This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
RE: Gprof can account for less than 1/3 of execution time?!?!
Hi Jon,
What does ldd says about your executable?
Thanks
++Cyrille
-----Original Message-----
From: gcc-owner@gcc.gnu.org [mailto:gcc-owner@gcc.gnu.org] On Behalf Of Jon Turner
Sent: Monday, February 22, 2010 3:44 PM
To: Joern Rennecke
Cc: Michael Matz; gcc@gcc.gnu.org
Subject: Re: Gprof can account for less than 1/3 of execution time?!?!
You're not listening. I am using -pg and the program is statically
linked. The concern I am raising is not about the function counting,
but the reported running times, which are determined by sampling
(read the gprof manual, if this is news to you).
In this case, the mcount overhead cannot account for the discrepancy,
since that would cause gprof to OVER-estimate the run time, while
in this case it is UNDER-estimating. It's missing about 70% of the
actual running time in the program.
It conceivably I am doing something wrong. I hope so, since
once I know what it is, I can fix it. But at the moment,
it's hard to avoid the suspicion that something about the
gprof implementation is deeply flawed.
Jon
Joern Rennecke wrote:
> Quoting Michael Matz <matz@suse.de>:
>
>> Hi,
>>
>> On Sun, 21 Feb 2010, Jon Turner wrote:
>>
>>> I have recently encountered a gross inaccuracy in gprof that
>>> I can't explain. Yes, I know gprof uses a sampling technique
>>
>> This is incorrect. Code compiled with -pg will call mcount on each
>> function entry. If there are many calls (compared to other computations)
>> the mcount overhead might become fairly large.
>
> The mcount overhead actually depends on the machine description, although
> most ports have standardized on a very runtime profligate scheme.