From: Frank Hirtz <fhirtz@redhat.com>
I'd like to submit the following patch for consideration. It documents the
newly added "CommitLimit" and "CommitAvail" items in /proc/meminfo as well
as cleaning up the "Committed_AS" description to be in line with the
additions.
Signed-off-by: <fhirtz@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---
25-akpm/Documentation/filesystems/proc.txt | 54 ++++++++++++++++++-------
25-akpm/Documentation/vm/overcommit-accounting | 4 +
2 files changed, 44 insertions(+), 14 deletions(-)
diff -puN Documentation/filesystems/proc.txt~add-documentation-for-new-commitlimit-and-commitavail-meminfo Documentation/filesystems/proc.txt
--- 25/Documentation/filesystems/proc.txt~add-documentation-for-new-commitlimit-and-commitavail-meminfo Wed Oct 6 16:11:56 2004
+++ 25-akpm/Documentation/filesystems/proc.txt Wed Oct 6 16:11:56 2004
@@ -394,7 +394,9 @@ Dirty: 968 kB
Writeback: 0 kB
Mapped: 280372 kB
Slab: 684068 kB
-Committed_AS: 1576424 kB
+CommitLimit: 7669796 kB
+Committed_AS: 100056 kB
+CommitAvail: 7569740 kB
PageTables: 24448 kB
ReverseMaps: 1080904
VmallocTotal: 112216 kB
@@ -434,19 +436,43 @@ VmallocChunk: 111088 kB
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
-Committed_AS: An estimate of how much RAM you would need to make a
- 99.99% guarantee that there never is OOM (out of memory)
- for this workload. Normally the kernel will overcommit
- memory. That means, say you do a 1GB malloc, nothing
- happens, really. Only when you start USING that malloc
- memory you will get real memory on demand, and just as
- much as you use. So you sort of take a mortgage and hope
- the bank doesn't go bust. Other cases might include when
- you mmap a file that's shared only when you write to it
- and you get a private copy of that data. While it normally
- is shared between processes. The Committed_AS is a
- guesstimate of how much RAM/swap you would need
- worst-case.
+ 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').
+ The CommitLimit is calculated with the following formula:
+ CommitLimit = ('vm.overcommit_ratio' * Physical RAM) + Swap
+ 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.
+ CommitAvail: Based on the current overcommit ratio
+ ('vm.overcommit_ratio'), this is the amount of memory
+ currently available to be allocated under the overcommit
+ limit (the CommitLimit above). This is calculated as:
+ CommitAvail = CommitLimit - Committed_AS
+ This limit is only enforced if strict overcommit accounting
+ is enabled (mode 2 in 'vm.overcommit_memory'). CommitAvail
+ may be a negative number if strict accounting is not enabled
+ and the system's memory is currently overcommitted.
+ For more details, see the memory overcommit documentation
+ in vm/overcommit-accounting.
PageTables: amount of memory dedicated to the lowest level of page
tables.
ReverseMaps: number of reverse mappings performed
diff -puN Documentation/vm/overcommit-accounting~add-documentation-for-new-commitlimit-and-commitavail-meminfo Documentation/vm/overcommit-accounting
--- 25/Documentation/vm/overcommit-accounting~add-documentation-for-new-commitlimit-and-commitavail-meminfo Wed Oct 6 16:11:56 2004
+++ 25-akpm/Documentation/vm/overcommit-accounting Wed Oct 6 16:11:56 2004
@@ -22,6 +22,10 @@ The overcommit policy is set via the sys
The overcommit percentage is set via `vm.overcommit_ratio'.
+The current overcommit limit, amount used, and amount remaining below
+the limit are viewable in /proc/meminfo as CommitLimit, Committed_AS, and
+CommitAvail respectively.
+
Gotchas
-------
_