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Date: 15 Jun 2014 05:07:29 -0400 From: "George Spelvin" <linux@...izon.com> To: linux-kernel@...r.kernel.org, linux@...izon.com, tytso@....edu Subject: Re: Surprising 64-bit performance anomaly (was Re: [PATCH] random: use an improved fast_mix() function) That is just *weird*. I'm testing on bare metal, and I keep measuring significantly better performance for the 64-bit code. Here's an i7-2700K @ 3.50GHz. (I added a "rolled x2" measurement just for interest's sake.) The idle performance is very similar for the original and unrolled x2 versions, but under load the original degrades far more. (174.5 cycles vs. 132.1 for fast_mix3.) And the 64-bit code is less than 2/3 the time of the 32-bit code under load. (84.3 cycles average = 132.1 * 63.8%) Idle make -j9 Rolled x3: 27043 1212 62897 28037 26910 1547 58415 25790 27095 1302 59158 26083 30684 6233 37242 15848 26979 1339 59683 24658 27042 1534 61609 27537 27069 1402 56379 25877 27301 1162 56782 26379 27300 1535 55344 25003 26873 1507 48627 22207 Rolled x2: 25697 2859 34779 11771 27097 5065 31805 12125 25564 2646 54448 22853 25123 2191 57469 29098 24931 2106 57621 27855 25223 2207 63852 31484 24956 2112 74393 34080 25889 2340 65434 33316 25170 2031 68246 31109 25541 2296 70891 34294 Unrolled x3: 23066 1358 50698 29156 22990 1569 59566 28935 22797 1269 58363 28813 22321 1004 54652 30148 23122 1545 52651 29113 22704 875 57904 33033 22793 979 51882 28880 22413 891 55537 30635 22814 997 52760 27188 22998 1020 59592 30133 Original: 22038 2303 26656 18045 21209 2082 37559 22256 21134 1626 40082 23261 20689 1436 34227 19133 20804 1503 48391 28563 20979 1617 48235 28095 20752 1427 52101 29817 21046 1830 56703 30179 20828 1362 51196 28658 20865 1659 51641 29680 Unrolled x2: 21051 1667 29000 17989 20889 1381 28328 16046 21244 1664 33015 21364 21130 1628 32588 21417 20753 1497 42376 25412 21943 2917 39879 24932 20880 1279 24064 15512 21303 1389 38724 24107 21416 1712 36570 23549 20956 1652 33578 22983 64-bit, unrolled x2: 16037 1031 16651 9740 16664 2094 19199 12689 16129 1057 20216 8590 16211 905 18382 9964 16637 2056 20125 8495 16229 1218 24190 13176 16515 1632 28717 18022 16220 1096 23762 13147 16307 1103 19599 5382 16326 1145 24844 12837 -- 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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