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Performance Tracing on Linux

When a performance problem is encountered on Linux, these instructions can be used to gather detailed information about what was happening on the machine at the time of the performance problem.

Required Tools

  • perfcollect: Bash script that automates data collection.
  • PerfView: Windows-based performance tool that can also analyze trace files collected with Perfcollect.

Preparing Your Machine

Follow these steps to prepare your machine to collect a performance trace.

  1. Download Perfcollect.

    curl -OL http://aka.ms/perfcollect

  2. Make the script executable.

    chmod +x perfcollect

  3. Install tracing prerequisites - these are the actual tracing libraries. For details on prerequisites, see below.

    sudo ./perfcollect install

Collecting a Trace

  1. Have two shell windows available - one for controlling tracing, referred to as [Trace], and one for running the application, referred to as [App].

  2. [App] Setup the application shell - this enables tracing configuration inside of CoreCLR.

    export COMPlus_PerfMapEnabled=1
export COMPlus_EnableEventLog=1

  3. [Trace] Start collection.

    sudo ./perfcollect collect sampleTrace

    Expected Output:

    Collection started.  Press CTRL+C to stop.

  4. [App] Run the app - let it run as long as you need to in order to capture the performance problem. Generally, you don't need very long. As an example, for a CPU investigation, 5-10 seconds of the high CPU situation is usually enough.

    dotnet run

  5. [Trace] Stop collection - hit CTRL+C.

    ^C
...STOPPED.

Starting post-processing. This may take some time.

Generating native image symbol files
...SKIPPED
Saving native symbols
...FINISHED
Exporting perf.data file
...FINISHED
Compressing trace files
...FINISHED
Cleaning up artifacts
...FINISHED

Trace saved to sampleTrace.trace.zip

    The compressed trace file is now stored in the current working directory.

Collecting in a Docker Container

Perfcollect can be used to collect data for an application running inside a Docker container. The main thing to know is that collecting a trace requires elevated privileges because the default seccomp profile blocks a required syscall - perf_events_open.

In order to use the instructions in this document to collect a trace, spawn a new shell inside the container that is privileged.

docker exec -it --privileged <container_name> /bin/bash

Even though the application hosted in the container isn't privileged, this new shell is, and it will have all the privileges it needs to collect trace data. Now, simply follow the instructions in Collecting a Trace using the privileged shell.

If you want to try tracing in a container, we've written a demo Dockerfile that installs all of the performance tracing pre-requisites, sets the environment up for tracing, and starts a sample CPU-bound app.

Filtering

Filtering is implemented on Windows through the latest mechanisms provided with the EventSource class.

On Linux those mechanisms are not available yet. Instead, there are two environment variables that exist just on linux to do some basic filtering.

  • COMPLUS_EventSourceFilter – filter event sources by name
  • COMPLUS_EventNameFilter – filter events by name

Setting one or both of these variables will only enable collecting events that contain the name you specify as a substring. Strings are treated as case insensitive.

Viewing a Trace

Traces are best viewed using PerfView on Windows. Note that we're currently looking into porting the analysis pieces of PerfView to Linux so that the entire investigation can occur on Linux.

Open the Trace File

  1. Copy the trace.zip file from Linux to a Windows machine.

  2. Download PerfView from http://aka.ms/perfview.

  3. Run PerfView.exe

    PerfView.exe <path to trace.zip file>

Select a View

PerfView will display the list of views that are supported based on the data contained in the trace file.

  • For CPU investigations, choose CPU stacks.
  • For very detailed GC information, choose GCStats.
  • For per-process/module/method JIT information, choose JITStats.
  • If there is not a view for the information you need, you can try looking for the events in the raw events view. Choose Events.

For more details on how to interpret views in PerfView, see help links in the view itself, or from the main window in PerfView choose Help->Users Guide.

Extra Information

This information is not strictly required to collect and analyze traces, but is provided for those who are interested.

Prerequisites

Perfcollect will alert users to any prerequisites that are not installed and offer to install them. Prerequisites can be installed automatically by running:

sudo ./perfcollect install

The current prerequisites are:

  1. perf: Also known as perf_event, the Linux Performance Events sub-system and companion user-mode collection/viewer application. perf is part of the Linux kernel source, but is not usually installed by default.
  2. LTTng: Stands for "Linux Tracing Toolkit Next Generation", and is used to capture event data emitted at runtime by CoreCLR. This data is then used to analyze the behavior of various runtime components such as the GC, JIT and thread pool.