TL;DR This blog post dives into an interesting finding: two frequently used system calls (gettimeofday, clock_gettime) are much slower on AWS EC2. Linux provides a mechanism for speeding up those two frequently used system calls by implementing the system call code in userland and avoiding the switch to the kernel entirely. This is done via a virtual shared library provided by the kernel that is mapped into the address space of every running program. The two system calls listed cannot use the vDSO as they normally would on any other system. This is because the virtualized clock source on xen (and some kvm configurations) do not support reading the time in userland via the vDSO. There is no safe workaround for this; the user may decide to change their clock source to tsc by writing to file in sysfs, but this is considered dangerous. Continue reading to learn more and the results of a microbenchmark.