From 72d3fd17d14073087f8a17e094a2b67199c3cae4 Mon Sep 17 00:00:00 2001 From: Andy Grove Date: Tue, 3 Mar 2026 17:59:35 -0700 Subject: [PATCH 1/3] docs: expand profiling guide with JVM and async-profiler coverage Rename profiling_native_code.md to profiling.md and add sections for async-profiler (unified JVM + native flame graphs), Java Flight Recorder, a tool comparison table, and practical tips for profiling Comet's mixed JVM/Rust execution. --- docs/source/contributor-guide/index.md | 2 +- docs/source/contributor-guide/profiling.md | 293 ++++++++++++++++++ .../profiling_native_code.md | 94 ------ 3 files changed, 294 insertions(+), 95 deletions(-) create mode 100644 docs/source/contributor-guide/profiling.md delete mode 100644 docs/source/contributor-guide/profiling_native_code.md diff --git a/docs/source/contributor-guide/index.md b/docs/source/contributor-guide/index.md index 2b6842e449..3e3a059cfe 100644 --- a/docs/source/contributor-guide/index.md +++ b/docs/source/contributor-guide/index.md @@ -35,7 +35,7 @@ Benchmarking Guide Adding a New Operator Adding a New Expression Tracing -Profiling Native Code +Profiling Spark SQL Tests SQL File Tests Roadmap diff --git a/docs/source/contributor-guide/profiling.md b/docs/source/contributor-guide/profiling.md new file mode 100644 index 0000000000..f4018138f1 --- /dev/null +++ b/docs/source/contributor-guide/profiling.md @@ -0,0 +1,293 @@ + + +# Profiling + +This guide covers profiling tools and techniques for Comet development. Because Comet +spans JVM (Spark) and native (Rust) code, choosing the right profiler depends on what +you are investigating. + +## Choosing a Profiling Tool + +| Tool | JVM Frames | Native (Rust) Frames | Install Required | Best For | +| --- | --- | --- | --- | --- | +| [async-profiler](https://github.com/async-profiler/async-profiler) | Yes | Yes | Yes | End-to-end Comet profiling with unified JVM + native flame graphs | +| [Java Flight Recorder (JFR)](https://docs.oracle.com/en/java/javase/17/jfapi/) | Yes | No | No (JDK 11+) | GC pressure, allocations, thread contention, I/O — any JVM-level investigation | +| [cargo-flamegraph](https://github.com/flamegraph-rs/flamegraph) | No | Yes | Yes | Isolated Rust micro-benchmarks without a JVM | + +**Recommendation:** Use **async-profiler** when profiling Spark queries with Comet enabled — +it is the only tool that shows both JVM and native frames in a single flame graph. +Use **JFR** when you need rich JVM event data (GC, locks, I/O) without installing anything. +Use **cargo-flamegraph** when working on native code in isolation via `cargo bench`. + +## Profiling with async-profiler + +[async-profiler](https://github.com/async-profiler/async-profiler) captures JVM and +native code in the same flame graph by using Linux `perf_events` or macOS `dtrace`. +This makes it ideal for profiling Comet, where hot paths cross the JNI boundary between +Spark and Rust. + +### Installation + +Download a release from the [async-profiler releases page](https://github.com/async-profiler/async-profiler/releases): + +```shell +# Linux x64 +wget https://github.com/async-profiler/async-profiler/releases/download/v3.0/async-profiler-3.0-linux-x64.tar.gz +tar xzf async-profiler-3.0-linux-x64.tar.gz -C /opt/async-profiler --strip-components=1 +export ASYNC_PROFILER_HOME=/opt/async-profiler +``` + +On macOS, download the appropriate `macos` archive instead. + +### Attaching to a running Spark application + +Use the `asprof` launcher to attach to a running JVM by PID: + +```shell +# Start CPU profiling for 30 seconds, output an HTML flame graph +$ASYNC_PROFILER_HOME/bin/asprof -d 30 -f flamegraph.html + +# Wall-clock profiling +$ASYNC_PROFILER_HOME/bin/asprof -e wall -d 30 -f flamegraph.html + +# Start profiling (no duration limit), then stop later +$ASYNC_PROFILER_HOME/bin/asprof