[pornin]

Measuring the total runtime is only one of many kinds of timing attacks. The most effective timing attacks do not do that; instead, they measure time taken to access some specific memory areas _after_ the execution of the target code, in order to work out which cache lines were loaded by that code (this applies to both data and code caches, btw). These are "timing attacks" since they ultimately come from a measure of elapsed time, but not necessarily the time taken by the target code itself.

Also, when a thread is sleeping, the OS will schedule other threads, which opens many additional ways for attackers to notice that sleeping started.