/proc/zoneinfo
zoneinfo能够看到看到内存使用的细节。 输出信息太多，这里只关注内存水位(watermark)。
[root@host143 ~]# cat /proc/zoneinfo
Node 0, zone DMA
pages free 3960
min 16
low 20
high 24
scanned 0
spanned 4095
present 3997
managed 3976
nr_free_pages 3960
nr_alloc_batch 4
nr_inactive_anon 0
nr_active_anon 0
nr_inactive_file 0
nr_active_file 0
nr_unevictable 0
nr_mlock 0
nr_anon_pages 0
nr_mapped 0
nr_file_pages 0
nr_dirty 0
nr_writeback 0
nr_slab_reclaimable 0
nr_slab_unreclaimable 16
nr_page_table_pages 0
nr_kernel_stack 0
nr_unstable 0
nr_bounce 0
nr_vmscan_write 0
nr_vmscan_immediate_reclaim 0
nr_writeback_temp 0
nr_isolated_anon 0
nr_isolated_file 0
nr_shmem 0
nr_dirtied 0
nr_written 0
numa_hit 2
numa_miss 0
numa_foreign 0
numa_interleave 0
numa_local 2
numa_other 0
workingset_refault 0
workingset_activate 0
workingset_nodereclaim 0
nr_anon_transparent_hugepages 0
nr_free_cma 0
# 和 lowmem_reserve_ratio 有关
protection: (0, 2830, 15850, 15850)
pagesets
cpu: 0
count: 0
high: 0
batch: 1
vm stats threshold: 6
cpu: 1
count: 0
high: 0
batch: 1
vm stats threshold: 6
cpu: 2
count: 0
high: 0
batch: 1
vm stats threshold: 6
cpu: 3
count: 0
high: 0
batch: 1
vm stats threshold: 6
all_unreclaimable: 0
start_pfn: 1
inactive_ratio: 1
linux内存使用水位方式控制：
low：当剩余内存慢慢减少，触到这个水位时，就会触发kswapd线程的内存回收。 min：如果剩余内存减少到触及这个水位，可认为内存严重不足，当前进程就会被堵住，kernel会直接在这个进程的进程上下文里面做内存回收（direct reclaim）。 high: 进行内存回收时，内存慢慢增加，触到这个水位时，就停止回收。
min下的内存是保留给内核使用的；当到达min，会触发内存的direct reclaim。
可以看到kswapd线程的工作区间是min和low之间。低于low启动，低于min触发direct reclaim。
每个ZONE都有这三个水位。
如果lowmem被使用殆尽，触及low或min水位，内核的普通kmalloc就申请不到内存了，就会触发cache/buffers的回收和匿名页swap，再不行就OOM了。
lowmen_reserve
So the Linux page allocator has a mechanism which prevents allocations which could use highmem from using too much lowmem. This means that a certain amount of lowmem is defended from the possibility of being captured into pinned user memory. The `lowmem_reserve_ratio’ tunable determines how aggressive the kernel is in defending these lower zones.
lowmem_reserve是给更高位的zones预留的内存，作用是防止高端zone在没内存的情况下过度使用低端zone的内存资源。。
[root@host143 ~]# cat /proc/sys/vm/lowmem_reserve_ratio
256 256 32
zone[i]’s protection[j] is calculated by following expression.
(i < j):
zone[i]->protection[j]
= (total sums of managed_pages from zone[i+1] to zone[j] on the node)
/ lowmem_reserve_ratio[i];
(i = j):
(should not be protected. = 0;
(i > j):
(not necessary, but looks 0)
The default values of lowmem_reserve_ratio[i] are 256 (if zone[i] means DMA or DMA32 zone) 32 (others). 预留内存值是ratio的倒数关系。
/proc/sys/vm/min_free_kbytes
vm.min_free_kbytes可以调节watermark[min]。
内核对min_free_kbytes的解释如下：
This is used to force the Linux VM to keep a minimum number of kilobytes free. The VM uses this number to compute a watermark[WMARK_MIN] value for each lowmem zone in the system. Each lowmem zone gets a number of reserved free pages based proportionally on its size. Some minimal amount of memory is needed to satisfy PF_MEMALLOC allocations; if you set this to lower than 1024KB, your system will become subtly broken, and prone to deadlock under high loads. Setting this too high will OOM your machine instantly.
划重点：不要随便调大min_free_kbytes，容易导致OOM。
初始化条件，总的"min"值约等于所有zones可用内存的总和乘以16再开平方的大小，同时设有min和max的阈值。
int __meminit init_per_zone_wmark_min(void)
{
min_free_kbytes = int_sqrt(lowmem_kbytes * 16);
if (min_free_kbytes < 128)
min_free_kbytes = 128;
if (min_free_kbytes > 65536)
min_free_kbytes = 65536;
...
}
/proc/sys/vm/swapness
TODO