On Wed, Aug 17, 2011 at 8:35 AM, Mike Hommey <mhom...@mozilla.com> wrote: > On Thu, Aug 11, 2011 at 09:27:23AM -0700, Xinliang David Li wrote: >> > Maybe I have an idea as to why FDO doesn't work so well. Does the >> > instrumentation code support running several times in parallel (as in, >> > several processes with the instrumented code running concurrently)? >> >> gcc supports profile merging from multiple runs -- but there is no >> synchronization on profile file update, but it will become a problem >> only when all the concurrently running processes are existing at the >> same time (and dumping profile data to the same gcda file at the same >> time). Similarly, there are data races in profile counter update for >> multi-threaded programs, in practice it is not a big issue >> (-fprofile-correction option needs to be turned on in profile-use). > > Are there known quirks with threads and dlopen() ?
threads causes data races in counter update, but the problem is minor in practice. dlopen should also be fine. libgcov is linked in statically into each shared library, so dlopen/dlclose won't mess up other shared lib or main executable's module list. > > To avoid the problem mentioned above, I configured Firefox to use only > one process. But in that setup, I only get partial instrumentation data. > For example, I get instrumentation data for cairo, but not for our > layout engine. I do however get layout engine instrumentation data with > the same build if I enable multiple processes (but then I hit the > problem that multiple processes updating the same profile data is not > properly supported). Are they exiting at the same time? If not, it should be fine. >The main difference between the main and the child > process is that the main process starts as a dalvik instance, dlopen()s > native code, which itself dlopen()s all our libraries. The child process > is just an executable that is linked against all these libraries. > If the child process's life time is properly controlled, looks like it can be made to work. David > Cheers, > > Mike >
