On Sat, Mar 19, 2016 at 09:06:26AM +0100, Alexandre Ratchov wrote:
> The browsers problems seem caused by the way pthreads behave;
> browsers appear to spin. With the proposed scheduler they spin
> less. But the real question is why they spin at all?
Inspired by this comment, I set out to see if I could find *where* the browsers
are spinning. "Just build the browsers with profiling instrumentation and look
where the time goes when playing a youtube video" I thought. "Easy" I thought.
Well no. We build our chrome and firefox with clang, which doesn't
support gprof style profiling.
Clang does have this -fprofile-instr-generate flag, which throws out some
profiling data at runtime, but it appears it is designed to be used by the
compiler itself as compile-time optimisation hints[1]. It's not even clear if
there is any timing data in there.
There's another clang profiling mode which depends upon Linux perf, which is
obviously not an option for us.
Nevertheless, I decided to try it on the off-chance that clang's profiling data
could be useful (and I'm totally accepting that, if it is, I will probably have
to write a python script to make sense of the output). Sadly I stumbled at the
first hurdle:
---8<---
$ cat a.c
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char **argv)
{
return (EXIT_SUCCESS);
}
$clang -fprofile-instr-generate a.c -lclang
/usr/local/lib/libclang.so.2.0: warning: warning: sprintf() is often misused,
please use snprintf()
/tmp/a-f91c15.o: In function `__llvm_profile_register_functions':
a.c:(.text+0x3c): undefined reference to `__llvm_profile_register_function'
/tmp/a-f91c15.o: In function `__llvm_profile_runtime_user':
a.c:(.text+0x53): undefined reference to `__llvm_profile_runtime'
clang-3.7: error: linker command failed with exit code 1 (use -v to see
invocation)
--->8---
Any clang afficionados know what is missing here? Or is there a better way to
find the spinning code? There has to be a better way.
A potential option would be to port and use the profiling portion of [2].
[1] http://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization
[2] https://github.com/gperftools/gperftools
--
Best Regards
Edd Barrett
http://www.theunixzoo.co.uk
