Interpreting /proc/meminfo and free output for Red Hat Enterprise Linux 5, 6 and 7

Solution Verified - UpdatedOctober 2 2018 at 6:35 AM -

Environment

  • Red Hat Enterprise Linux (RHEL) 5
  • Red Hat Enterprise Linux (RHEL) 6
  • Red Hat Enterprise Linux (RHEL) 7

Issue

  • I need an interpretation of /proc/meminfo output.
  • I want to compare the output of free -k to cat /proc/meminfo.

Resolution

Comparing the output

  • free -k output (RHEL 5 and RHEL 6):
             total       used       free     shared    buffers     cached
Mem: 7778104 2971960 4806144 0 211756 1071092
-/+ buffers/cache: 1689112 6088992
Swap: 4194296 0 4194296
  • free -k output (RHEL 7):
              total        used        free      shared  buff/cache   available
Mem: 1012952 252740 158732 11108 601480 543584
Swap: 1048572 5380 1043192
  • Relevant fields from /proc/meminfo to match them against the output of free -k:
MemTotal:        7778104 kB
MemFree: 4806144 kB
Buffers: 211756 kB
Cached: 1071092 kB
SwapTotal: 4194296 kB
SwapFree: 4194296 kB
  • For RHEL 7 there is an additional field available, which is used instead of the calculation for -/+ buffers/cache line:
MemAvailable:     543584 kB

Matching output of free -k to /proc/meminfo

The following table shows how to get the free output matched to the /proc/meminfo fields in Red Hat Enterprise Linux 5, 6 and 7.0.

free output coresponding /proc/meminfo fields
Mem: total MemTotal
Mem: used MemTotal - MemFree
Mem: free MemFree
Mem: shared (can be ignored nowadays. It has no meaning.) N/A
Mem: buffers Buffers
Mem: cached Cached
-/+ buffers/cache: used MemTotal - (MemFree + Buffers + Cached)
-/+ buffers/cache: free MemFree + Buffers + Cached
Swap: total SwapTotal
Swap: used SwapTotal - SwapFree
Swap: free SwapFree

The following table shows how to get the free output matched to the /proc/meminfo fields in Red Hat Enterprise Linux 7.1 or later (procps-ng 3.3.10).

free output coresponding /proc/meminfo fields
Mem: total MemTotal
Mem: used MemTotal - MemFree - Buffers - Cached - Slab
Mem: free MemFree
Mem: shared Shmem
Mem: buff/cache Buffers + Cached + Slab
Mem:available MemAvailable
Swap: total SwapTotal
Swap: used SwapTotal - SwapFree
Swap: free SwapFree

Root Cause

  • Analyzing memory consumption

Diagnostic Steps

Most stuff is taken from the kernel documentation (Documentation/filesystems/proc.txt and Documentation/vm/hugetlbpage.txt)

High Level statistics

RHEL 5, RHEL 6 and RHEL 7
  • MemTotal: Total usable memory
  • MemFree: The amount of physical memory not used by the system
  • Buffers: Memory in buffer cache, so relatively temporary storage for raw disk blocks. This shouldn't get very large.
  • Cached: Memory in the pagecache (Diskcache and Shared Memory)
  • SwapCached: Memory that is present within main memory, but also in the swapfile. (If memory is needed this area does not need to be swapped out AGAIN because it is already in the swapfile. This saves I/O and increases performance if machine runs short on memory.)
RHEL 7 only
  • MemAvailable: An estimate of how much memory is available for starting new applications, without swapping.

Detailed Level statistics

RHEL 5, RHEL 6 and RHEL 7
  • Active: Memory that has been used more recently and usually not swapped out or reclaimed
  • Inactive: Memory that has not been used recently and can be swapped out or reclaimed
RHEL 6 and RHEL 7 only
  • Active(anon): Anonymous memory that has been used more recently and usually not swapped out
  • Inactive(anon): Anonymous memory that has not been used recently and can be swapped out
  • Active(file): Pagecache memory that has been used more recently and usually not reclaimed until needed
  • Inactive(file): Pagecache memory that can be reclaimed without huge performance impact
  • Unevictable: Unevictable pages can't be swapped out for a variety of reasons
  • Mlocked: Pages locked to memory using the mlock() system call. Mlocked pages are also Unevictable.

