Howto: Profiling the TCP IP stack with LTTng

Antti Kantee edited this page May 4, 2015 · 5 revisions


The purpose of this exercise is to lay groundwork for the performance optimization of the NetBSD TCP/IP stack running in a rump kernel in userspace. While a rump kernel always runs directly on host threads, synchronization constructs within the rump kernel limit which host thread can run. To understand how to optimize the host's scheduling, we need to understand what is happening.

For packet generation, we will use rump-pktgenif. For tracing, we will use LTTng, and will run the rump kernel in userspace on Linux.

Getting LTTng

The sane procedure for this will vary depending on what your system is running, and will not be exhaustively documented here. I was running on Ubuntu, and did not have huge success with the LTTng 2.1 offered in Ubuntu 13.10. After switching to LTTng 2.4.0 offered by an Ubuntu PPA, as detailed here, the rate of success grew. Your mileage may vary.

Compiling rump-pktgenif

As a prerequisite, you need The latest version is recommended.

To fetch and build, simply run:

    git clone
    cd rump-pktgenif
    rumpmake USE_LTTNG=1 dependall

(see for more information on how to find rumpmake and place it in your $PATH)

Building with USE_LTTNG=1 will add lttng-ust probes into the code, making it easier to track kernel events.

Running rump-pktgenif

For the packet generator to be able to run, you must configure the TCP/IP stack that it is using (cf. ifconfig etc.). The easiest way to do this is to use rumpctrl. Edit the path at the top of to point to your rumpctrl directory, and you are good to go.

Run the tool with for example:

    ./testtool -r ./<configname> recv

Press ctrl-c to end the run.

Capturing a trace

We will capture a basic trace which includes context switches, cache misses, and the pktgenif tool specified tracepoints. You are of course free to adjust this list however you want. Run the commands with appropriate privileges (root or being in the group "tracing" was enough for me, but consult LTTng documentation to be sure).

    lttng create testtool
    lttng enable-event -k sched_switch
    lttng add-context -k -t perf:cache-misses
    lttng enable-event -u 'pktgenif:*'

Now, run the packet generator in another window and then type:

    lttng start ; sleep 1 ; lttng stop

The packet generator can be stopped now. You can view the raw capture data with lttng view. A visualization tool is helpful in getting a better idea of what is going on.

Visualizing the trace

If you have Eclipse, you can try using the LTTng plugin. I did not have Eclipse and did not have good luck installing it and the plugin, so I used the standalone version available from this page.

To view the trace:

  • File -> Import, enter the path that lttng create printed
  • click the checkpoint next to the box, click Finish
  • open the Traces tree on the left-hand side
  • right-click the items (64-bit and kernel for me), select Open for both

You should now have the event information and the scheduling information on the screen. To make the scheduling information a bit more palatable, restrict threads only to the ones we are interested in. On the top right, click Show View Filters, click Uncheck All, then select anything the the name testtool, pktgen and rsi?/3. The number of rsi? depends on the number of cores configured into the rump kernel (default: 2). Notably, everything we selected is in a single process. If someone knows an easier way of selecting threads in a single process, please edit the above.

You should see something like: lttng pktgenif


Run lttng destroy to remove your session.

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