Linux' hrtimer - microsecond precision?

Question

Is it possible to execute tasks on a Linux host with microsecond precision? I.e., I\'d like to execute a task at a specific instant of time. I know, Linux is no real-time sy

Answer 1

You can do what you want from user space

1. use clock_gettime() with CLOCK_REALTIME to get the time-of-day with nano-second resolution
2. use nanosleep() to yield the CPU until you are close to the time you need to execute your task (it is at least milli-second resolution).
3. use a spin loop with clock_gettime() until you reach the desired time
4. execute your task

The clock_gettime() function is implemented as a VDSO in recent kernels and modern x86 processors - it takes 20-30 nanoseconds to get the time-of-day with nano-second resolution - you should be able to call clock_gettime() over 30 times per micro-second. Using this method your task should dispatch within 1/30th of a micro-second of the intended time.

Answer 2

The default Linux kernel timer ticks each millisecond. Microseconds is way beyond anything current user hardware is capable of.

The jitter you see is due to a host of factors, like interrupt handling and servicing higher priority tasks. You can cut that down somewhat by selecting hardware carefully, only enabling what is really needed. The real-time patchseries to the kernel (see the HOWTO) might be an option to reduce it a bit further.

Always keep in mind that any gain has a definite cost in terms of interactiveness, stability, and (last, but by far not least) your time in building, tuning, troubleshooting, and keeping the house of cards from falling apart.