add docs

benches/thread_eager_init.rs

@@ -4,8 +4,12 @@ use std::time::{Duration, Instant};
 use wasmtime::*;
 
 fn measure_execution_time(c: &mut Criterion) {
+    // Baseline performance: a single measurment covers both initializing
+    // thread local resources and executing the first call.
+    //
+    // The other two bench functions should sum to this duration.
     c.bench_function("lazy initialization at call", move |b| {
-        let (engine, module) = test_engine();
+        let (engine, module) = test_setup();
         b.iter_custom(move |iters| {
             (0..iters)
                 .into_iter()
@@ -14,8 +18,10 @@ fn measure_execution_time(c: &mut Criterion) {
         })
     });
 
+    // Using Engine::tls_eager_initialize: measure how long eager
+    // initialization takes on a new thread.
     c.bench_function("eager initialization", move |b| {
-        let (engine, module) = test_engine();
+        let (engine, module) = test_setup();
         b.iter_custom(move |iters| {
             (0..iters)
                 .into_iter()
@@ -26,8 +32,11 @@ fn measure_execution_time(c: &mut Criterion) {
                 .sum()
         })
     });
+
+    // Measure how long the first call takes on a thread after it has been
+    // eagerly initialized.
     c.bench_function("call after eager initialization", move |b| {
-        let (engine, module) = test_engine();
+        let (engine, module) = test_setup();
         b.iter_custom(move |iters| {
             (0..iters)
                 .into_iter()
@@ -40,8 +49,47 @@ fn measure_execution_time(c: &mut Criterion) {
     });
 }
 
-fn test_engine() -> (Engine, Module) {
-    let pool_count = 1000;
+/// Creating a store and measuring the time to perform a call is the same behavior
+/// in both setups.
+fn duration_of_call(engine: &Engine, module: &Module) -> Duration {
+    let mut store = Store::new(engine, ());
+    let inst = Instance::new(&mut store, module, &[]).expect("instantiate");
+    let f = inst.get_func(&mut store, "f").expect("get f");
+    let f = f.typed::<(), (), _>(&store).expect("type f");
+
+    let call = Instant::now();
+    f.call(&mut store, ()).expect("call f");
+    call.elapsed()
+}
+
+/// When wasmtime first runs a function on a thread, it needs to initialize
+/// some thread-local resources and install signal handlers. This benchmark
+/// spawns a new thread, and returns the duration it took to execute the first
+/// function call made on that thread.
+fn lazy_thread_instantiate(engine: Engine, module: Module) -> Duration {
+    thread::spawn(move || duration_of_call(&engine, &module))
+        .join()
+        .expect("thread joins")
+}
+/// This benchmark spawns a new thread, and records the duration to eagerly
+/// initializes the thread local resources. It then creates a store and
+/// instance, and records the duration it took to execute the first function
+/// call.
+fn eager_thread_instantiate(engine: Engine, module: Module) -> (Duration, Duration) {
+    thread::spawn(move || {
+        let init_start = Instant::now();
+        Engine::tls_eager_initialize().expect("eager init");
+        let init_duration = init_start.elapsed();
+
+        (init_duration, duration_of_call(&engine, &module))
+    })
+    .join()
+    .expect("thread joins")
+}
+
+fn test_setup() -> (Engine, Module) {
+    // We only expect to create one Instance at a time, with a single memory.
+    let pool_count = 10;
 
     let mut config = Config::new();
     config.allocation_strategy(InstanceAllocationStrategy::Pooling {
@@ -55,45 +103,12 @@ fn test_engine() -> (Engine, Module) {
             memory_reservation_size: 1,
         },
     });
-
     let engine = Engine::new(&config).unwrap();
+
+    // The module has a memory (shouldn't matter) and a single function which is a no-op.
     let module = Module::new(&engine, r#"(module (memory 1) (func (export "f")))"#).unwrap();
     (engine, module)
 }
 
-fn lazy_thread_instantiate(engine: Engine, module: Module) -> Duration {
-    thread::spawn(move || {
-        let mut store = Store::new(&engine, ());
-        let inst = Instance::new(&mut store, &module, &[]).expect("instantiate");
-        let f = inst.get_func(&mut store, "f").expect("get f");
-        let f = f.typed::<(), (), _>(&store).expect("type f");
-
-        let call = Instant::now();
-        f.call(&mut store, ()).expect("call f");
-        call.elapsed()
-    })
-    .join()
-    .expect("thread joins")
-}
-
-fn eager_thread_instantiate(engine: Engine, module: Module) -> (Duration, Duration) {
-    thread::spawn(move || {
-        let init_start = Instant::now();
-        Engine::tls_eager_initialize().expect("eager init");
-        let init_duration = init_start.elapsed();
-
-        let mut store = Store::new(&engine, ());
-        let inst = Instance::new(&mut store, &module, &[]).expect("instantiate");
-        let f = inst.get_func(&mut store, "f").expect("get f");
-        let f = f.typed::<(), (), _>(&store).expect("type f");
-
-        let call = Instant::now();
-        f.call(&mut store, ()).expect("call f");
-        (init_duration, call.elapsed())
-    })
-    .join()
-    .expect("thread joins")
-}
-
 criterion_group!(benches, measure_execution_time);
 criterion_main!(benches);