Swift Profile Recorder, an in-process sampling profiler

Want to profile your software in restricted Kubernetes or Docker containers or other environments where you don't have CAP_SYS_PTRACE? Look no further.

What is this?

This is a sampling profiler (like sample on macOS) with the special twist that it runs inside the process that gets sampled. This means that it doesn't need CAP_SYS_PTRACE or any other privileges to work.

You can pull it in as a fully self-contained Swift Package Manager dependency and then use it in your app.

Swift Profile Recorder is an on- and off-CPU profiler, which means that it records waiting threads (e.g., sleeps, locks, blocking system calls) as well as running (i.e., computing) threads.

Supported OSes

At the moment, it only supports Linux and macOS. It could also support other operating systems, but that's not implemented at this point in time.

How can I use it?

Via Swift Profile Recorder Server

The easiest way to use Swift Profile Recorder in your application is to run the Swift Profile Recorder Server. This allows you to retrieve symbolicated samples with a single curl (or any other HTTP client) command.

Using the Sampling Server

One-off setup to get your application ready for sampling

• Add a swift-profile-recorder dependency: .package(url: "https://github.com/apple/swift-profile-recorder.git", .upToNextMinor(from: "0.3.0"))
• Make your main executableTarget depend on ProfileRecorderServer: .product(name: "ProfileRecorderServer", package: "swift-profile-recorder"),
• Add the following few lines at the very beginning of your main function (static func main() or func run()):

import ProfileRecorderServer

[...]

@main
struct YourApp {
   func run() async throws {
       // Run `ProfileRecorderServer` in the background if enabled via environment variable. Ignore failures.
       // It will be automatically cancelled if this function returns.
       //
       // Example:
       //   PROFILE_RECORDER_SERVER_URL_PATTERN='unix:///tmp/my-app-samples-{PID}.sock' ./my-app
       async let _ = ProfileRecorderServer(configuration: .parseFromEnvironment()).runIgnoringFailures(logger: logger)

       [... your regular main function ...]
    }
}

Using the profiling server

Once you added the profile recorder server to your app, you can enable it using an environment variable (assuming you passed configuration: .parseFromEnvironment()):

# Request the profile recording server to listen on a UNIX Domain Socket at path `/tmp/my-app-samples-{PID}.sock`.
# `{PID}` will automatically be replaced with your process's process ID.
PROFILE_RECORDER_SERVER_URL_PATTERN=unix:///tmp/my-app-samples-{PID}.sock .build/release/MyApp

After that, you're ready to request samples:

curl --unix-socket /tmp/my-app-samples-62012.sock -sd '{"numberOfSamples":10,"timeInterval":"100 ms"}' http://localhost/sample | swift demangle --compact > /tmp/samples.perf

Now, a file called /tmp/samples.perf should have been created. This file is in the standard Linux perf format.

Visualisation

Whilst .perf files are plain text files, they are most easily digested in a visual form such as FlameGraphs.

Below, some compatible visualisation tools:

• Speedscope (speedscope.app), simply drag a .perf file (such as /tmp/samples.perf in the example above) onto the Speedscope website.
• Firefox Profiler (profiler.firefox.com), simply drag a .perf file (such as /tmp/samples.perf in the example above) onto the Firefox Profiler website.
• The original FlameGraph tooling. Try for example ./stackcollapse-perf.pl < /tmp/samples.perf | swift demangle --compact | ./flamegraph.pl > /tmp/samples.svg && open -a Safari /tmp/samples.svg.

Compatibility

Formats

• The Linux perf script format (.perf, like what perf record && perf script would emit)
• The pprof format (.pprof, like what Golang's pprof emits)
• The "collapsed" format (like what FlameGraph's stackcollapse* scripts emit)

Profile recording server URL endpoints

• pprof's /debug/pprof/profile endpoint
• Swift Profile Recorder's own /sample endpoint

Example profiles

• Hummingbird's hello example load-tested by wrk -T50s -c 20000 -t 200 http://127.0.0.1:8080 running on macOS

  • Applied a small diff to enable Swift Profile Recorder in Humminbird's hello example
  • Server started with just one SwiftNIO thread for a cleaner profile: NIO_SINGLETON_BLOCKING_POOL_THREAD_COUNT=1 NIO_SINGLETON_GROUP_LOOP_COUNT=1 PROFILE_RECORDER_SERVER_URL_PATTERN=unix:///tmp/swipr-{PID}.sock .build/release/App = Samples received using curl -sd '{"numberOfSamples":1000,"timeInterval":"10ms"}' --unix-socket /tmp/swipr-SERVER_PID.sock http://unix | swift demangle --compact > /tmp/samples.perf
  • View profile in Firefox Profiler
  • Screenshot of speedscope.app:

• Hummingbird's hello example load-tested by wrk -T50s -c 20000 -t 200 http://127.0.0.1:8080 running on Linux (Ubuntu 20.04, Swift 6.2, unprivileged container)

  • Applied a small diff to enable Swift Profile Recorder in Humminbird's hello example
  • Server started with just one SwiftNIO thread for a cleaner profile: NIO_SINGLETON_BLOCKING_POOL_THREAD_COUNT=1 NIO_SINGLETON_GROUP_LOOP_COUNT=1 PROFILE_RECORDER_SERVER_URL_PATTERN=unix:///tmp/swipr-{PID}.sock .build/release/App = Samples received using curl -sd '{"numberOfSamples":1000,"timeInterval":"10ms"}' --unix-socket /tmp/swipr-SERVER_PID.sock http://unix | swift demangle --compact > /tmp/samples.perf
  • View profile in Firefox Profiler
  • Screenshot of speedscope.app:

Example diffs

Add Swift Profile Recorder to hummingbird-examples/hello

Expand here to see git diff -U1 onto commit 97a09f0664679f017616a82894848b267c5e7068

diff --git a/hello/Package.swift b/hello/Package.swift
index ae0b6d2..33b24ed 100644
--- a/hello/Package.swift
+++ b/hello/Package.swift
@@ -11,2 +11,3 @@ let package = Package(
         .package(url: "https://github.com/apple/swift-argument-parser.git", from: "1.4.0"),
+        .package(url: "[email protected]:apple/swift-profile-recorder.git", branch: "main"),
     ],
@@ -18,2 +19,3 @@ let package = Package(
                 .product(name: "Hummingbird", package: "hummingbird"),
+                .product(name: "ProfileRecorderServer", package: "swift-profile-recorder"),
             ],
diff --git a/hello/Sources/App/app.swift b/hello/Sources/App/app.swift
index 13131d9..95b114a 100644
--- a/hello/Sources/App/app.swift
+++ b/hello/Sources/App/app.swift
@@ -1,2 +1,3 @@
 import ArgumentParser
+import ProfileRecorderServer

@@ -17,2 +18,5 @@ struct HummingbirdArguments: AsyncParsableCommand {
         )
+        async let _ = ProfileRecorderServer(configuration: .parseFromEnvironment()).runIgnoringFailures(
+            logger: app.logger
+        )
         try await app.runService()