"A vDSO (virtual dynamic shared object) is an alternative to the
somewhat cycle-expensive system call interface that the GNU/Linux
The traditional mechanism of communication between userland
applications and the kernel is something called a system call.
Syscalls are implemented as software interrupts providing the
userland application with some kernel functionality. For instance,
gettimeofday() and fork() are both system calls. The reason
syscalls exist is due to the fact that the Linux kernel is divided
into two primary segments of memory: userland and kernel land.