When vfork is called and the current process has other running child threads,
for Linux, the other threads remain running. For Occlum, this behavior is
different. All the other threads will be frozen until the vfork returns
or execve is called in the child process.
The reason is that since Occlum doesn't support fork, many applications will
use vfork to replace fork. For multi-threaded applications, if vfork doesn't
stop other child threads, the application will be more likely to fail because
the child process directly uses the VM and the file table of the parent process.
Occlum is a single-address-space library OS. Previously, userspace memory are divided for each process.
And all the memory are allocated when the process is created, which leads to a lot of wasted space and
complicated configuration.
In the current implementation, the whole userspace is managed as a memory pool that consists of chunks. There
are two kinds of chunks:
(1) Single VMA chunk: a chunk with only one VMA. Should be owned by exactly one process.
(2) Multi VMA chunk: a chunk with default chunk size and there could be a lot of VMAs in this chunk. Can be used
by different processes.
This design can help to achieve mainly two goals:
(1) Simplify the configuration: Users don't need to configure the process.default_mmap_size anymore. And multiple processes
running in the same Occlum instance can use dramatically different sizes of memory.
(2) Gain better performance: Two-level management(chunks & VMAs) reduces the time for finding, inserting, deleting, and iterating.
