老版的free命令长成这个样子：
较新版本的Ubuntun、CentOS等中的free命令是这个样子的：
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：
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：
