( Sorry about the delay in answering this. I could blame the delay on the merge
window, but in reality I've been procrastinating this is due to the permanent,
non-trivial impact PIE has on generated C code. )
* Thomas Garnier <thgar...@google.com> wrote:
> 1) PIE sometime needs two instructions to represent a single
> instruction on mcmodel=kernel.
What again is the typical frequency of this occurring in an x86-64 defconfig
kernel, with the very latest GCC?
Also, to make sure: which unwinder did you use for your measurements,
frame-pointers or ORC? Please use ORC only for future numbers, as
frame-pointers is obsolete from a performance measurement POV.
> 2) GCC does not optimize switches in PIE in order to reduce relocations:
Hopefully this can either be fixed in GCC or at least influenced via a compiler
switch in the future.
> The switches are the biggest increase on small functions but I don't
> think they represent a large portion of the difference (number 1 is).
Ok.
> A side note, while testing gcc 7.2.0 on hackbench I have seen the PIE
> kernel being faster by 1% across multiple runs (comparing 50 runs done
> across 5 reboots twice). I don't think PIE is faster than a
> mcmodel=kernel but recent versions of gcc makes them fairly similar.
So I think we are down to an overhead range where the inherent noise (both
random
and systematic one) in 'hackbench' overwhelms the signal we are trying to
measure.
So I think it's the kernel .text size change that is the best noise-free proxy
for
the overhead impact of PIE.
It doesn't hurt to double check actual real performance as well, just don't
expect
there to be much of a signal for anything but fully cached microbenchmark
workloads.
Thanks,
Ingo
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