Почему процесс в Linux может получить out-of-memory, даже если память показывает free?

In Linux, a process can encounter an out-of-memory (OOM) error even if the free command shows that there is free memory. This is related to the memory management features of the Linux kernel:

• Memory overcommit: By default, Linux allows processes to request more memory than is physically available, assuming not all allocated pages will be used at once.

• OOM Killer: When the system actually runs out of memory, the kernel invokes the OOM Killer to terminate one or more processes to free up memory.

• Free memory vs. available memory: The free command shows several metrics. Free memory (free) might be small, but there is also cache and buffers that can be freed. However, if processes require more memory than is available considering overcommit, an OOM can occur.

• cgroups or ulimit limits: A process might have a memory limit, and exceeding it can trigger an OOM, even if there is overall memory available in the system.

• Memory fragmentation: Sometimes, memory exists but is fragmented, making it impossible to allocate a large contiguous block.

Thus, having free memory according to free does not guarantee that a process can allocate the needed amount, and an OOM can occur.