[lttng-dev] Restriction on channel sub-buffer size?

[Sean Heelan]

Hi all,

I recently encountered a situation whereby tracing in a tight loop was
generating a very large number of events, thus resulting in quite a few
discarded events. To resolve this I attempted to create a new channel with
a larger buffer size as follows:

lttng create cc_session -o $trace_output_path
lttng enable-channel -u custom-channel--subbuf-size 4M
lttng enable-event -u -c custom-channel cc_provider:*
lttng start

/run/target

lttng stop
lttng destroy

After running the above the trace is empty. However, if I change the size
to 1M or 2M it works fine. Is there a restriction on the sub-buffer size? I
also notice a similar behaviour if I increase the number of subbuffers
above 8 while using a 1M buffer size.

I'm still losing around 80k events at a sub-buffer size of 2M (down from
1.2 million at the default buffer size buffer size of 4096) so ideally I'd
like to use a larger buffer size (assuming it's possible).

Some potentially relevant details:
* I'm running the target inside docker, version 1.10.3
* The container is Ubuntu 14.04
* I'm using LTTng from git HEAD

If I can provide any other useful information let me know!

Thanks,
Sean
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