LTTng » LTTng-tools

The following max stream buffer size calculation is wrong.

max_stream_size = output->max_size / nb_streams; (lttng-tools/src/bin/lttng-sessiond/ust-app.c, ln4933)

This assumes that all streams in a session are of the same size and will fail when its not the case. The same logic is duplicated on the kernel.c side.

Example on a machine sporting an i7 quad-core (with HT):

$ lttng create --snapshot

# Creating a channel containing 8 streams, so 16 * 8 = 128M
$ lttng enable-channel chan1 -u --num-subbuf 1 --subbuf-size 16M

# Creating a channel containing 8 streams, so 4 * 8 = 32M
$ lttng enable-channel chan2 -u --num-subbuf 1 --subbuf-size 4M
$ lttng start

# Starting an instrumented application in another terminal
# The snapshot should fit in 160M...
$ lttng snapshot record -m 160M
Error: Invalid parameter

The session daemon gives this error message:

DEBUG3 [21150/21153]: UST app snapshot record maximum stream size 5242880 is smaller than subbuffer size of 16777216 (in ust_app_snapshot_record() at ust-app.c:4976)