benches: Add support to generate flamegraphs when we profile our benchmarks

matrix_sdk_crypto/Cargo.toml

@@ -52,6 +52,7 @@ http = "0.2.3"
 matrix-sdk-test = { version = "0.2.0", path = "../matrix_sdk_test" }
 indoc = "1.0.3"
 criterion = { version = "0.3.4", features = ["async", "async_futures", "html_reports"] }
+pprof = { version = "0.4.2", features = ["flamegraph"] }
 
 [[bench]]
 name = "crypto_bench"

matrix_sdk_crypto/benches/crypto_bench.rs

@@ -1,9 +1,8 @@
+mod perf;
+
 use std::convert::TryFrom;
 
-use criterion::{
-    async_executor::FuturesExecutor, criterion_group, criterion_main, BatchSize, BenchmarkId,
-    Criterion, Throughput,
-};
+use criterion::{async_executor::FuturesExecutor, *};
 
 use futures::executor::block_on;
 use matrix_sdk_common::{
@@ -101,5 +100,13 @@ pub fn keys_claiming(c: &mut Criterion) {
     group.finish()
 }
 
-criterion_group!(benches, keys_query, keys_claiming);
+fn criterion() -> Criterion {
+    Criterion::default().with_profiler(perf::FlamegraphProfiler::new(100))
+}
+
+criterion_group! {
+    name = benches;
+    config = criterion();
+    targets = keys_query, keys_claiming
+}
 criterion_main!(benches);

matrix_sdk_crypto/benches/perf.rs

@@ -0,0 +1,78 @@
+//! This is a simple Criterion Profiler implementation using pprof.
+//!
+//! It's mostly a direct copy from here: https://www.jibbow.com/posts/criterion-flamegraphs/
+use std::{fs::File, os::raw::c_int, path::Path};
+
+use criterion::profiler::Profiler;
+use pprof::ProfilerGuard;
+
+/// Small custom profiler that can be used with Criterion to create a flamegraph for benchmarks.
+/// Also see [the Criterion documentation on this][custom-profiler].
+///
+/// ## Example on how to enable the custom profiler:
+///
+/// ```
+/// mod perf;
+/// use perf::FlamegraphProfiler;
+///
+/// fn fibonacci_profiled(criterion: &mut Criterion) {
+///     // Use the criterion struct as normal here.
+/// }
+///
+/// fn custom() -> Criterion {
+///     Criterion::default().with_profiler(FlamegraphProfiler::new())
+/// }
+///
+/// criterion_group! {
+///     name = benches;
+///     config = custom();
+///     targets = fibonacci_profiled
+/// }
+/// ```
+///
+/// The neat thing about this is that it will sample _only_ the benchmark, and not other stuff like
+/// the setup process.
+///
+/// Further, it will only kick in if `--profile-time <time>` is passed to the benchmark binary.
+/// A flamegraph will be created for each individual benchmark in its report directory under
+/// `profile/flamegraph.svg`.
+///
+/// [custom-profiler]: https://bheisler.github.io/criterion.rs/book/user_guide/profiling.html#implementing-in-process-profiling-hooks
+pub struct FlamegraphProfiler<'a> {
+    frequency: c_int,
+    active_profiler: Option<ProfilerGuard<'a>>,
+}
+
+impl<'a> FlamegraphProfiler<'a> {
+    pub fn new(frequency: c_int) -> Self {
+        FlamegraphProfiler {
+            frequency,
+            active_profiler: None,
+        }
+    }
+}
+
+impl<'a> Profiler for FlamegraphProfiler<'a> {
+    fn start_profiling(&mut self, _benchmark_id: &str, _benchmark_dir: &Path) {
+        self.active_profiler = Some(ProfilerGuard::new(self.frequency).unwrap());
+    }
+
+    fn stop_profiling(&mut self, _benchmark_id: &str, benchmark_dir: &Path) {
+        std::fs::create_dir_all(benchmark_dir)
+            .expect("Can't create a directory to store the benchmarking report");
+
+        let flamegraph_path = benchmark_dir.join("flamegraph.svg");
+
+        let flamegraph_file = File::create(&flamegraph_path)
+            .expect("File system error while creating flamegraph.svg");
+
+        if let Some(profiler) = self.active_profiler.take() {
+            profiler
+                .report()
+                .build()
+                .expect("Can't build profiling report")
+                .flamegraph(flamegraph_file)
+                .expect("Error writing flamegraph");
+        }
+    }
+}