Interpretation

output free	corresponding fields / proc / meminfo
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

Diagnostic steps

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

High level statistics

RHEL 5, RHEL 6 and RHEL 7/8

• MemTotal: Total used memory

• MemFree: The amount of physical memory not used by the system

• Buffers: Memory in the buffer cache, that is, relatively temporary storage for unprocessed disk blocks. It doesn’t have to be very big.

• Cached: Memory in page cache (pagecache) (disk cache and shared memory).

• SwapCached: Memory that is present inside the main and also in the paging file. (If memory becomes necessary, this area does not need to be swapped out AGAIN because it is already in the paging file. This saves I / O and improves performance if the machine runs out of memory.)

RHEL 7 only

• MemAvailable: An estimate of how much memory is available to run new applications without swapping.

Detailed level statistics

RHEL 5, RHEL 6 and RHEL 7/8

• Active: Memory that has been used recently and is usually not swapped out or freed

• Inactive: Memory that not has been used recently and can be unloaded or released.

For RHEL 6 and RHEL 7/8 only

• Active (anon): Anonymous memory that has been used recently and is not usually 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 recently and is usually not freed until needed.

• Inactive (file): Pagecache memory that can be reclaimed without significantly impacting performance

• Unevictable: Unrecoverable pages cannot be unloaded for a number of reasons.

• Mlocked: Pages locked in memory using a system call mlock()… Blocked pages are also Unrecoverable.

Memory statistics

RHEL 5, RHEL 6 and RHEL 7/8

• SwapTotal: Total available swap space.

• SwapFree: Remaining available swap space.

• Dirty: Memory waiting to be written to disk

• Writeback: Memory that is actively being written to disk

• AnonPages: Non-file supported pages mapped to user-space page tables

• Mapped: Files that have been displayed, such as libraries.

• Slab: Cache of data structures inside the kernel

• PageTables: The amount of memory reserved for the lowest level of the page tables. This parameter can grow to a high value if many processes are attached to the same shared memory segment.

• NFS_Unstable: NFS pages sent to the server but not yet committed to storage.

• Bounce: Memory used for bounce buffers block devices

• CommitLimit: Based on the memory over-allocation ratio (vm.overcommit_ratio), this is the total amount of memory that can currently be allocated on the system. This limit is respected only if strict overcommit accounting is enabled (mode 2 in vm.overcommit_memory).

• Committed_AS: The amount of memory currently allocated on the system. Committed memory is the sum of all memory that has been committed by processes, even if it has not yet been “used” by them.

• VmallocTotal: total size of the memory area vmalloc…

• VmallocUsed: area volume vmallocwhich is used

• VmallocChunk: the largest contiguous block of an area vmallocwhich is free

• HugePages_Total: number of huge pages allocated by the kernel (Determined by vm.nr_hugepages)

• HugePages_Free: The number of huge pages not allocated by the process.

• HugePages_Rsvd: The number of huge pages that have been committed to provisioning from the standby pool but have not yet been provisioned.

• Hugepagesize: The size hugepage (typically 2 MB on an Intel based system).

For RHEL 6 and RHEL 7/8 only

• Shmem: Total used shared memory (shared by multiple processes including RAM disks, SYS-V-IPC, and BSDs like SHMEM)

• SReclaimable: The part of the Slab that can be released (for example, the cache).

• SUnreclaim: the part of the Slab that cannot be released under the pressure of memory.

• KernelStack: Memory used by the kernel stack. It cannot be released.

• WritebackTmp: Memory used by FUSE for temporary write-back buffers.

• HardwareCorrupted: The amount of RAM that the kernel has identified as damaged / non-working.

• AnonHugePages: Non-file huge pages mapped to user space page tables.

• HugePages_Surp: Number of huge pages in the pool exceeding the value in vm.nr_hugepages… The maximum number of extra huge pages is controlled vm.nr_overcommit_hugepages…

• DirectMap4k: The amount of memory used to display standard 4k pages.

• DirectMap2M: amount of memory used for display hugepages (usually 2 MB in size).

• DirectMap1G… Memory used for display hugepages (usually 1 GB in size).

Note for DirectMap1G: This value will be available in /proc/meminfo only if the appropriate processor hardware supports it. You can verify this by checking for the ‘pdpe1gb’ flag in /proc/cpuinfo…

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The material was prepared within the framework of the specialization Administrator Linux.

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