[IMPALA-2312] Timing bug in both MonotonicStopWatch and StopWatch

• Type: Bug
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: Impala 2.2.4
• Fix Version/s: Impala 3.4.0
• Component/s: Backend
• Target Version: Product Backlog
• Assignee: Tim Armstrong
• Reporter: Henry Robinson
• Created: 08/Sep/15 22:27
• Updated: 26/Sep/19 23:05
• Resolved: 26/Sep/19 23:05

Description

Both MonotonicStopWatch and StopWatch underestimate the total time if the stopwatch is running while ElapsedTime() is called. For example:

uint64_t ElapsedTime() const {
    if (!running_) return total_time_;
    timespec end;
    clock_gettime(CLOCK_MONOTONIC, &end);
    // Should include total_time_, but does not
    return (end.tv_sec - start_.tv_sec) * 1000L * 1000L * 1000L +
        (end.tv_nsec - start_.tv_nsec);
  }

The effect is that we could have:

MonotonicStopWatch sw;
sw.Start();
sw.Stop();
uint64_t total = sw.ElapsedTime();
sw.Start();
// With the bug, this could fail.
ASSERT_GE(sw.ElapsedTime(), total);