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Date: 12 Jun 2014 00:13:18 -0400 From: "George Spelvin" <linux@...izon.com> To: linux@...izon.com, tytso@....edu Cc: hpa@...ux.intel.com, linux-kernel@...r.kernel.org, mingo@...nel.org, price@....edu Subject: random: Benchamrking fast_mix2 > I redid my numbers, and I can no longer reproduce the 7x slowdown. I > do see that if you compile w/o -O2, fast_mix2 is twice as slow. But > it's not 7x slower. For my single-round, I needed to drop to 2 loops rather than 3 to match the speed. That's in the source I posted, but I didn't point it out. (It wasn't an attempt to be deceptive, that's just how I happened to have left the file when I was experimenting with various options. I figured if we were looking for 7x, 1.5x wasn't all that important.) That explains some of the residual difference between our figures. When developing, I was using a many-iteration benchmark, and I suspect it fitted in the Ivy Bridge uop cache, which let it saturate the execution resources. Sorry for the premature alarm; I'll go back to work and find something better. I still get comparable speed for 2 loops and -O2: $ cc -W -Wall -m32 -O2 -march=native random.c -o random32 # ./perftest ../spooky/random32 pool 1 = 85670974 e96b1f8f 51244abf 5863283f pool 2 = 03564c6c eba81d03 55c77fa1 760374a7 0: 148 124 (-24) 1: 48 36 (-12) 2: 40 36 (-4) 3: 44 40 (-4) 4: 44 40 (-4) 5: 36 36 (+0) 6: 52 36 (-16) 7: 44 32 (-12) 8: 44 36 (-8) 9: 48 36 (-12) $ cc -W -Wall -m64 -O2 -march=native random.c -o random64 # ./perftest ../spooky/random64 pool 1 = 85670974 e96b1f8f 51244abf 5863283f pool 2 = 03564c6c eba81d03 55c77fa1 760374a7 0: 132 104 (-28) 1: 40 40 (+0) 2: 36 44 (+8) 3: 32 40 (+8) 4: 40 36 (-4) 5: 32 40 (+8) 6: 36 44 (+8) 7: 40 40 (+0) 8: 36 44 (+8) 9: 40 36 (-4) $ cc -W -Wall -m32 -O3 -march=native random.c -o random32 # ./perftest ./random32 pool 1 = 85670974 e96b1f8f 51244abf 5863283f pool 2 = 03564c6c eba81d03 55c77fa1 760374a7 0: 88 48 (-40) 1: 36 40 (+4) 2: 36 44 (+8) 3: 32 40 (+8) 4: 36 40 (+4) 5: 96 40 (-56) 6: 40 40 (+0) 7: 36 40 (+4) 8: 28 48 (+20) 9: 28 40 (+12) $ cc -W -Wall -m64 -O3 -march=native random.c -o random64 # ./perftest ./random64 pool 1 = 85670974 e96b1f8f 51244abf 5863283f pool 2 = 03564c6c eba81d03 55c77fa1 760374a7 0: 72 80 (+8) 1: 36 52 (+16) 2: 32 36 (+4) 3: 32 36 (+4) 4: 28 40 (+12) 5: 32 40 (+8) 6: 32 40 (+8) 7: 32 36 (+4) 8: 28 44 (+16) 9: 36 36 (+0) $ cc -W -Wall -m32 -Os -march=native random.c -o random32 # ./perftest ./random32 pool 1 = 85670974 e96b1f8f 51244abf 5863283f pool 2 = 03564c6c eba81d03 55c77fa1 760374a7 0: 108 132 (+24) 1: 44 44 (+0) 2: 76 40 (-36) 3: 44 48 (+4) 4: 36 40 (+4) 5: 32 44 (+12) 6: 40 56 (+16) 7: 44 36 (-8) 8: 44 40 (-4) 9: 32 40 (+8) $ $ cc -W -Wall -m64 -Os -march=native random.c -o random64 # ./perftest ./random64 pool 1 = 85670974 e96b1f8f 51244abf 5863283f pool 2 = 03564c6c eba81d03 55c77fa1 760374a7 0: 96 108 (+12) 1: 44 52 (+8) 2: 40 40 (+0) 3: 40 36 (-4) 4: 40 32 (-8) 5: 36 36 (+0) 6: 44 32 (-12) 7: 36 36 (+0) 8: 40 36 (-4) 9: 40 36 (-4) Yours looks much more careful about the timing. A few GCC warnings I ended up fixing: 