richardstartin commented on issue #7866:
URL: https://github.com/apache/pinot/issues/7866#issuecomment-985992967
Hi @mqliang, there’s a problem with your benchmark: unless you present the
produced value to a blackhole (or return the value if there’s only one) the JIT
compiler may notice that the value is never used, and then eliminate all the
code used to produce the value. That is, the code you’re measuring may not
execute. There may be some a side effect in the invocation of the method which
prevents dead code elimination.
A good way to spot this is to ask yourself how many cycles must the measured
code have taken at your processor frequency. Assuming your processor frequency
is 4GHz, 0.5ns is 2 cycles, which isn’t enough to have made a even a userspace
function call to to read a clock.
You will get different results if you return the value:
@Benchmark
public long benchmarkThreadCpuTimer() {
ThreadTimer threadTimer = new ThreadTimer();
long totalThreadCpuTimeNs = threadTimer.getThreadTimeNs();
return totalThreadCpuTimeNs;
}
@Benchmark
public long benchmarkSystemCurrentTimeMillis() {
long startWallClockTimeMs = System.currentTimeMillis();
long totalWallClockTimeMs = System.currentTimeMillis() -
startWallClockTimeMs;
long totalWallClockTimeNs =
TimeUnit.MILLISECONDS.toNanos(totalWallClockTimeMs);
return totalWallClockTimeNs;
}
@Benchmark
public long benchmarkSystemNanoTime() {
long startWallClockTimeNs = System.nanoTime();
long totalWallClockTimeNs = System.nanoTime() - startWallClockTimeNs;
return startWallClockTimeNs;
}
System.currentTimeMills() and System.nanoTime() generally do *not* require a
syscall because they call in to clock_gettime(CLOCK_REALTIME) and
clock_gettime(CLOCK_MONOTONIC) respectively which have been in vDSO for a very
long time. When these clocks are userspace calls, you should expect latencies
of 20-30ns. When they are syscalls expect hundreds of nanoseconds. To see what
actual syscall overhead looks like, run the fixed benchmark on a virtualised
instance using a ‘xen’ clocksource and you will see the difference.
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