meminfo 详解

1、meminfo统计信息原文  "cat /proc/meminfo":

MemTotal:  1031016 kB

MemFree:  13548 kB

MemShared:  0 kB

Buffers:  98064 kB

Cached:   692320 kB

SwapCached:  2244 kB

Active:   563112 kB

Inact_dirty:  309584 kB

Inact_clean:  79508 kB

Inact_target:  190440 kB

HighTotal:  130992 kB

HighFree:  1876 kB

LowTotal:  900024 kB

LowFree:  11672 kB

SwapTotal:  1052248 kB

SwapFree:  1043908 kB

Committed_AS:  332340 kB

The information comes in the form of both high-level and low-level statistics. At the top you see a quick summary of the most common values people would like to look at. Below you find the individual values we will discuss. First we will discuss the high-level statistics.

High-Level Statistics

MemTotal: Total usable ram (i.e. physical ram minus a few reserved bits and the kernel binary code)

MemFree: Is sum of LowFree+HighFree (overall stat)

MemShared: 0; is here for compat reasons but always zero.

Buffers: Memory in buffer cache. mostly useless as metric nowadays

Cached: Memory in the pagecache (diskcache) minus SwapCache

SwapCache: Memory that once was swapped out, is swapped back in but still also is in the swapfile (if memory is needed it doesn't need to be swapped out AGAIN because it is already in the swapfile. This saves I/O)

Detailed Level Statistics

VM Statistics

VM splits the cache pages into "active" and "inactive" memory. The idea is that if you need memory and some cache needs to be sacrificed for that, you take it from inactive since that's expected to be not used. The vm checks what is used on a regular basis and moves stuff around.

When you use memory, the CPU sets a bit in the pagetable and the VM checks that bit occasionally, and based on that, it can move pages back to active. And within active there's an order of "longest ago not used" (roughly, it's a little more complex in reality). The longest-ago used ones can get moved to inactive. Inactive is split into two in the above kernel (2.4.18-24.8.0). Some have it three.

Active: Memory that has been used more recently and usually not reclaimed unless absolutely necessary.

Inact_dirty: Dirty means "might need writing to disk or swap." Takes more work to free. Examples might be files that have not been written to yet. They aren't written to memory too soon in order to keep the I/O down. For instance, if you're writing logs, it might be better to wait until you have a complete log ready before sending it to disk.

Inact_clean: Assumed to be easily freeable. The kernel will try to keep some clean stuff around always to have a bit of breathing room.

Inact_target: Just a goal metric the kernel uses for making sure there are enough inactive pages around. When exceeded, the kernel will not do work to move pages from active to inactive. A page can also get inactive in a few other ways, e.g. if you do a long sequential I/O, the kernel assumes you're not going to use that memory and makes it inactive preventively. So you can get more inactive pages than the target because the kernel marks some cache as "more likely to be never used" and lets it cheat in the "last used" order.

Memory Statistics

HighTotal: is the total amount of memory in the high region. Highmem is all memory above (approx) 860MB of physical RAM. Kernel uses indirect tricks to access the high memory region. Data cache can go in this memory region.

LowTotal: The total amount of non-highmem memory.

LowFree: The amount of free memory of the low memory region. This is the memory the kernel can address directly. All kernel datastructures need to go into low memory.

SwapTotal: Total amount of physical swap memory.

SwapFree: Total amount of swap memory free.

Committed_AS: An estimate of how much RAM you would need to make a 99.99% guarantee that there never is OOM (out of memory) for this workload. Normally the kernel will overcommit memory. That means, say you do a 1GB malloc, nothing happens, really. Only when you start USING that malloc memory you will get real memory on demand, and just as much as you use. So you sort of take a mortgage and hope the bank doesn't go bust. Other cases might include when you mmap a file that's shared only when you write to it and you get a private copy of that data. While it normally is shared between processes. The Committed_AS is a guesstimate of how much RAM/swap you would need worst-case.

