Messages in this thread | | | Date | Sun, 12 Apr 2015 10:07:04 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86: Turn off GCC branch probability heuristics |
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* Thomas Gleixner <tglx@linutronix.de> wrote:
> > How much of that 5% comes from code alignment? Or was this on > > *top* of the 1-byte alignment testt? > > I thinks its just the no-guess one: > > text data dec patch reduction > > 7563475 1781048 10302987 > > 7192973 1780024 9931461 no-guess -4.8% > > 7354819 1781048 958464 align-1 -2.7% > > 7192973 1780024 9931461 no-guess + align-1 -4.8% > > So with the no-guess applied the align-1 does not matter anymore.
What kernel config and GCC version did you use?
I used x86-64 defconfig+kvmconfig with GCC 4.9.1, and it gave me:
text filename reduction 12566383 vmlinux.expect=10 [==vanilla] 11923529 vmlinux.-fno-guess-branch-probability -5.4% 11903663 vmlinux.align=1 -5.6% 11646102 vmlinux.align=1+fno-guess-branch-probability -7.9%
So there's still appears to be an additional -2.5% to be gained from turning off GCC branch heuristics.
x86 defconfig was derived from a distro kernel config and is still pretty close to what distros typically enable, so it's a good 'typical' config to use for testing.
To double check that assumption I also tested a distro kernel .config and looked at vmlinux sizes:
text filename reduction 12612201 vmlinux.expect=10 [==vanilla] 12107614 vmlinux.-fno-guess-branch-probability -4.1% 12021817 vmlinux.align=1 -4.9% 11846771 vmlinux.align=1+fno-guess-branch-probability -6.5%
this was cloned from a different major Linux distro than the one the original x86 defconfig was derived from - still the vmlinux sizes are pretty similar.
So x86 'make defconfig' measurements are pretty representative if you are looking for a quick, independent way to measure 'typical Linux distro' kernel characteristics.
( The only bigger difference is that FUNCTION_TRACER was turned on in the distro config - which bloats function prologues a bit and thus reduces the relative savings a bit. )
Thanks,
Ingo
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