Re: x86/asm: __clear_user() micro-optimization (was: "Re: [GIT PULL] x86/asm changes for v4.18")
From: Alexey Dobriyan
Date: Tue Jun 05 2018 - 18:42:01 EST
On Tue, Jun 05, 2018 at 10:32:55AM -0700, Linus Torvalds wrote:
> On Tue, Jun 5, 2018 at 10:22 AM Alexey Dobriyan <adobriyan@xxxxxxxxx> wrote:
> >
> > Tested? :^) I had P4 maybe ~15(?) years ago.
>
> Did you EVEN test it on what you have today?
>
> Do you have any numbers at all, in other words?
>
> Micro-optimizations need numbers. Otherwise they aren't
> micro-optimizations, they are just "change code randomly".
On my potato performance increase is 33%, sheesh.
And CPU starts doing 3 instructions per cycle vs 2.
benchmark is "clear_user(p + 4096 - 4068, 4068)"
4068 comes from booting Debian 8 with printk.
f0(4068) (old clear_user)
--------
$ taskset -c 15 perf stat -r 16 ./a.out
Performance counter stats for './a.out' (16 runs):
2033.189084 task-clock (msec) # 1.000 CPUs utilized ( +- 0.41% )
2 context-switches # 0.001 K/sec ( +- 11.11% )
0 cpu-migrations # 0.000 K/sec
46 page-faults # 0.023 K/sec ( +- 0.91% )
4,268,425,486 cycles # 2.099 GHz ( +- 0.41% )
8,672,326,256 instructions # 2.03 insn per cycle ( +- 0.00% )
2,169,900,710 branches # 1067.240 M/sec ( +- 0.00% )
4,226,258 branch-misses # 0.19% of all branches ( +- 0.01% )
2.033700109 seconds time elapsed ( +- 0.41% )
f1(4068) (new clear_user)
$ taskset -c 15 perf stat -r 16 ./a.out
Performance counter stats for './a.out' (16 runs):
1345.149992 task-clock (msec) # 1.000 CPUs utilized ( +- 0.01% )
2 context-switches # 0.002 K/sec ( +- 8.35% )
0 cpu-migrations # 0.000 K/sec
46 page-faults # 0.034 K/sec ( +- 0.82% )
2,823,965,728 cycles # 2.099 GHz ( +- 0.01% )
8,661,733,733 instructions # 3.07 insn per cycle ( +- 0.00% )
2,169,437,410 branches # 1612.785 M/sec ( +- 0.00% )
4,216,469 branch-misses # 0.19% of all branches ( +- 0.01% )
1.345375114 seconds time elapsed ( +- 0.01% )
-------------------------------------
CFLAGS = -Wall -fno-strict-aliasing -fno-common -fshort-wchar -std=gnu89 -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -m64 -falign-jumps=1 -falign-loops=1 -mno-80387 -mno-fp-ret-in-387 -mpreferred-stack-boundary=3 -mskip-rax-setup -mtune=generic -mno-red-zone -funit-at-a-time -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -fno-delete-null-pointer-checks -O2 --param=allow-store-data-races=0 -fno-stack-protector -fomit-frame-pointer -fno-var-tracking-assignments -g -femit-struct-debug-baseonly -fno-var-tracking -fno-strict-overflow -fno-merge-all-constants -fmerge-constants -fno-stack-check -fconserve-stack
0000000000000780 <f0>:
780: mov rax,rsi
783: mov rcx,rsi
786: xor edx,edx
788: and eax,0x7
78b: shr rcx,0x3
78f: mov esi,0x8
794: test rcx,rcx
797: je 7a3 <f0+0x23>
799: mov QWORD PTR [rdi],rdx
79c: add rdi,rsi
79f: dec ecx
7a1: jne 799 <f0+0x19>
7a3: mov rcx,rax
7a6: test ecx,ecx
7a8: je 7b3 <f0+0x33>
7aa: mov BYTE PTR [rdi],dl
7ac: inc rdi
7af: dec ecx
7b1: jne 7aa <f0+0x2a>
7b3: mov rax,rcx
7b6: ret
00000000000007c0 <f1>:
7c0: mov rax,rsi
7c3: shr rsi,0x3
7c7: and eax,0x7
7ca: mov rcx,rsi
7cd: test rcx,rcx
7d0: je 7e1 <f1+0x21>
7d2: mov QWORD PTR [rdi],0x0
7d9: add rdi,0x8
7dd: dec ecx
7df: jne 7d2 <f1+0x12>
7e1: mov rcx,rax
7e4: test ecx,ecx
7e6: je 7f2 <f1+0x32>
7e8: mov BYTE PTR [rdi],0x0
7eb: inc rdi
7ee: dec ecx
7f0: jne 7e8 <f1+0x28>
7f2: mov rax,rcx
7f5: ret