start -e cpu +# ... run your query ... +$ASYNC_PROFILER_HOME/bin/asprof stop -f flamegraph.html +``` + +Find the Spark driver/executor PID with `jps` or `pgrep -f SparkSubmit`. + +### Passing profiler flags via spark-submit + +You can also attach async-profiler as a Java agent at JVM startup: + +```shell +spark-submit \ + --conf "spark.driver.extraJavaOptions=-agentpath:/opt/async-profiler/lib/libasyncProfiler.so=start,event=cpu,file=driver.html" \ + --conf "spark.executor.extraJavaOptions=-agentpath:/opt/async-profiler/lib/libasyncProfiler.so=start,event=cpu,file=executor.html" \ + ... +``` + +### Choosing an event type + +| Event | When to use | +| --- | --- | +| `cpu` | Default. Shows where CPU cycles are spent. Use for compute-bound queries. | +| `wall` | Wall-clock time including blocked/waiting threads. Use to find JNI boundary overhead and I/O stalls. | +| `alloc` | Heap allocation profiling. Use to find JVM allocation hotspots around Arrow FFI and columnar conversions. | +| `lock` | Lock contention. Use when threads appear to spend time waiting on synchronized blocks or locks. | + +### Output formats + +| Format | Flag | Description | +| --- | --- | --- | +| HTML flame graph | `-f out.html` | Interactive flame graph (default and most useful). | +| JFR | `-f out.jfr` | Viewable in JDK Mission Control or IntelliJ. | +| Collapsed stacks | `-f out.collapsed` | For use with Brendan Gregg's FlameGraph scripts. | +| Text summary | `-o text` | Flat list of hot methods. | + +### Platform notes + +**Linux:** Set `perf_event_paranoid` to allow profiling: + +```shell +sudo sysctl kernel.perf_event_paranoid=1 # or 0 / -1 for full access +sudo sysctl kernel.kptr_restrict=0 # optional: enable kernel symbols +``` + +**macOS:** async-profiler uses `dtrace` on macOS, which requires running as root or +with SIP (System Integrity Protection) adjustments. Native Rust frames may not be fully +resolved on macOS; Linux is recommended for the most complete flame graphs. + +### Integrated benchmark profiling + +The TPC benchmark scripts in `benchmarks/tpc/` have built-in async-profiler support via +the `--async-profiler` flag. See [benchmarks/tpc/README.md](../../benchmarks/tpc/README.md) +for details. + +## Profiling with Java Flight Recorder + +[Java Flight Recorder (JFR)](https://docs.oracle.com/en/java/javase/17/jfapi/) is built +into JDK 11+ and collects detailed JVM runtime data with very low overhead. It does not +see native Rust frames, but is excellent for diagnosing GC pressure, thread contention, +I/O latency, and JVM-level allocation patterns. + +### Adding JFR flags to spark-submit + +```shell +spark-submit \ + --conf "spark.driver.extraJavaOptions=-XX:StartFlightRecording=duration=120s,filename=driver.jfr" \ + --conf "spark.executor.extraJavaOptions=-XX:StartFlightRecording=duration=120s,filename=executor.jfr" \ + ... +``` + +For continuous recording without a fixed duration: + +```shell +--conf "spark.driver.extraJavaOptions=-XX:StartFlightRecording=disk=true,maxsize=500m,filename=driver.jfr" +``` + +You can also start and stop recording dynamically using `jcmd`: + +```shell +jcmd JFR.start name=profile +# ... run your query ... +jcmd JFR.stop name=profile filename=recording.jfr +``` + +### Viewing recordings + +- **[JDK Mission Control (JMC)](https://jdk.java.net/jmc/)** — the most comprehensive viewer. + Shows flame graphs, GC timeline, thread activity, I/O, and allocation hot spots. +- **IntelliJ IDEA** — open `.jfr` files directly in the built-in profiler + (Run → Open Profiler Snapshot). +- **`jfr` CLI** — quick summaries from the command line: `jfr summary driver.jfr` + +### Useful JFR events for Comet debugging + +| Event | What it shows | +| --- | --- | +| `jdk.GCPhasePause` | GC pause durations — helps identify memory pressure from