> 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 _______________________________________________ ltt-dev mailing list [email protected] http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev
