Hi Jose,
* Jose R. Santos ([email protected]) wrote:
>
> The problem now is how do we define "high event rate". This is
> something that is highly dependent on the workload being run as well as
> the system configuration for such workload. There are a lot of places
> in the kernel that can be turned into high event rates with with the
> right workload even though the may not represent 99% of most user cases.
>
> I would guess that anything above 500 event/s per-CPU on several
> realistic workloads is a good place to start.
>
Yes, it seems like a good starting point. But besides the event rate, just
having the most widely used events marked in the code should also be the
target. The markup mechanism serves two purposes :
1 - identify important events in a way that follows code change.
2 - speed up instrumentation.
>
> >On the macro-benchmark side, no significant difference in performance has
> >been
> >found between the vanilla kernel and a kernel "marked" with the standard
> >LTTng
> >instrumentation.
> >
>
> Out of curiosity, how many cycles does it take to process a complete
> LTTng event up until the point were it has been completely stored into
> the trace buffer. Since this should take a lot more than 55.74 cycles,
> it would be interesting to know at what event rate would a static marker
> stop showing as big of a performance advantage compared to dynamic probing.
>
In my OLS paper, I pointed out that, in its current state, LTTng took about 278
cycles on the same Pentium 4. I think I could lower that by implementing per-cpu
atomic operations (removing the LOCK prefix, as the data is not shared between
the CPUs; the atomic operations are only useful to protect from higher priority
execution contexts on the same CPU).
Regards,
Mathieu
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