* Jes Sorensen ([email protected]) wrote:
> Mathieu Desnoyers wrote:
> >Please Ingo, stop repeating false argument without taking in account
> >people's
> >corrections :
> >
> >* Ingo Molnar ([email protected]) wrote:
> >>sorry, but i disagree. There _is_ a solution that is superior in every
> >>aspect: kprobes + SystemTap. (or any other equivalent dynamic tracer)
> >>
> >I am sorry to have to repeat myself, but this is not true for heavy loads.
>
> Alan pointed out earlier in the thread that the actual kprobe is noise
> in this context, and I have seen similar issues on real workloads. Yes
> kprobes are probably a little higher overhead in real life, but you have
> to way that up against the rest of the system load.
>
> If you want to prove people wrong, I suggest you do some real life
> implementation and measure some real workloads with a predefined set of
> tracepoints implemented using kprobes and LTT and show us that the
> benchmark of the user application suffers in a way that can actually be
> measured. Argueing that a syscall takes an extra 50 instructions
> because it's traced using kprobes rather than LTT doesn't mean it
> actually has any real impact.
>
> "The 'kprobes' are too high overhead that makes them unusable" is one of
> these classic myths that the static tracepoint advocates so far have
> only been backing up with rhetoric. Give us some hard evidence or stop
> repeating this argument please. Just because something is repeated
> constantly doesn't transform it into truth.
>
Hi,
Here we go. I made a test that we can consider a lower bound for kprobes impact.
Two tests per run.
Simulation of high speed network traffic :
time ping -f localhost
First run : without any tracing activated, LTTng probes compiled in :
39457 packets received in 2.021 seconds : 19523.50 packets/s
142672 packets received in 7.237 seconds : 19714.24 packets/s
Second run : LTTng tracing activated (traces system calls, interrupts and
packet in/out...) :
93051 packets received in 7.395 seconds : 12582.96 packets/s
121585 packets received in 9.703 seconds : 12530.66 packets/s
Third run : same LTTng instrumentation, with a kprobe handler triggered by each
event traced.
56643 packets received in 11.152 seconds : 5079.17 packets/s
50150 packets received in 9.593 seconds : 5227.77 packets/s
The bottom line is :
LTTng impact on the studied phenomenon : 35% slower
LTTng+kprobes impact on the studied phenomenon : 73% slower
Therefore, I conclude that on this type of high event rate workload, kprobes
doubles the tracer impact on the system.
Mathieu
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