First of all: LTTng seems to be a great piece of software. Great docs
and good visualization tools (Eclipse) are available. I mention this to
clarify that I like LTTng...
... unfortunately, for the issue I've started using it, it doesn't work
very well. I contact the list because I hope you have some tips and
tricks for me.
I have a Ubuntu 14.04/32, i5 Quadcore. The goal is to run an existing
application as jitter less as possible. To tune the system, I've
programmed a simple test-application which sends a UDP frame to an
embedded system and waits for the response. If the time until test-app
receives the response is bigger than 5ms, it consider that a big jitter
and exits. In that case, I want to find out what caused the jitter of >
5ms. NOTE: I run my application with real-time prio 99 (which could
matter in this case, I guess)
Here's how I initially run my measurement (all as sudo):
lttng create
lttng enable-channel channel0 -k
lttng enable-event -k --all -c channel0
lttng start
chrt -f 50 ./run_my_test-application_here
lttng stop
lttng destroy
Then, I start compiling a kernel image and run bonnie++ to generate
system load. It takes about 1h until a jitter > 5ms occurs... but it
happens.
Unfortunately, whenever the big jitter (which are around 6ms up to 25ms)
occur, LTTng is not able to record the events around the time the delay
happened. Means: I got really nice graphs in Eclipse - but not at the
points where it would be useful... I also verified this with "babeltrace
.... > /dev/null" which reported the loss of various events.
I read the docs and then tried to increase the buffers. I tried several
numbers, the final version is:
lttng enable-channel channel0 -k --num-subbuf 32 --subbuf-size 1M
But it doesn't help. From my point of view, the result is still the same
as before it doesn't obviously loose less events than before. (Side
note: Actually, I also tried with "subbuf 16" and "subbuf-size 10M" but
then my system crashed: X gone, no keyboard input possible anymore but
(surprisingly) no kernel dump)
(Side note: There's one more thing to mention: The system is setup with
isolcpu=1,2,3 (because core 1..3 run real-time apps other than the one
mentioned here) - thus everything, including lttng daemons, ran on CPU0
so far.)
Finally, I tried to run lttng-sessiond and lttng-consumerd on a separate
CPU core (CPU3) and even changed policy to SCHED_FIFO and prio to 99 -
but lttng still looses events. This really surpises me...
I do understand that it is a good feature that lttng discards events
when buffers are full... but in my case, it seems to render lttng useless.
(Note: I did not yes use the "buffer overwrite" feature, but I guess the
events of interest will still be lost)
My question: Is there anything else I can do on my system (which is a
not a production system, just a developer machine) to make lttng work
for my scenario?
Any input is very welcome!
Raphael
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