[krallin]

(Note: the following is only applicable to Linux)

The default for overcommitting on Linux is heuristic; it doesn't always succeed: if you try and allocate several exabytes or RAM, allocation will definitely fail (in fact, trying to allocate e.g. 2GB of RAM if you only have 1 free will usually fail just the same).

There is an option for "always overcommit" (incidentally; the one Redis recommends you use), in which case allocation will always succeed provided the Kernel can represent what you're trying to allocate (what you're describing), but it's definitely not the default

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

[ryao]

malloc() should fail even with always over commit enabled if you try to malloc more memory than there is virtual address space, although that needs testing to confirm it. The alternative to failing would be hanging inside the kernel, which would be a DoS attack.

[krallin]

Yes, you're entirely correct; that's actually what I meant by "provided the Kernel can represent [it]" (albeit perhaps not very clearly!).

Cheers,

[ryao]

There is also the case of the sum of the allocations of several malloc operations exceeding that threshold. The case of just 1 malloc operation being that large is just a special case of that. That is what I meant, but reading what Ibwrore made me realize that I was not clear about that.

[spoiler]

Thanks for clarifying that up! :)