Re: gcov in multithreaded applications

[Dr Adrian Wrigley <amtw@linuxchip.demon.co.uk>]

daelen@my-deja.com wrote:
> 
> I am working with Gnat 3.13a on Windows NT 4.0.
> 
> I would like to use gcov to profile our application containing several tasks.

If I my experience profiling code with multiple tasks is virtually futile.

At least on the Gnat 3.11/Linux on Intel architecture, the basic block
counting code is not thread-safe :(  This stops reliable block profiling
with gcov under gcc.  I was using native (Linux) threads.

Further, the function/procedure counting was not thread safe either
(the mcount does not protect the count).  This leads to spurious
values in a "gprof" profile of function counts/timing.

I was unable to resolve the problem under support from ACT, as it
seems to be a problem with the "mcount" in the profiling library,
and gcov style block profiling doesn't seem to be supported.

I had some success with my own thread-safe alternative to "mcount",
but the performance drag was very significant (worse than 3x for
heavily nested calling).  Also, some success enabling profiling
dynamically on a per function basis, by one thread at a time.

All this is from memory of over a year ago.  Things may have changed.
I may have misremembered. Profiling on NT may have fixed the gcov bug.

Did you get the results you wanted, Jorg?  Were they reliable?
--
Adrian Wrigley.