cat /proc/meminfo what do all those numbers mean

In reading the man page on the free command in Linux. I found that is gets its info from /proc/meminfo.

I understand a few of the entries, like MemTotal and MemFree. What do the rest mean.

cat /proc/meminfo
MemTotal: 3973736 kB
MemFree: 431064 kB
Buffers: 46604 kB
Cached: 494648 kB
SwapCached: 11360 kB
Active: 2322760 kB
Inactive: 933028 kB
Active(anon): 2057952 kB
Inactive(anon): 679956 kB
Active(file): 264808 kB
Inactive(file): 253072 kB
Unevictable: 16 kB
Mlocked: 16 kB
SwapTotal: 4096568 kB
SwapFree: 3961748 kB
Dirty: 236 kB
Writeback: 0 kB
AnonPages: 2704520 kB
Mapped: 182240 kB
Shmem: 23372 kB
Slab: 93848 kB
SReclaimable: 52044 kB
SUnreclaim: 41804 kB
KernelStack: 5064 kB
PageTables: 64928 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 6083436 kB
Committed_AS: 7327800 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 321156 kB
VmallocChunk: 34359411708 kB
HardwareCorrupted: 0 kB
AnonHugePages: 0 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
DirectMap4k: 225280 kB
DirectMap2M: 3895296 kB

1 Answer

I got the following from the CentOS documentation:

Much of the information here is used by the free, top, and pscommands. In fact, the output of the free command is similar in appearance to the contents and structure of /proc/meminfo. But by looking directly at /proc/meminfo, more details are revealed:

MemTotal — Total amount of physical RAM, in kilobytes.

MemFree — The amount of physical RAM, in kilobytes, left unused by the system.

Buffers — The amount of physical RAM, in kilobytes, used for file buffers.

Cached — The amount of physical RAM, in kilobytes, used as cache memory.

SwapCached — The amount of swap, in kilobytes, used as cache memory.

Active — The total amount of buffer or page cache memory, in kilobytes, that is in active use. This is memory that has been recently used and is usually not reclaimed for other purposes.

Inactive — The total amount of buffer or page cache memory, in kilobytes, that are free and available. This is memory that has not been recently used and can be reclaimed for other purposes.

HighTotal and HighFree — The total and free amount of memory, in kilobytes, that is not directly mapped into kernel space. The HighTotal value can vary based on the type of kernel used.

LowTotal and LowFree — The total and free amount of memory, in kilobytes, that is directly mapped into kernel space. The LowTotal value can vary based on the type of kernel used.

SwapTotal — The total amount of swap available, in kilobytes.

SwapFree — The total amount of swap free, in kilobytes.

Dirty — The total amount of memory, in kilobytes, waiting to be written back to the disk.

Writeback — The total amount of memory, in kilobytes, actively being written back to the disk.

Mapped — The total amount of memory, in kilobytes, which have been used to map devices, files, or libraries using the mmap command.

Slab — The total amount of memory, in kilobytes, used by the kernel to cache data structures for its own use.

Committed_AS — The total amount of memory, in kilobytes, estimated to complete the workload. This value represents the worst case scenario value, and also includes swap memory.

PageTables — The total amount of memory, in kilobytes, dedicated to the lowest page table level.

VMallocTotal — The total amount of memory, in kilobytes, of total allocated virtual address space.

VMallocUsed — The total amount of memory, in kilobytes, of used virtual address space.

VMallocChunk — The largest contiguous block of memory, in kilobytes, of available virtual address space.

HugePages_Total — The total number of hugepages for the system. The number is derived by dividing Hugepagesize by the megabytes set aside for hugepages specified in /proc/sys/vm/hugetlb_pool. This statistic only appears on the x86, Itanium, and AMD64 architectures.

HugePages_Free — The total number of hugepages available for the system. This statistic only appears on the x86, Itanium, and AMD64 architectures.

Hugepagesize — The size for each hugepages unit in kilobytes. By default, the value is 4096 KB on uniprocessor kernels for 32 bit architectures. For SMP, hugemem kernels, and AMD64, the default is 2048 KB. For Itanium architectures, the default is 262144 KB. This statistic only appears on the x86, Itanium, and AMD64 architectures.

These articles give a great explanation of the information in /proc/meminfo: