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_ratioto 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.