1) "volatile" on rdtsc is meaningless and ignore (with a warning) 2) fast_mix2() needs a void return type; it defaults to int. 3) int main() needs a "return 0" Here's what I got running *your* program, unmodified except for the above (meaning 3 inner loop iterations). Compiled with GCC 4.9.0 (Devian 4.9.0-6), -O2. i7-4940K# ./perftest ./ted32 fast_mix: 430 fast_mix2: 431 fast_mix: 442 fast_mix2: 464 fast_mix: 442 fast_mix2: 465 fast_mix: 442 fast_mix2: 431 fast_mix: 442 fast_mix2: 465 fast_mix: 431 fast_mix2: 430 fast_mix: 442 fast_mix2: 431 fast_mix: 431 fast_mix2: 465 fast_mix: 431 fast_mix2: 465 fast_mix: 431 fast_mix2: 431 i7-4940K# ./perftest ./ted64 fast_mix: 454 fast_mix2: 465 fast_mix: 453 fast_mix2: 465 fast_mix: 442 fast_mix2: 464 fast_mix: 453 fast_mix2: 464 fast_mix: 454 fast_mix2: 465 fast_mix: 453 fast_mix2: 465 fast_mix: 442 fast_mix2: 464 fast_mix: 453 fast_mix2: 464 fast_mix: 453 fast_mix2: 464 fast_mix: 453 fast_mix2: 465 In other words, pretty damn near the same speed (with 3 loops). So we still have some discrepancy to track down. A few other machines. i5-3330$ /tmp/ted32 fast_mix: 226 fast_mix2: 277 fast_mix: 561 fast_mix2: 429 fast_mix: 156 fast_mix2: 406 fast_mix: 504 fast_mix2: 534 fast_mix: 579 fast_mix2: 270 fast_mix: 240 fast_mix2: 270 fast_mix: 494 fast_mix2: 270 fast_mix: 240 fast_mix2: 138 fast_mix: 750 fast_mix2: 277 fast_mix: 124 fast_mix2: 270 i5-3330$ /tmp/ted64 fast_mix: 224 fast_mix2: 277 fast_mix: 226 fast_mix2: 312 fast_mix: 646 fast_mix2: 276 fast_mix: 233 fast_mix2: 456 fast_mix: 591 fast_mix2: 570 fast_mix: 413 fast_mix2: 563 fast_mix: 584 fast_mix2: 270 fast_mix: 231 fast_mix2: 261 fast_mix: 233 fast_mix2: 459 fast_mix: 528 fast_mix2: 277 Pentium4$ /tmp/ted32 fast_mix: 912 fast_mix2: 396 fast_mix: 792 fast_mix2: 160 fast_mix: 524 fast_mix2: 160 fast_mix: 1460 fast_mix2: 440 fast_mix: 496 fast_mix2: 160 fast_mix: 672 fast_mix2: 160 fast_mix: 700 fast_mix2: 160 fast_mix: 336 fast_mix2: 540 fast_mix: 896 fast_mix2: 160 fast_mix: 1052 fast_mix2: 156 Phemom9850$ /tmp/ted32 fast_mix: 463 fast_mix2: 158 fast_mix: 276 fast_mix2: 174 fast_mix: 194 fast_mix2: 135 fast_mix: 620 fast_mix2: 424 fast_mix: 584 fast_mix2: 424 fast_mix: 610 fast_mix2: 418 fast_mix: 651 fast_mix2: 1107 fast_mix: 634 fast_mix2: 439 fast_mix: 632 fast_mix2: 456 fast_mix: 534 fast_mix2: 205 Phemom9850$ /tmp/ted64 fast_mix: 783 fast_mix2: 185 fast_mix: 903 fast_mix2: 144 fast_mix: 955 fast_mix2: 178 fast_mix: 515 fast_mix2: 437 fast_mix: 642 fast_mix2: 580 fast_mix: 610 fast_mix2: 525 fast_mix: 523 fast_mix2: 119 fast_mix: 180 fast_mix2: 315 fast_mix: 596 fast_mix2: 570 fast_mix: 598 fast_mix2: 775 AthlonXP$ /tmp/ted32 fast_mix: 119 fast_mix2: 113 fast_mix: 139 fast_mix2: 109 fast_mix: 155 fast_mix2: 123 fast_mix: 134 fast_mix2: 140 fast_mix: 126 fast_mix2: 154 fast_mix: 134 fast_mix2: 113 fast_mix: 176 fast_mix2: 140 fast_mix: 145 fast_mix2: 113 fast_mix: 134 fast_mix2: 144 fast_mix: 155 fast_mix2: 112 So I'm still a bit confused. Would any bystanders like to chip in? Ted, shall I send you some binaries? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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