2、meminfo统计信息译文  "cat /proc/meminfo":

MemTotal: 507480 kB

MemFree: 10800 kB

Buffers: 34728 kB

Cached: 98852 kB

SwapCached: 128 kB

Active: 304248 kB

Inactive: 46192 kB

HighTotal: 0 kB

HighFree: 0 kB

LowTotal: 507480 kB

LowFree: 10800 kB

SwapTotal: 979956 kB

SwapFree: 941296 kB

Dirty: 32 kB

Writeback: 0 kB

AnonPages: 216756 kB

Mapped: 77560 kB

Slab: 22952 kB

SReclaimable: 15512 kB

SUnreclaim: 7440 kB

PageTables: 2640 kB

NFS_Unstable: 0 kB

Bounce: 0 kB

CommitLimit: 1233696 kB

Committed_AS: 828508 kB

VmallocTotal: 516088 kB

VmallocUsed: 5032 kB

VmallocChunk: 510580 kB

MemTotal: 所有可用RAM大小 （即物理内存减去一些预留位和内核的二进制代码大小）

MemFree: LowFree与HighFree的总和，被系统留着未使用的内存

Buffers: 用来给文件做缓冲大小

Cached: 被高速缓冲存储器（cache memory）用的内存的大小（等于 diskcache minus SwapCache ）.

SwapCached:被高速缓冲存储器（cache memory）用的交换空间的大小已经

被交换出来的内存，但仍然被存放在swapfile中。用来在需要的时候很快的

被替换而不需要再次打开I/O端口。

Active: 在活跃使用中的缓冲或高速缓冲存储器页面文件的大小，除非非常必要否则不会被移作他用.

Inactive: 在不经常使用中的缓冲或高速缓冲存储器页面文件的大小，可能被用于其他途径.

HighTotal:

HighFree: 该区域不是直接映射到内核空间。内核必须使用不同的手法使用该段内存。

LowTotal:

LowFree: 低位可以达到高位内存一样的作用，而且它还能够被内核用来记录

一些自己的数据结构。Among many other things, it is where

everything from the Slab is allocated.  Bad things happen

when you're out of lowmem.

SwapTotal: 交换空间的总大小

SwapFree: 未被使用交换空间的大小

Dirty: 等待被写回到磁盘的内存大小。

Writeback: 正在被写回到磁盘的内存大小。

AnonPages：未映射页的内存大小

Mapped: 设备和文件等映射的大小。

Slab: 内核数据结构缓存的大小，可以减少申请和释放内存带来的消耗。

SReclaimable:可收回Slab的大小

SUnreclaim：不可收回Slab的大小（SUnreclaim+SReclaimable＝Slab）

PageTables：管理内存分页页面的索引表的大小。

NFS_Unstable:不稳定页表的大小

Bounce:

CommitLimit: Based on the overcommit ratio('vm.overcommit_ratio'),

this is the total amount of  memory currently available to

be allocated on the system. This limit is only adhered to

if strict overcommit accounting is enabled (mode 2 in

'vm.overcommit_memory').

The CommitLimit is calculated with the following formula:

CommitLimit = ('vm.overcommit_ratio' * Physical RAM) + Swap

For example, on a system with 1G of physical RAM and 7G

of swap with a `vm.overcommit_ratio` of 30 it would

yield a CommitLimit of 7.3G.

For more details, see the memory overcommit documentation

in vm/overcommit-accounting.

Committed_AS: The amount of memory presently allocated on

the system.

The committed memory is a sum of all of the memory which

has been allocated by processes, even if it has not been

"used" by them as of yet. A process which malloc()'s 1G

of memory, but only touches 300M of it will only show up

as using 300M of memory even if it has the address space

allocated for the entire 1G. This 1G is memory which has

been "committed" to by the VM and can be used at any time

by the allocating application. With strict overcommit

enabled on the system (mode 2 in 'vm.overcommit_memory'),

allocations which would exceed the CommitLimit (detailed

above) will not be permitted. This is useful if one needs

to guarantee that processes will not fail due to lack of

memory once that memory has been successfully allocated.

VmallocTotal: 可以vmalloc虚拟内存大小

VmallocUsed: 已经被使用的虚拟内存大小。

VmallocChunk: largest contigious block of vmalloc area which is free

下面简单来个例子，看看已用内存和物理内存大小..

