about overcommit_memory

Since 2.1.27 there are a sysctl 

VM_OVERCOMMIT_MEMORY

 and
proc file 

/proc/sys/vm/overcommit_memory

 with
values 1: do overcommit, and 0 (default): don't. Unfortunately, this does not allow you to tell the kernel to be more careful, it only allows you to tell the kernel to be less careful. With 

overcommit_memory

 set
to 1 every 

malloc()

 will
succeed. When set to 0 the old heuristics are used, the kernel still overcommits.

Going in the right direction

Since 2.5.30 the values are: 0 (default): as before: guess about how much overcommitment is reasonable, 1: never refuse any 

malloc()

,
2: be precise about the overcommit - never commit a virtual address space larger than swap space plus a fraction 

overcommit_ratio

 of
the physical memory. Here 

/proc/sys/vm/overcommit_ratio

 (by
default 50) is another user-settable parameter. It is possible to set 

overcommit_ratio

to
values larger than 100. (See also 

Documentation/vm/overcommit-accounting

.)

After

1	# echo 2 > /proc/sys/vm/overcommit_memory

all three demo programs were able to obtain 498 MiB on this 2.6.8.1 machine (256 MiB, 539 MiB swap, lots of other active processes), very satisfactory.

 

However, without swap, no more processes could be started - already more than half of the memory was committed. After

1	# echo 80 > /proc/sys/vm/overcommit_ratio

all three demo programs were able to obtain 34 MiB. (Exercise: solve two equations with two unknowns and conclude that main memory was 250 MiB, and the other processes took 166 MiB.)

 

One can view the currently committed amount of memory in 

/proc/meminfo

,
in the field 

Committed_AS

.