Re: [DynInst_API:] DynInst Overhead

[Buddhika Chamith Kahawitage Don <budkahaw@xxxxxxxxxxxx>]

Please find my responses inline.

On Mon, Jul 21, 2014 at 1:48 PM, Bill Williams <bill@xxxxxxxxxxx> wrote:

On 07/21/2014 11:52 AM, Matthew LeGendre wrote:

Presumably you're running the CodeCoverage tool in two steps: 1)
Rewriting the binary 2) Running the rewritten binary. ÂAll of the
analysis/rewriting overheads are in step 1, and the instrumentation
overhead can be measured just by timing step 2.

That's true.
Â

If you're getting 50x overhead on just step 2 then something's very
wrong. I've got my own codeCoverage tool (which I unfortunately can't
share yet) and I only see 10% overhead.

Hrm. If this is with a prebuilt, statically linked binary and not with a build from source against current Dyninst, we may also be hitting traps in an inner loop. That's more the right order of magnitude than trampguards would be--trampguards would be in the 1.5-5x sort of neighborhood off the top of my head.

In fact that was the use case I had in my mind. But I was just checking the static rewriting case first up since it was readily available with code-coverage tool.

The source for CodeCoverage (which you can build against the latest Dyninst and be reasonably sure of *not* hitting traps in almost all of SPEC) is in our tools.git repository. I know we've fixed some performance regressions that turned up between the AWAT paper and 8.1.2.

I am using dyninst 8.1.2 which I built from source.

Just an educated guess--I frequently see big overheads associated with
trampoline guards. ÂDyninst should have realized trampoline guards are
unnecessary for codeCoverage and not emited them. ÂBut if something went
wrong you can manually turn them off by putting a call to:

 Âbpatch.setTrampRecursive(true);

Tried it without any success :(
Â

Near the top of codeCoverage.C's main() function. ÂIf that makes a
difference then let the list know. ÂThat implies there's a bug that
should be investigated.

Any ideas on how to debug this?

Thanks

Bud