From mboxrd@z Thu Jan 1 00:00:00 1970 From: roland@redhat.com (Roland McGrath) Date: Wed, 16 Feb 2011 10:19:43 -0800 (PST) Subject: [ltt-dev] LTTng-UST vs SystemTap userspace tracing benchmarks In-Reply-To: Tom Tromey's message of Wednesday, 16 February 2011 08:30:32 -0700 References: <4D5AA164.1050607@polymtl.ca> Message-ID: <20110216181943.E1E131806E0@magilla.sf.frob.com> > 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