Memory statistics

RHEL 5, RHEL 6 and RHEL 7
  • SwapTotal: Total swap space available
  • SwapFree: The remaining swap space available
  • Dirty: Memory waiting to be written back to disk
  • Writeback: Memory which is actively being written back to disk
  • AnonPages: Non-file backed pages mapped into userspace page tables
  • Mapped: Files which have been mmaped, such as libraries
  • Slab: In-kernel data structures cache
  • PageTables: Amount of memory dedicated to the lowest level of page tables. This can increase to a high value if a lot of processes are attached to the same shared memory segment.
  • NFS_Unstable: NFS pages sent to the server, but not yet commited to the storage
  • Bounce: Memory used for block device bounce buffers
  • 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).
  • 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.
  • VmallocTotal: total size of vmalloc memory area
  • VmallocUsed: amount of vmalloc area which is used
  • VmallocChunk: largest contiguous block of vmalloc area which is free
  • HugePages_Total: Number of hugepages being allocated by the kernel (Defined with vm.nr_hugepages)
  • HugePages_Free: The number of hugepages not being allocated by a process
  • HugePages_Rsvd: The number of hugepages for which a commitment to allocate from the pool has been made, but no allocation has yet been made.
  • Hugepagesize: The size of a hugepage (usually 2MB on an Intel based system)
RHEL 6 and RHEL 7 only
  • Shmem: Total used shared memory (shared between several processes, thus including RAM disks, SYS-V-IPC and BSD like SHMEM)
  • SReclaimable: The part of the Slab that might be reclaimed (such as caches)
  • SUnreclaim: The part of the Slab that can't be reclaimed under memory pressure
  • KernelStack: The memory the kernel stack uses. This is not reclaimable.
  • WritebackTmp: Memory used by FUSE for temporary writeback buffers
  • HardwareCorrupted: The amount of RAM the kernel identified as corrupted / not working
  • AnonHugePages: Non-file backed huge pages mapped into userspace page tables
  • HugePages_Surp: The number of hugepages in the pool above the value in vm.nr_hugepages. The maximum number of surplus hugepages is controlled by vm.nr_overcommit_hugepages.
  • DirectMap4k: The amount of memory being mapped to standard 4k pages
  • DirectMap2M: The amount of memory being mapped to hugepages (usually 2MB in size)
 
 

A quick rundown on /proc’s files:

  1. /proc/cmdline – Kernel command line information.
  2. /proc/console – Information about current consoles including tty.
  3. /proc/devices – Device drivers currently configured for the running kernel.
  4. /proc/dma – Info about current DMA channels.
  5. /proc/fb – Framebuffer devices.
  6. /proc/filesystems – Current filesystems supported by the kernel.
  7. /proc/iomem – Current system memory map for devices.
  8. /proc/ioports – Registered port regions for input output communication with device.
  9. /proc/loadavg – System load average.
  10. /proc/locks – Files currently locked by kernel.
  11. /proc/meminfo – Info about system memory (see above example).
  12. /proc/misc – Miscellaneous drivers registered for miscellaneous major device.
  13. /proc/modules – Currently loaded kernel modules.
  14. /proc/mounts – List of all mounts in use by system.
  15. /proc/partitions – Detailed info about partitions available to the system.
  16. /proc/pci – Information about every PCI device.
  17. /proc/stat – Record or various statistics kept from last reboot.
  18. /proc/swap – Information about swap space.
  19. /proc/uptime – Uptime information (in seconds).
  20. /proc/version – Kernel version, gcc version, and Linux distribution installed.

Within /proc’s numbered directories you will find a few files and links. Remember that these directories’ numbers correlate to the PID of the command being run within them. Let’s use an example. On my system, there is a folder name /proc/12:

# cd /proc/12
# ls
Sample Output
attr        coredump_filter  io         mounts      oom_score_adj  smaps    wchan
autogroup cpuset latency mountstats pagemap stack
auxv cwd limits net personality stat
cgroup environ loginuid ns root statm
clear_refs exe maps numa_maps sched status
cmdline fd mem oom_adj schedstat syscall
comm fdinfo mountinfo oom_score sessionid task

If I run:

# cat /proc/12/status

I get the following:

Name:	xenwatch
State: S (sleeping)
Tgid: 12
Pid: 12
PPid: 2
TracerPid: 0
Uid: 0 0 0 0
Gid: 0 0 0 0
FDSize: 64
Groups:
Threads: 1
SigQ: 1/4592
SigPnd: 0000000000000000
ShdPnd: 0000000000000000
SigBlk: 0000000000000000
SigIgn: ffffffffffffffff
SigCgt: 0000000000000000
CapInh: 0000000000000000
CapPrm: ffffffffffffffff
CapEff: ffffffffffffffff
CapBnd: ffffffffffffffff
Cpus_allowed: 1
Cpus_allowed_list: 0
Mems_allowed: 00000000,00000001
Mems_allowed_list: 0
voluntary_ctxt_switches: 84
nonvoluntary_ctxt_switches: 0

So, what does this mean? Well, the important part is at the top. We can see from the status file that this process belongs to xenwatch. Its current state is sleeping, and its process ID is 12, obviously. We also can see who is running this, as UID and GID are 0, indicating that this process belongs to the root user.

In any numbered directory, you will have a similar file structure. The most important ones, and their descriptions, are as follows:

  1. cmdline – command line of the process
  2. environ – environmental variables
  3. fd – file descriptors
  4. limits – contains information about the limits of the process
  5. mounts – related information

You will also notice a number of links in the numbered directory:

  1. cwd – a link to the current working directory of the process
  2. exe – link to the executable of the process
  3. root – link to the work directory of the process

This should get you started with familiarizing yourself with the /proc directory. It should also provide insight to how a number of commands obtain their info, such as uptime, lsof, mount, and ps, just to name a few.

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