Bizarre profiler results

[Chris]

I'm using a CPU profiler to find the bottlenecks in my app. When I run my
app normally, a particularly time-critical part takes 58 seconds to run.
When I use a CPU profiler on it, and set the profiling method to "samples",
the execution time is 40 seconds.

That's right -- it runs faster with the profiler going in the background.
This makes no sense.

Perhaps someone can speculate as to why this is happening.

I had thought that perhaps the profiler was messing with the system clock
and it wasn't *really* going faster, but I timed it with a wall clock.
Nope -- it's really faster.

I had thought that it was an IDE thing. Nope -- if I run the app on the
command line, it's slow, and if I add the -Xrunhprof:cpu=samples option to
the command line, it runs fast.

I'm using JDK 1.3.1

 

[Jon Skeet]

I'm using a CPU profiler to find the bottlenecks in my app. When I run my
app normally, a particularly time-critical part takes 58 seconds to run.
When I use a CPU profiler on it, and set the profiling method to "samples",
the execution time is 40 seconds.

That's right -- it runs faster with the profiler going in the background.
This makes no sense.

Perhaps someone can speculate as to why this is happening.

Completely wild speculation: perhaps running in the profiler turns off
some JIT compiler optimisations which happen to unfortunately be
"pessimisations" on your code.

 

[Daniel Dyer]

I'm using a CPU profiler to find the bottlenecks in my app. When I run my
app normally, a particularly time-critical part takes 58 seconds to run.
When I use a CPU profiler on it, and set the profiling method to
"samples",
the execution time is 40 seconds.

That's right -- it runs faster with the profiler going in the background.
This makes no sense.

Perhaps someone can speculate as to why this is happening.

I had thought that perhaps the profiler was messing with the system clock
and it wasn't *really* going faster, but I timed it with a wall clock.
Nope -- it's really faster.

I had thought that it was an IDE thing. Nope -- if I run the app on the
command line, it's slow, and if I add the -Xrunhprof:cpu=samples option
to
the command line, it runs fast.

I'm using JDK 1.3.1

I have no idea, but it certainly sounds interesting. What is your code
doing (is it CPU-bound or IO-bound)? What platform are you using? Is it a
multi-processor machine? What other switches are you using with Java?


Dan.

 

[Roedy Green]

I'm using a CPU profiler to find the bottlenecks in my app. When I run my
app normally, a particularly time-critical part takes 58 seconds to run.
When I use a CPU profiler on it, and set the profiling method to "samples",
the execution time is 40 seconds.

It is a well known effect that employees are more productive when the
boss is watching.

-)