LTTng » LTTng-tools

Consider the following (LTTng 2.3.0 running in a single-processor 1 GiB virtual machine):

$ lttng create test
Session test created.
Traces will be written in /home/daniel/lttng-traces/test-20131018-122002
$ lttng enable-channel ch -u --discard --num-subbuf 0x800
UST channel ch enabled for session test
$ lttng enable-event -c ch -u -a
All UST events are enabled in channel ch
$ lttng start
Tracing started for session test
$ lttng list test
Tracing session test: [active]
    Trace path: /home/daniel/lttng-traces/test-20131018-122002

=== Domain: UST global ===

Buffer type: per UID

Channels:
-------------
- ch: [enabled]

    Attributes:
      overwrite mode: 0
      subbufers size: 131072
      number of subbufers: 2048
      switch timer interval: 0
      read timer interval: 0
      output: mmap()

    Events:
      * (type: tracepoint) [enabled]

$ lttng destroy
Session test destroyed

$ lttng create test
Session test created.
Traces will be written in /home/daniel/lttng-traces/test-20131018-122241
$ lttng enable-channel ch -u --discard --num-subbuf 0x1000
UST channel ch enabled for session test
$ lttng enable-event -c ch -u -a
All UST events are enabled in channel ch
$ lttng start
Tracing started for session test
$ lttng list test
Tracing session test: [active]
    Trace path: /home/daniel/lttng-traces/test-20131018-122241

=== Domain: UST global ===

Buffer type: per UID

Channels:
-------------
- ch: [enabled]

    Attributes:
      overwrite mode: 0
      subbufers size: 131072
      number of subbufers: 4096
      switch timer interval: 0
      read timer interval: 0
      output: mmap()

    Events:
      * (type: tracepoint) [enabled]

$ lttng destroy
Session test destroyed

The first session generates a trace as expected. The second does not: the trace folder remains stubbornly empty.

This is puzzling from a memory management point of view (the first session grabs 256 MiB of buffers, the second should grab 512 MiB), since the system's capacity was apparently not reached.

The one error captured in the session log (attached) is:

libringbuffer[14690/14695]: Error: zero_file: No space left on device (in _shm_object_table_alloc_shm() at shm.c:173)

The error needs to be properly captured and passed on to the lttng client.