Arrow allocations. | +| `jdk.ObjectAllocationInNewTLAB` / `jdk.ObjectAllocationOutsideTLAB` | Allocation hot spots. | +| `jdk.JavaMonitorWait` / `jdk.ThreadPark` | Thread contention and lock waits. | +| `jdk.FileRead` / `jdk.FileWrite` / `jdk.SocketRead` | I/O latency. | +| `jdk.ExecutionSample` | CPU sampling (method profiling, similar to a flame graph). | + +### Integrated benchmark profiling + +The TPC benchmark scripts support `--jfr` for automatic JFR recording during benchmark +runs. See [benchmarks/tpc/README.md](../../benchmarks/tpc/README.md) for details. + +## Profiling Native Code with cargo-flamegraph + +For profiling Rust code in isolation — without a JVM — use `cargo bench` with +[cargo-flamegraph](https://github.com/flamegraph-rs/flamegraph). + +### Running micro benchmarks with cargo bench + +When implementing a new operator or expression, it is good practice to add a new microbenchmark under `core/benches`. + +It is often easiest to copy an existing benchmark and modify it for the new operator or expression. It is also +necessary to add a new section to the `Cargo.toml` file, such as: + +```toml +[[bench]] +name = "shuffle_writer" +harness = false +``` + +These benchmarks are useful when for comparing performance between releases or between feature branches and the +main branch to help prevent regressions in performance when adding new features or fixing bugs. + +Individual benchmarks can be run by name with the following command. + +```shell +cargo bench shuffle_writer +``` + +Here is some sample output from running this command. + +``` + Running benches/shuffle_writer.rs (target/release/deps/shuffle_writer-e37b59e37879cce7) +Gnuplot not found, using plotters backend +shuffle_writer/shuffle_writer + time: [2.0880 ms 2.0989 ms 2.1118 ms] +Found 9 outliers among 100 measurements (9.00%) + 3 (3.00%) high mild + 6 (6.00%) high severe +``` + +### Profiling with cargo-flamegraph + +Install cargo-flamegraph: + +```shell +cargo install flamegraph +``` + +Follow the instructions in [cargo-flamegraph](https://github.com/flamegraph-rs/flamegraph) for your platform for +running flamegraph. + +Here is a sample command for running `cargo-flamegraph` on MacOS. + +```shell +cargo flamegraph --root --bench shuffle_writer +``` + +This will produce output similar to the following. + +``` +dtrace: system integrity protection is on, some features will not be available + +dtrace: description 'profile-997 ' matched 1 probe +Gnuplot not found, using plotters backend +Testing shuffle_writer/shuffle_writer +Success + +dtrace: pid 66402 has exited +writing flamegraph to "flamegraph.svg" +``` + +The generated flamegraph can now be opened in a browser that supports svg format. + +Here is the flamegraph for this example: + +![flamegraph](../_static/images/flamegraph.png) + +## Tips for Profiling Comet + +### Use wall-clock profiling to spot JNI boundary overhead + +When profiling Comet with async-profiler, `wall` mode is often more revealing than `cpu` +because it captures time spent crossing the JNI boundary, waiting for native results, +and blocked on I/O — none of which show up in CPU-only profiles. + +```shell +$ASYNC_PROFILER_HOME/bin/asprof -e wall -d 60 -f wall-profile.html +``` + +### Use alloc profiling around Arrow FFI + +JVM allocation profiling can identify hotspots in the Arrow FFI path where temporary +objects are created during data transfer between JVM and native code: + +```shell +$ASYNC_PROFILER_HOME/bin/asprof -e alloc -d 60 -f alloc-profile.html +``` + +Look for allocations in `CometExecIterator`, `CometBatchIterator`, and Arrow vector +classes. + +### Isolate Rust-only performance issues + +If a flame graph shows the hot path is entirely within native code, switch to +`cargo-flamegraph` to get better symbol resolution and avoid JVM noise: + +```shell +cd native +cargo flamegraph --root --bench +``` + +### Correlating JVM and native frames + +In async-profiler flame graphs, native Rust frames appear below JNI entry points like +`Java_org_apache_comet_Native_*`. Look for these transition points to understand how +time is split between Spark's JVM code and Comet's native execution. diff --git a/docs/source/contributor-guide/profiling_native_code.md b/docs/source/contributor-guide/profiling_native_code.md deleted file mode 100644 index ec3349b2e8..0000000000 --- a/docs/source/contributor-guide/profiling_native_code.md +++ /dev/null @@ -1,94 +0,0 @@ - - -# Profiling Native Code - -We use `cargo bench` to run benchmarks to measure the performance of individual operators and expressions -and [cargo-flamegraph](https://github.com/flamegraph-rs/flamegraph) for profiling. - -## Running micro benchmarks with cargo bench - -When implementing a new operator or expression, it is good practice to add a new microbenchmark under `core/benches`. - -It is often easiest to copy an existing benchmark and modify it for the new operator or expression. It is also -necessary to add a new section to the `Cargo.toml` file, such as: - -```toml -[[bench]] -name = "shuffle_writer" -harness = false -``` - -These benchmarks are useful when for comparing performance between releases or between feature branches and the -main branch to help prevent regressions in performance when adding new features or fixing bugs. - -Individual benchmarks can be run by name with the following command. - -```shell -cargo bench shuffle_writer -``` - -Here is some sample output from running this command. - -``` - Running benches/shuffle_writer.rs (target/release/deps/shuffle_writer-e37b59e37879cce7) -Gnuplot not found, using plotters backend -shuffle_writer/shuffle_writer - time: [2.0880 ms 2.0989 ms 2.1118 ms] -Found 9 outliers among 100 measurements (9.00%) - 3 (3.00%) high mild - 6 (6.00%) high severe -``` - -## Profiling with cargo-flamegraph - -Install cargo-flamegraph: - -```shell -cargo install flamegraph -``` - -Follow the instructions in [cargo-flamegraph](https://github.com/flamegraph-rs/flamegraph) for your platform for -running flamegraph. - -Here is a sample command for running `cargo-flamegraph` on MacOS. - -```shell -cargo flamegraph --root --bench shuffle_writer -``` - -This will produce output similar to the following. - -``` -dtrace: system integrity protection is on, some features will not be available - -dtrace: description 'profile-997 ' matched 1 probe -Gnuplot not found, using plotters backend -Testing shuffle_writer/shuffle_writer -Success - -dtrace: pid 66402 has exited -writing flamegraph to "flamegraph.svg" -``` - -The generated flamegraph can now be opened in a browser that supports svg format. - -Here is the flamegraph for this example: - -![flamegraph](../_static/images/flamegraph.png) From fa01fa978bd51f4dacadcb8844daef5fffbb5ee3 Mon Sep 17 00:00:00 2001 From: Andy Grove Date: Tue, 3 Mar 2026 18:08:13 -0700 Subject: [PATCH 2/3] style: format profiling guide with prettier --- docs/source/contributor-guide/profiling.md | 46 +++++++++++----------- 1 file changed, 23 insertions(+), 23 deletions(-) diff --git a/docs/source/contributor-guide/profiling.md b/docs/source/contributor-guide/profiling.md index f4018138f1..7f128937df 100644 --- a/docs/source/contributor-guide/profiling.md +++ b/docs/source/contributor-guide/profiling.md @@ -25,11 +25,11 @@ you are investigating. ## Choosing a Profiling Tool -| Tool | JVM Frames | Native (Rust) Frames | Install Required | Best For | -| --- | --- | --- | --- | --- | -| [async-profiler](https://github.com/async-profiler/async-profiler) | Yes | Yes | Yes | End-to-end Comet profiling with unified JVM + native flame graphs | -| [Java Flight Recorder (JFR)](https://docs.oracle.com/en/java/javase/17/jfapi/) | Yes | No | No (JDK 11+) | GC pressure, allocations, thread contention, I/O — any JVM-level investigation | -| [cargo-flamegraph](https://github.com/flamegraph-rs/flamegraph) | No | Yes | Yes | Isolated Rust micro-benchmarks without a JVM | +| Tool | JVM Frames | Native (Rust) Frames | Install Required | Best For | +| ------------------------------------------------------------------------------ | ---------- | -------------------- | ---------------- | ------------------------------------------------------------------------------ | +| [async-profiler](https://github.com/async-profiler/async-profiler) | Yes | Yes | Yes | End-to-end Comet profiling with unified JVM + native flame graphs | +| [Java Flight Recorder (JFR)](https://docs.oracle.com/en/java/javase/17/jfapi/) | Yes | No | No (JDK 11+) | GC pressure, allocations, thread contention, I/O — any JVM-level investigation | +| [cargo-flamegraph](https://github.com/flamegraph-rs/flamegraph) | No | Yes | Yes | Isolated Rust micro-benchmarks without a JVM | **Recommendation:** Use **async-profiler** when profiling Spark queries with Comet enabled — it is the only tool that shows both JVM and native frames in a single flame graph. @@ -88,21 +88,21 @@ spark-submit \ ### Choosing an event type -| Event | When to use | -| --- | --- | -| `cpu` | Default. Shows where CPU cycles are spent. Use for compute-bound queries. | -| `wall` | Wall-clock time including blocked/waiting threads. Use to find JNI boundary overhead and I/O stalls. | +| Event | When to use | +| ------- | --------------------------------------------------------------------------------------------------------- | +| `cpu` | Default. Shows where CPU cycles are spent. Use for compute-bound queries. | +| `wall` | Wall-clock time including blocked/waiting threads. Use to find JNI boundary overhead and I/O stalls. | | `alloc` | Heap allocation profiling. Use to find JVM allocation hotspots around Arrow FFI and columnar conversions. | -| `lock` | Lock contention. Use when threads appear to spend time waiting on synchronized blocks or locks. | +| `lock` | Lock contention. Use when threads appear to spend time waiting on synchronized blocks or locks. | ### Output formats -| Format | Flag | Description | -| --- | --- | --- | -| HTML flame graph | `-f out.html` | Interactive flame graph (default and most useful). | -| JFR | `-f out.jfr` | Viewable in JDK Mission Control or IntelliJ. | -| Collapsed stacks | `-f out.collapsed` | For use with Brendan Gregg's FlameGraph scripts. | -| Text summary | `-o text` | Flat list of hot methods. | +| Format | Flag | Description | +| ---------------- | ------------------ | -------------------------------------------------- | +| HTML flame graph | `-f out.html` | Interactive flame graph (default and most useful). | +| JFR | `-f out.jfr` | Viewable in JDK Mission Control or IntelliJ. | +| Collapsed stacks | `-f out.collapsed` | For use with Brendan Gregg's FlameGraph scripts. | +| Text summary | `-o text` | Flat list of hot methods. | ### Platform notes @@ -163,13 +163,13 @@ jcmd JFR.stop name=profile filename=recording.jfr ### Useful JFR events for Comet debugging -| Event | What it shows | -| --- | --- | -| `jdk.GCPhasePause` | GC pause durations — helps identify memory pressure from Arrow allocations. | -| `jdk.ObjectAllocationInNewTLAB` / `jdk.ObjectAllocationOutsideTLAB` | Allocation hot spots. | -| `jdk.JavaMonitorWait` / `jdk.ThreadPark` | Thread contention and lock waits. | -| `jdk.FileRead` / `jdk.FileWrite` / `jdk.SocketRead` | I/O latency. | -| `jdk.ExecutionSample` | CPU sampling (method profiling, similar to a flame graph). | +| Event | What it shows | +| ------------------------------------------------------------------- | --------------------------------------------------------------------------- | +| `jdk.GCPhasePause` | GC