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

Posted 2021-01-23 数据库技术和故事

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

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

https://access.redhat.com/solutions/406773

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

• For definition of /proc/meminfo fields in Red Hat Enterprise Linux (RHEL) releases prior to RHEL 5, please look at What is indicated by each value in /proc/meminfo?
• Each field of cat /proc/meminfo will be discussed in the Diagnostics Steps.
• The RHEL 5 output differs in some settings. This is also marked in the Diagnostic Steps.
• RHEL 5 also has some fields no longer present in RHEL 6. For explanation on this issue have a look at Why are LowTotal, LowFree, HighTotal, and HighFree missing from /proc/meminfo on x86_64 RHEL 6?
• For more information on the output of the free command see How do I view system memory utilization in Red Hat Enterprise Linux?
• RHEL 7 has an additional field called MemAvailable in /proc/meminfo
• RHEL 7 has a slightly changed output of the free command

Comparing the output

• free -k output (RHEL 5 and RHEL 6):

Raw

             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):

Raw

              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:

Raw

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:

Raw

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)