On Thu, Jun 06, 2013 at 01:58:06PM +0800, Yan, Zheng wrote: > From: "Yan, Zheng" <zheng.z....@intel.com> > > If perf event buffer is in overwrite mode, the kernel only updates > the data head when it overwrites old samples. The program that owns > the buffer need periodically check the buffer and update a variable > that tracks the date tail. If the program fails to do this in time, > the data tail can be overwritted by new samples. The program has to > rewind the buffer because it does not know where is the first vaild > sample. > > This patch makes the kernel update the date tail when it overwrites > old events. So the program that owns the event buffer can always > read the latest samples. This is convenient for programs that use > perf to do branch tracing. One use case is GDB branch tracing: > (http://sourceware.org/ml/gdb-patches/2012-06/msg00172.html) > It uses perf interface to read BTS, but only cares the branches > before the ptrace event. > > I added code to perf_output_{begin/end} to count how many cycles > are spent by sample output, then ran "perf record" to profile kernel > compilation 10 times on IvyBridge-EP. (perf record -a make -j 60) > The first number is scaled to 1000, the rest numbers are scaled by > the same factor. > > before overwrite mode after overwrite mode > AVG 1000 999 1046 1044 > STDEV 19.4 19.5 17.1 17.9 >
Right, so it all gets about 5% more expensive.. I don't suppose there's anything smart we can do to avoid this for the !overwrite mode? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
