[ltt-dev] LTTng-UST vs SystemTap userspace tracing benchmarks

[Roland McGrath]

> The sdt.h stuff has been rewritten at least once since then.  I'd
> suggest trying the latest.  I think it probably won't matter for the
> flight recorder mode, but it may matter for measuring overhead.

The v3 and v2 versions have the same runtime/code overhead.  The
difference is in the data overhead, which includes some startup-time
dynamic linking overhead for PIC code.  In v3, there is no data
overhead except possibly the one semaphore word, and none of that
startup-time overhead at all.

> I'd be interested to also see the numbers when the probes are in place
> in the source, but not enabled.  That is, what is the overhead of a
> disabled probe?
> 
> When doing this with SystemTap it would be interesting to try twice:
> once with a semaphore for each probe, and once without.

In v2 and v3, the only runtime overhead of a disabled probe is the
presence of a single 'nop' instruction in the code path.  AIUI, on
modern processors that costs less than one cycle.  The overhead of
a probe firing is unchanged, being a full breakpoint trap and the
kernel side of handling that and running the probe machinery.


Thanks,
Roland