pause durations — helps identify memory pressure from Arrow allocations. | +| `jdk.ObjectAllocationInNewTLAB` / `jdk.ObjectAllocationOutsideTLAB` | Allocation hot spots. | +| `jdk.JavaMonitorWait` / `jdk.ThreadPark` | Thread contention and lock waits. | +| `jdk.FileRead` / `jdk.FileWrite` / `jdk.SocketRead` | I/O latency. | +| `jdk.ExecutionSample` | CPU sampling (method profiling, similar to a flame graph). | ### Integrated benchmark profiling From 933ab7db0f087512d2d3c222700774f6be194047 Mon Sep 17 00:00:00 2001 From: Andy Grove Date: Sat, 7 Mar 2026 08:46:57 -0700 Subject: [PATCH 3/3] address feedback --- docs/source/contributor-guide/profiling.md | 17 ++++++++++------- 1 file changed, 10 insertions(+), 7 deletions(-) diff --git a/docs/source/contributor-guide/profiling.md b/docs/source/contributor-guide/profiling.md index 7f128937df..67729a235e 100644 --- a/docs/source/contributor-guide/profiling.md +++ b/docs/source/contributor-guide/profiling.md @@ -50,8 +50,9 @@ Download a release from the [async-profiler releases page](https://github.com/as ```shell # Linux x64 wget https://github.com/async-profiler/async-profiler/releases/download/v3.0/async-profiler-3.0-linux-x64.tar.gz -tar xzf async-profiler-3.0-linux-x64.tar.gz -C /opt/async-profiler --strip-components=1 -export ASYNC_PROFILER_HOME=/opt/async-profiler +mkdir -p $HOME/opt/async-profiler +tar xzf async-profiler-3.0-linux-x64.tar.gz -C $HOME/opt/async-profiler --strip-components=1 +export ASYNC_PROFILER_HOME=$HOME/opt/async-profiler ``` On macOS, download the appropriate `macos` archive instead. @@ -81,11 +82,13 @@ You can also attach async-profiler as a Java agent at JVM startup: ```shell spark-submit \ - --conf "spark.driver.extraJavaOptions=-agentpath:/opt/async-profiler/lib/libasyncProfiler.so=start,event=cpu,file=driver.html" \ - --conf "spark.executor.extraJavaOptions=-agentpath:/opt/async-profiler/lib/libasyncProfiler.so=start,event=cpu,file=executor.html" \ + --conf "spark.driver.extraJavaOptions=-agentpath:$ASYNC_PROFILER_HOME/lib/libasyncProfiler.so=start,event=cpu,file=driver.html,tree" \ + --conf "spark.executor.extraJavaOptions=-agentpath:$ASYNC_PROFILER_HOME/lib/libasyncProfiler.so=start,event=cpu,file=executor.html,tree" \ ... ``` +Note: If the executor is distributed then `executor.html` will be written on the remote node. + ### Choosing an event type | Event | When to use | @@ -120,7 +123,7 @@ resolved on macOS; Linux is recommended for the most complete flame graphs. ### Integrated benchmark profiling The TPC benchmark scripts in `benchmarks/tpc/` have built-in async-profiler support via -the `--async-profiler` flag. See [benchmarks/tpc/README.md](../../benchmarks/tpc/README.md) +the `--async-profiler` flag. See [benchmarks/tpc/README.md](https://github.com/apache/datafusion-comet/blob/main/benchmarks/tpc/README.md) for details. ## Profiling with Java Flight Recorder @@ -174,7 +177,7 @@ jcmd JFR.stop name=profile filename=recording.jfr ### Integrated benchmark profiling The TPC benchmark scripts support `--jfr` for automatic JFR recording during benchmark -runs. See [benchmarks/tpc/README.md](../../benchmarks/tpc/README.md) for details. +runs. See [benchmarks/tpc/README.md](https://github.com/apache/datafusion-comet/blob/main/benchmarks/tpc/README.md) for details. ## Profiling Native Code with cargo-flamegraph @@ -194,7 +197,7 @@ name = "shuffle_writer" harness = false ``` -These benchmarks are useful when for comparing performance between releases or between feature branches and the +These benchmarks are useful for comparing performance between releases or between feature branches and the main branch to help prevent regressions in performance when adding new features or fixing bugs. Individual benchmarks can be run by name with the following command.