#include

#include

#include

int MemInfo(char* Info, int len);

int main()

{

char buf[128];

memset(buf, 0, 128);

MemInfo(buf, 100);

printf("%s", buf);

return 0;

}

int MemInfo(char* Info, int len)

{

char sStatBuf[256];

FILE* fp;

int flag;

int TotalMem;

int UsedMem;

char* line;

if(system("free -m | awk '{print ,}' > mem"));

memset(sStatBuf, 0, 256);

fp = fopen("mem", "rb");

if(fp < 0)

{

return -1;

}

fread(sStatBuf,1, sizeof(sStatBuf) , fp);

line = strstr(sStatBuf, "n");

TotalMem = atoi(line);

line = strstr(line, " ");

UsedMem = atoi(line);

memset(sStatBuf, 0, 256);

sprintf(sStatBuf, "Used %dM/Total %dMn", UsedMem, TotalMem);

if(strlen(sStatBuf) > len)

{

return -1;

}

memcpy(Info, sStatBuf, strlen(sStatBuf));

return 0;

}

结果：Used 488M/Total 495M

3、meminfo 代码解析

那么内核怎么把meminfo信息动态反应到meminfo文件中呢

在内核 linux/fs/proc/proc_misc.c中

static int meminfo_read_proc(char *page, char **start, off_t off,

int count, int *eof, void *data)

{

struct sysinfo i;

int len;

unsigned long committed;

unsigned long allowed;

struct vmalloc_info vmi;

long cached;

#define K(x) ((x) << (PAGE_SHIFT - 10))

/**

*该宏作用把存储单位传换成 kb

*/

si_meminfo(&i);

si_swapinfo(&i);

/**

*这两个函数是对struct sysinfo结构进行初始化的

*/

committed = atomic_read(&vm_committed_space);

allowed = ((totalram_pages - hugetlb_total_pages())

* sysctl_overcommit_ratio / 100) + total_swap_pages;

/**

*其中这项根据上篇文章CommitLimit解释计算的

*/

cached = global_page_state(NR_FILE_PAGES) -

total_swapcache_pages - i.bufferram;

if (cached < 0)

cached = 0;

get_vmalloc_info(&vmi);

/*

* Tagged format, for easy grepping and expansion.

*/

len = sprintf(page,

"MemTotal: %8lu kBn"

"MemFree: %8lu kBn"

"Buffers: %8lu kBn"

"Cached: %8lu kBn"

"SwapCached: %8lu kBn"

"Active: %8lu kBn"

"Inactive: %8lu kBn"

#ifdef CONFIG_HIGHMEM

"HighTotal: %8lu kBn"

"HighFree: %8lu kBn"

"LowTotal: %8lu kBn"

"LowFree: %8lu kBn"

#endif

"SwapTotal: %8lu kBn"

"SwapFree: %8lu kBn"

"Dirty:     %8lu kBn"

"Writeback: %8lu kBn"

"AnonPages: %8lu kBn"

"Mapped: %8lu kBn"

"Slab:      %8lu kBn"

"SReclaimable: %8lu kBn"

"SUnreclaim: %8lu kBn"

"PageTables: %8lu kBn"

"NFS_Unstable: %8lu kBn"

"Bounce: %8lu kBn"

"CommitLimit: %8lu kBn"

"Committed_AS: %8lu kBn"

"VmallocTotal: %8lu kBn"

"VmallocUsed: %8lu kBn"

"VmallocChunk: %8lu kBn",

K(i.totalram),

K(i.freeram),

K(i.bufferram),

K(cached),

K(total_swapcache_pages),

K(global_page_state(NR_ACTIVE)),

K(global_page_state(NR_INACTIVE)),

#ifdef CONFIG_HIGHMEM

K(i.totalhigh),

K(i.freehigh),

K(i.totalram-i.totalhigh),

K(i.freeram-i.freehigh),

#endif

K(i.totalswap),

K(i.freeswap),

K(global_page_state(NR_FILE_DIRTY)),

K(global_page_state(NR_WRITEBACK)),

K(global_page_state(NR_ANON_PAGES)),

K(global_page_state(NR_FILE_MAPPED)),

K(global_page_state(NR_SLAB_RECLAIMABLE) +

global_page_state(NR_SLAB_UNRECLAIMABLE)),

K(global_page_state(NR_SLAB_RECLAIMABLE)),

K(global_page_state(NR_SLAB_UNRECLAIMABLE)),

K(global_page_state(NR_PAGETABLE)),

K(global_page_state(NR_UNSTABLE_NFS)),

K(global_page_state(NR_BOUNCE)),

K(allowed),

K(committed),

(unsigned long)VMALLOC_TOTAL >> 10,

vmi.used >> 10,

vmi.largest_chunk >> 10

);

len += hugetlb_report_meminfo(page + len);

return proc_calc_metrics(page, start, off, count, eof, len);

