available内存