Tracing Runtime Events in .NET Core on Linux

March 30, 2017

After exploring the basic profiling story, let’s turn to ETW events. On Windows, the CLR is instrumented with a myriad of ETW events, which can be used to tackle very hard problems at runtime. Here are some examples of these events:

  • Garbage collections
  • Assembly load/unload
  • Thread start/stop (including thread pool threads)
  • Object allocations
  • Exceptions thrown, caught, filtered
  • Methods compiled (JIT)

By collecting all of, or a subset of, these events, you can get a very nice picture of what your .NET application is doing. By combining these with Windows kernel events for CPU sampling, file accesses, process creations and more — you have a fairly complete tool for performance investigations. You might find my recorded DotNext talk on using PerfView for .NET performance investigations useful — it shows how ETW and pretty much nothing else but ETW can be used to solve a huge variety of performance problems.

Unfortunately, ETW is a Windows-only mechanism. Event Tracing for Windows, you know? It is implemented in the Windows kernel, and that’s partly why it is so efficient and powerful. While looking for an equivalent Linux framework, the CLR team considered multiple alternatives, and decided to go with LTTNG. it’s not that Linux doesn’t have enough tracers — quite the opposite — they had to choose which tracer is most appropriate for the CLR’s needs.

Hello, LTTNG

LTTNG is similar in spirit to ETW. It is a lightweight tracing framework that can process events in real-time or record them to a file for later processing. It supports multiple simultaneous trace sessions, and each session can have multiple providers enabled — a system call provider (which requires a kernel module to be installed) alongside with user-space providers, such as the CoreCLR. Traces can be analyzed on the same machine or on another machine, and there are viewers available for more advanced visualization. Custom EventSource-based providers are also supported.

The massive, unfortunate, painful, unforgivable downside is that LTTNG doesn’t have stack trace support for user-space events. And this hurts more than you can imagine:

  • You can collect GC events, but you can’t aggregate stack traces to figure out where the GCs are coming from
  • You can collect exception events, but you don’t have the exception call stack
  • You can collect assembly load events (and potential failures), but you don’t know what’s triggering that assembly load
  • You can collect object allocations, but you can’t aggregate statistics to indicate which code paths are causing lots of allocations (and thereby garbage collections)

Effectively, not having stack trace support makes LTTNG for CoreCLR a logging framework, which can be used to record and investigate logs, but not a tracing framework, which can help diagnose performance problems and troubleshoot hard issues in the field.

Collecting LTTNG Traces

Still, for the sake of completeness, let’s take a look at collecting LTTNG traces from CoreCLR. As in the previous post, you could use the perfcollect tool — but it is typically overkill. First, perfcollect’s current implementation always turns on CPU sampling, which takes a massive amount of space and introduces a certain overhead. Second, perfcollect doesn’t have an event filter — it has exactly three modes for CoreCLR events: everything, GC only, and GC collect only. If you only care about assembly load events, or GC events, or JIT events, you’re not in luck.

Fortunately, it’s very easy to roll your own LTTNG collection (you’ll need to install lttng-tools to record, and babeltrace to view):

# lttng create exceptions-trace
# lttng add-context --userspace --type vpid
# lttng add-context --userspace --type vtid
# lttng add-context --userspace --type procname
# lttng enable-event --userspace --tracepoint DotNETRuntime:Exception*
# lttng start

In the preceding commands, lttng create creates a trace session that you then add providers to. The add-context command makes sure each event will have, in addition to the provider data, the PID, TID, and process name. Then, enable-event adds a specific event set from the CoreCLR provider — note that LTTNG doesn’t need any metadata about these events ahead of time. To get a list of all the possible events, one easy way is to just read the perfcollect script — it declares them all, e.g.:

declare -a DotNETRuntime_NoKeyword=(

Finally, lttng start starts the trace session with the enabled providers. By default, the traces are written to ~/session-name-timestamp — in our case, it’s going to be /root/exceptions-trace-20170330-something.

Now you run your scenario (the target application should have the COMPlus_EnableEventLog environment variable set to 1), and stop the trace when you’re done:

# lttng stop
# lttng destroy
# babeltrace ~/exceptions-trace
[07:31:11.751548909] (+?.?????????) ubuntu-16 DotNETRuntime:ExceptionThrown_V1: { cpu_id = 0 }, { ExceptionType = "System.NotSupportedException", ExceptionMessage = "Sample exception.", ExceptionEIP = 139767278604807, ExceptionHRESULT = 2148734229, ExceptionFlags = 16, ClrInstanceID = 0 }
[07:31:11.751603953] (+0.000055044) ubuntu-16 DotNETRuntime:ExceptionCatchStart: { cpu_id = 0 }, { EntryEIP = 139765244804131, MethodID = 139765233785640, MethodName = "void [Runny] Runny.Program::Main(string[])", ClrInstanceID = 0 }

babeltrace is a simple trace viewer, but there are also UI tools like Trace Compass and other visualization tools that can parse the CTF (Common Trace Format) specification.

What Now?

We have the fundamentals for collecting LTTNG traces from CoreCLR and from custom providers, but the lack of stack traces is gnawing at me. In the next post, we will explore a hacky way of getting stack traces from interesting events, and even using BCC/SystemTap for tracing them.

You can also follow me on Twitter, where I put stuff that doesn’t necessarily deserve a full-blown blog post.

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  1. Jonathan RajotteMarch 31, 2017 ב 5:44 PM


    Nice blog post! Glad to see LTTng used for CoreCLR.

    “The massive, unfortunate, painful, unforgivable downside is that LTTNG doesn’t have stack trace support for user-space events.”

    We are always looking for ways to improve so it is duly noted. Feel free to drop by our irc channel or mailing list to discuss any issues.


    1. Sasha Goldshtein
      Sasha GoldshteinApril 2, 2017 ב 9:27 AM

      Thanks Jonathan, I’ve also seen that there’s planned support for stack traces in LTTng 2.10, which is absolutely amazing! By the way, I hope it was clear that I wasn’t picking on LTTng — it has a set of design goals and it meets them very gracefully. Rather, I was trying to say that the CoreCLR team might have considered a tracing mechanism that has stack trace support built-in — so it is a downside in CoreCLR’s tracing, not in LTTng.