Mathieu Desnoyers wrote:
However, the performance impact for using a kprobe is non negligible when
activated. Assuming that kprobes would have a mechanism to get the variables
from the caller's stack, it would perform the same task in at least 4178.23
cycles vs 55.74 for a marker and a probe (ratio : 75). While kprobes are very
useful for the reason explained earlier, the high event rate paths in the kernel
would clearly benefit from a marker mechanism when the are probed.
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.
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.
-JRS
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