[LordGrey]

For anyone not familiar with the meaning of '2' in this context:

The Linux kernel supports the following overcommit handling modes

0 - Heuristic overcommit handling. Obvious overcommits of address space are refused. Used for a typical system. It ensures a seriously wild allocation fails while allowing overcommit to reduce swap usage. root is allowed to allocate slightly more memory in this mode. This is the default.

1 - Always overcommit. Appropriate for some scientific applications. Classic example is code using sparse arrays and just relying on the virtual memory consisting almost entirely of zero pages.

2 - Don't overcommit. The total address space commit for the system is not permitted to exceed swap + a configurable amount (default is 50%) of physical RAM. Depending on the amount you use, in most situations this means a process will not be killed while accessing pages but will receive errors on memory allocation as appropriate. Useful for applications that want to guarantee their memory allocations will be available in the future without having to initialize every page.

[dbdr]

> exceed swap + a configurable amount (default is 50%) of physical RAM

Naive question: why is this default 50%, and more generally why is this not the entire RAM, what happens to the rest?

[godelski]

There's a lot of options. If you want to go down the rabbithole try typing `sysctl -a | grep -E "^vm"` and that'll give you a lot of things to google ;)

[vin10]

it's a (then-)safe default from the age when having 1GB of RAM and 2GB of swap was the norm: https://linux-kernel.vger.kernel.narkive.com/U64kKQbW/should...

[dasil003]

Not sure if I understand your question but nothing "happens to the rest", overcommitting just means processes can allocate memory in excess of RAM + swap. The percentage is arbitrary, could be 50%, 100% or 1000%. Allocating additional memory is not a problem per se, it only becomes a problem when you try to actually write (and subsequently read) more than you have.

[crote]

Just a guess, but I reckon it doesn't account for things like kernel memory usage, such as caches and buffers. Assigning 100% of physical RAM to applications is probably going to have a Really Bad Outcome.

[throw0101c]

> For anyone not familiar with the meaning of '2' in this context:

Source:

* https://www.kernel.org/doc/Documentation/vm/overcommit-accou...

[sidewndr46]

Do any of the settings actually result in "malloc" or a similar function returning NULL?

[LordGrey]

malloc() and friends may always return NULL. From the man page:

If successful, calloc(), malloc(), realloc(), reallocf(), valloc(), and aligned_alloc() functions return a pointer to allocated memory. If there is an error, they return a NULL pointer and set errno to ENOMEM.

In practice, I find a lot of code that does not check for NULL, which is rather distressing.

[themafia]

malloc() is an interface. There are many implementations.

[fulafel]

Yes.