[LITMUS^RT] Feather-Trace scalability & TSC calibration patches

[Björn Brandenburg]

On 14 Jan 2014, at 10:56, Björn Brandenburg <bbb at mpi-sws.org> wrote:

> Hi everyone,
> 
> here's another set of patches extracted from our RTAS'14 branch. When tracing LITMUS^RT on a 64-core platform, we ran into two issues:
> 
> 	1) Feather-Trace's global timestamp buffer became a major scalability bottleneck, and
> 
> 	2) non-synchronized TSCs with a constant offset.
> 
> (1) is a serious problem because it distorts the measurements (i.e, the overhead Feather-Trace itself becomes much larger than the overhead that it is supposed to measure). (2) is not a problem for most measurements (CPU-local measurements are not affected by cross-CPU skew), but measuring IPI latencies becomes difficult if TSCs do not share a common time zero.
> 
> The following patches address (1) by changing Feather-Trace to record all timestamps into processor-local trace buffers, and provide a workaround for (2) by adding some benchmarking code that determines the offset between any two cores, which can then be used to patch up measurements to refer to a common time base.
> 
> 	https://github.com/LITMUS-RT/litmus-rt/commits/wip-ft-pcpu
> 
> The corresponding userspace patches can be found here:
> 
> 	https://github.com/LITMUS-RT/feather-trace-tools/commits/wip-ft-pcpu
> 
> This of course breaks userspace scripts in all sorts of ways because /litmus/dev/ft_trace0 goes away, new devices appear, etc. Nonetheless, the pain is worth it I believe because otherwise we won't be able to derive meaningful measurements on large multicore platforms. I'd appreciate help with testing and feedback.

Now that we are all out of ECRTS mode, are there any objections to or comments on merging this?

Thanks,
Björn