Auto merge of rust-lang#2816 - oli-obk:💤, r=RalfJung

Update the virtual clock in isolation mode to step forward with around the same speed as the host system.

Before this, the 1s sleep test took around 4 minutes on my machine.

Author: bors

src/tools/miri/src/clock.rs

@@ -3,7 +3,10 @@ use std::time::{Duration, Instant as StdInstant};
 
 /// When using a virtual clock, this defines how many nanoseconds we pretend are passing for each
 /// basic block.
-const NANOSECONDS_PER_BASIC_BLOCK: u64 = 10;
+/// This number is pretty random, but it has been shown to approximately cause
+/// some sample programs to run within an order of magnitude of real time on desktop CPUs.
+/// (See `tests/pass/shims/time-with-isolation*.rs`.)
+const NANOSECONDS_PER_BASIC_BLOCK: u64 = 5000;
 
 #[derive(Debug)]
 pub struct Instant {

src/tools/miri/tests/pass/concurrency/thread_park_isolated.rs

@@ -7,6 +7,6 @@ fn main() {
 
     thread::park_timeout(Duration::from_millis(200));
 
-    // Thanks to deterministic execution, this will wiat *exactly* 200ms (rounded to 1ms).
-    assert!((200..201).contains(&start.elapsed().as_millis()));
+    // Thanks to deterministic execution, this will wait *exactly* 200ms, plus the time for the surrounding code.
+    assert!((200..210).contains(&start.elapsed().as_millis()), "{}", start.elapsed().as_millis());
 }

src/tools/miri/tests/pass/shims/time-with-isolation.rs

@@ -22,14 +22,23 @@ fn test_time_passes() {
     let diff = now2.duration_since(now1);
     assert_eq!(now1 + diff, now2);
     assert_eq!(now2 - diff, now1);
-    // The virtual clock is deterministic and I got 29us on a 64-bit Linux machine. However, this
+    // The virtual clock is deterministic and I got 15ms on a 64-bit Linux machine. However, this
     // changes according to the platform so we use an interval to be safe. This should be updated
     // if `NANOSECONDS_PER_BASIC_BLOCK` changes.
-    assert!(diff.as_micros() > 10);
-    assert!(diff.as_micros() < 40);
+    assert!(diff.as_millis() > 5);
+    assert!(diff.as_millis() < 20);
+}
+
+fn test_block_for_one_second() {
+    let end = Instant::now() + Duration::from_secs(1);
+    // This takes a long time, as we only increment when statements are executed.
+    // When we sleep on all threads, we fast forward to the sleep duration, which we can't
+    // do with busy waiting.
+    while Instant::now() < end {}
 }
 
 fn main() {
     test_time_passes();
+    test_block_for_one_second();
     test_sleep();
 }

src/tools/miri/tests/pass/shims/time-with-isolation2.rs

@@ -0,0 +1,8 @@
+use std::time::Instant;
+
+fn main() {
+    let begin = Instant::now();
+    for _ in 0..100_000 {}
+    let time = begin.elapsed();
+    println!("The loop took around {}s", time.as_secs());
+}

src/tools/miri/tests/pass/shims/time-with-isolation2.stdout

@@ -0,0 +1 @@
+The loop took around 13s