#undef K

}

其中sysinfo结构在 linux/kernel.h  定义：

struct sysinfo {

long uptime; /* 启动到现在经过的时间 */

unsigned long loads[3];

/* 1, 5, and 15 minute load averages */

unsigned long totalram; /* 总的可用的内存大小 */

unsigned long freeram; /* 还未被使用的内存大小 */

unsigned long sharedram; /* 共享的存储器的大小*/

unsigned long bufferram; /* 的存储器的大小 */

unsigned long totalswap; /* 交换区大小 */

unsigned long freeswap; /* 还可用的交换区大小 */

unsigned short procs; /* 当前进程数目 */

unsigned short pad; /* explicit padding for m68k */

unsigned long totalhigh; /* 总的高内存大小 */

unsigned long freehigh; /* 可用的高内存大小 */

unsigned int mem_unit; /* 以字节为单位的内存大小 */

char _f[20-2*sizeof(long)-sizeof(int)];

};

而global_page_state()函数中的常量定义在 linux/mmzone.h

enum zone_stat_item {

/* First 128 byte cacheline (assuming 64 bit words) */

NR_FREE_PAGES,

NR_INACTIVE,

NR_ACTIVE,

NR_ANON_PAGES, /* Mapped anonymous pages */

NR_FILE_MAPPED, /* pagecache pages mapped into pagetables.

only modified from process context */

NR_FILE_PAGES,

NR_FILE_DIRTY,

NR_WRITEBACK,

/* Second 128 byte cacheline */

NR_SLAB_RECLAIMABLE,

NR_SLAB_UNRECLAIMABLE,

NR_PAGETABLE,      /* used for pagetables */

NR_UNSTABLE_NFS,     /* NFS unstable pages */

NR_BOUNCE,

NR_VMSCAN_WRITE,

#ifdef CONFIG_NUMA

NUMA_HIT,      /* allocated in intended node */

NUMA_MISS,      /* allocated in non intended node */

NUMA_FOREIGN,      /* was intended here, hit elsewhere */

NUMA_INTERLEAVE_HIT, /* interleaver preferred this zone */

NUMA_LOCAL,      /* allocation from local node */

NUMA_OTHER,      /* allocation from other node */

#endif

NR_VM_ZONE_STAT_ITEMS

};

其中通过global_page_state()函数根据 zone_stat_item 结构的常量得到不同区大小，会跟 vm_stat[NR_VM_ZONE_STAT_ITEMS]对应起来。

vm_stat[]是统计各区的大小。

内核 linux/vmstat.h定义：

static inline unsigned long global_page_state(enum zone_stat_item item)

{

long x = atomic_long_read(&vm_stat[item]);

#ifdef CONFIG_SMP

if (x < 0)

x = 0;

#endif

return x;

}

下面根据struct sysinfo结构，简单分析CPU和内存使用信息。

#include

#include /* 包含调用 _syscallX 宏等相关信息*/

#include /* 包含sysinfo结构体信息*/

int main(int argc, char *agrv[])

{

struct sysinfo s_info;

int error;

error = sysinfo(&s_info);

printf("nncode error=%dn",error);

printf("Uptime = %dsnLoad: 1 min%d / 5 min %d / 15 min %dn"

"RAM: total %d / free %d /shared%dn"

"Memory in buffers = %dnSwap:total%d/free%dn"

"Number of processes = %dn",

s_info.uptime, s_info.loads[0],

s_info.loads[1], s_info.loads[2],

s_info.totalram, s_info.freeram,

s_info.totalswap, s_info.freeswap,

s_info.procs);

return 0;

}

结果：

code error=0

Uptime = 8329s

Load: 1 min37152 / 5 min 37792 / 15 min 48672

RAM: total 519659520 / free 9031680 /shared1003474944

Memory in buffers = 937451520

Swap:total223/free-1078732672

Number of processes = -1078732608

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