Thursday, January 10, 2008


/proc/meminfo annotated:

fs/proc/proc_misc.c: meminfo_read_proc()

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

MemFree: The sum of LowFree+HighFree

Buffers: Relatively temporary storage for raw disk blocks
shouldn't get tremendously large (20MB or so)

Cached: in-memory cache for files read from the disk (the
pagecache). Doesn't include SwapCached

SwapCached: 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)

(All page frames are linked in inactive or active list)
Active: Memory that has been used more recently and usually not
reclaimed unless absolutely necessary.
Inactive: Memory which has been less recently used. It is more
eligible to be reclaimed for other purposes

HighFree: Highmem is all memory above ~860MB of physical memory
Highmem areas are for use by userspace programs, or
for the pagecache. The kernel must use tricks to access
this memory, making it slower to access than lowmem.
LowFree: Lowmem is memory which can be used for everything that
highmem can be used for, but it is also available for the
kernel's use for its own data structures. Among many
other things, it is where everything from the Slab is
allocated. Bad things happen when you're out of lowmem.

SwapTotal: total amount of swap space available
SwapFree: Memory which has been evicted from RAM, and is temporarily
on the disk
Dirty: Memory which is waiting to get written back to the disk
Writeback: Memory which is actively being written back to the disk
Mapped: files which have been mmaped, such as libraries
Slab: in-kernel data structures cache
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
The CommitLimit is calculated with the following formula:
CommitLimit = ('vm.overcommit_ratio' * Physical RAM) +
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.
PageTables: amount of memory dedicated to the lowest level of page
VmallocTotal: total size of vmalloc memory area
VmallocUsed: amount of vmalloc area which is used
VmallocChunk: largest contigious block of vmalloc area which is free

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