This ticket is created on behalf of Jaromir Hamala:

We can additionally optimize os::current_thread_cpu_time (used by ThreadMXBean.getCurrentThreadUserTime()) on Linux by using the kernel's fast-path for the calling thread.

The current code has 2 phases:

- Create a timer bound to the current Thread ID (TID) + massage the timer to switch from CPUCLOCK_SCHED timer to CPUCLOCK_VIRT
- Use the timer in clock_gettime()

Within the kernel, a routine, pid_for_clock, exists to extract the TID from the timer, followed by a radix lookup to find booking structures for the specified TID.

However, the kernel also has a fast-path: When TID is 0, then the kernel avoids the radix lookup and instead treats the timer as bound to 'the current thread' (or a process, depending on lower bits of the clock). The current thread/task has bookkeeping structures more readily available and the radix lookup is avoided. The result is around 13% faster getCurrentThreadUserTime().