Re: micro-benchmarking
On Sat, 2 May 2009, Arne Vajh?j wrote:
Tom Anderson wrote:
On Fri, 1 May 2009, Arne Vajh?j wrote:
Mark Space wrote:
Arne Vajh?j wrote:
It does not matter. If milliseconds is not granular enough, then
the results are worthless on OS's like Windows and *nix.
currentTimeMillis(), on Windows at least, could use a very inaccurate
timer that doesn't even compute milliseconds correctly. It's always off
by like 18 milliseconds one way or the other (I'd have to look up the
exact value). nanoTime() isn't accurate to nanoseconds, but it's usually
accurate to at least 1 millisecond.
Sure.
But given the unpredictable scheduling of other processes, then you can
still not meaningful measure so small intervals.
Sure you can, you just have to do a lot of runs. This is why when i do
benchmarks, i do 100 runs, and ignore the five slowest, on the grounds that
they probably took so long because either another process was scheduled in
the middle, or GC kicked in or something. For this to be effective, you
actually want to keep the individual runs quite short, to minimise the
chance of them being disrupted, and do a huge number of them, and for this,
a high-resolution timer is essential.
1) That is actually very close to what I suggest. You are just making
it rather cumbersome for yourself by having many short runs and
add instead of a few long runs.
No, the whole point is that more runs gives you more numbers to do
statistics on, which gives you a better chance to detect and discard
outliers.
2) And you don't want to ignore GC time. Because GC will also hit the
real app.
It will. But if we're talking about microbenchmarking, i don't think you
necessarily want to measure it. If the problem is one which itself
involves allocating memory, then yes, you do, but if not, i think you
don't. If you're doing macrobenchmarking, then yes, you do want to measure
it, of course.
tom
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