Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

Ingo Molnar
2006-09-16T19:19:39Z

* Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:

> See http://ltt.polymtl.ca/svn/tests/kernel/test-kprobes.c to insert
> the kprobe. Tests done on LTTng 0.5.111, on a x86 3GHz with
> hyperthreading.

i have done a bit of kprobes and djprobes testing on a 2160 MHz Athlon64
CPU, UP. I have tested 2 types of almost-NOP tracepoints (on 2.6.17),
where the probe function only increases a counter:

static int counter;

static void probe_func(struct djprobe *djp, struct pt_regs *regs)
{
counter++;
}

and have measured the overhead of an unmodified, kprobes-probed and
djprobes-probed sys_getpid() system-call:

sys_getpid() unmodified latency: 317 cycles [ 0.146 usecs ]
sys_getpid() kprobes latency: 815 cycles [ 0.377 usecs ]
sys_getpid() djprobes latency: 380 cycles [ 0.176 usecs ]

i.e. the kprobes overhead is +498 cycles (+0.231 usecs), the djprobes
overhead is +63 cycles (+0.029 usecs).

what do these numbers tell us? Firstly, on this CPU the kprobes overhead
is not 1000-2000 cycles but 500 cycles. Secondly, if that's not fast
enough, the "next-gen kprobes" code, djprobes have a really small
overhead of 63 cycles.

Ingo