Messages in this thread | | | Subject | Re: [PATCH] x86: Turn off GCC branch probability heuristics | From | Mike Galbraith <> | Date | Sun, 12 Apr 2015 09:56:11 +0200 |
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On Sun, 2015-04-12 at 07:47 +0200, Ingo Molnar wrote: > * Linus Torvalds <torvalds@linux-foundation.org> wrote: > > > On Sat, Apr 11, 2015 at 11:57 AM, Thomas Gleixner < > > tglx@linutronix.de> wrote: > > > > > > 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% > > > > Yeah, a 5% code expansion is a big deal. Sadly, it looks like > > 'no-guess' also disables our explicit likely/unlikely handling. > > So I spent some time trying to get as much code size reduction as > possible via GCC optimization options, and the total savings > possible > are 10.1%: > > text data bss dec filename > 12566391 1617840 1089536 15273767 vmlinux.vanilla > 11416805 1617840 1089536 14124181 vmlinux.combo > 10532552 1596080 1089536 13218168 vmlinux.Os > > (combo patch attached below.) > > The -Os savings are 19% total - but as you mentioned before, it's > sometimes achieved through unacceptable techniques. > > Unfortunately I found no other GCC options to achieve what -Os does - > > the missing 9% can purely be achieved via -Os, with no cherry- > picking > possible. > > The other, smaller savings are: > > + # Reduces vmlinux size by 0.25%: > + KBUILD_CFLAGS += -fno-caller-saves > + > + # Reduces vmlinux size by 1.10%: > + KBUILD_CFLAGS += -fno-inline-small-functions > + > + # Reduces vmlinux size by about 0.95%: > + KBUILD_CFLAGS += -fno-tree-ch > > (each of them has to be double checked to make sure it leads to > nothing silly and unacceptable - I just blindly tried to find GCC > options that impacted kernel code size.)
Ew, my i4790 really hated this patch.
taskset 0xc pipe-test 1 avg 733.2 727.1 743.1 746.6 737.1 737 KHz 1.000 -gcc_twiddle 713.7 717.9 715.5 718.0 708.7 714 KHz .968 +gcc_twiddle
tbench.sh 8 30
3566.14 3560.91 3566.35 3556.57 3549.69 3559.93 MB/S 1.000 -gcc_twiddle 2862.18 2899.51 2888.74 2897.18 2878.63 2885.25 MB/S .810 +gcc_twiddle
-Mike
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