Merge pull request #1465 from rust-lang/runtime-measurement-precision

Split measurement of walltime and perf. counters

Author: Kobzol

collector/benchlib/src/benchmark.rs

@@ -34,17 +34,13 @@ impl BenchmarkSuite {
 
     /// Registers a single benchmark.
     /// `func` should return a closure that will be benchmarked.
-    pub fn register<F: Fn() -> Bench + 'static, R, Bench: FnOnce() -> R + 'static>(
+    pub fn register<F: Fn() -> Bench + Clone + 'static, R, Bench: FnOnce() -> R + 'static>(
         &mut self,
         name: &'static str,
-        func: F,
+        constructor: F,
     ) {
-        // We want to monomorphize the target `func` and then wrap it in a Box, to avoid going
-        // through a vtable when we execute the benchmarked function.
-        let benchmark_func = Box::new(move || {
-            let bench_fn = func();
-            benchmark_function(name, bench_fn)
-        });
+        // We want to type-erase the target `func` by wrapping it in a Box.
+        let benchmark_func = Box::new(move || benchmark_function(name, constructor.clone()));
         let benchmark_def = BenchmarkWrapper {
             func: benchmark_func,
         };

collector/benchlib/src/measure/perf_counter/unix.rs

@@ -12,33 +12,42 @@ struct Counters {
     cache_references: Counter,
 }
 
-/// Benchmarks a single function.
-pub fn benchmark_function<F: FnOnce() -> R, R>(
+/// Benchmarks a single function generated by `benchmark_constructor`.
+/// The function is executed twice, once to gather wall-time measurement and the second time to
+/// gather perf. counters.
+pub fn benchmark_function<F: Fn() -> Bench + 'static, R, Bench: FnOnce() -> R + 'static>(
     name: &'static str,
-    func: F,
+    benchmark_constructor: F,
 ) -> anyhow::Result<BenchmarkResult> {
     let mut group = create_group()?;
     let counters = prepare_counters(&mut group)?;
 
-    // FIXME: don't run perf counters and time measurements together, run the benchmark twice
-    // instead.
-    let start = Instant::now();
+    // Measure perf. counters.
+    let func = benchmark_constructor();
 
     // Do not act on the return value to avoid including the branch in the measurement
     let enable_ret = group.enable();
     let output = func();
     group.disable()?;
 
-    let duration = start.elapsed();
+    // Try to avoid optimizing the result out.
+    black_box(output);
 
     // Check if we have succeeded before
     enable_ret?;
 
-    // Try to avoid optimizing the result out
-    black_box(output);
-
     let measurement = group.read()?;
 
+    // Measure wall time.
+    let func = benchmark_constructor();
+
+    let start = Instant::now();
+    let output = func();
+    let duration = start.elapsed();
+
+    // Try to avoid optimizing the result out.
+    black_box(output);
+
     let result = BenchmarkResult {
         name: String::from(name),
         cycles: measurement[&counters.cycles],