* Paolo Abeni <pab...@redhat.com> wrote: > The 'rep' prefix suffers for a relevant "setup cost"; as a result > string copies with unrolled loops are faster than even > optimized string copy using 'rep' variant, for short string. > > This change updates __copy_user_generic() to use the unrolled > version for small string length. The threshold length for short > string - 64 - has been selected with empirical measures as the > larger value that still ensure a measurable gain. > > A micro-benchmark of __copy_from_user() with different lengths shows > the following: > > string len vanilla patched delta > bytes ticks ticks tick(%) > > 0 58 26 32(55%) > 1 49 29 20(40%) > 2 49 31 18(36%) > 3 49 32 17(34%) > 4 50 34 16(32%) > 5 49 35 14(28%) > 6 49 36 13(26%) > 7 49 38 11(22%) > 8 50 31 19(38%) > 9 51 33 18(35%) > 10 52 36 16(30%) > 11 52 37 15(28%) > 12 52 38 14(26%) > 13 52 40 12(23%) > 14 52 41 11(21%) > 15 52 42 10(19%) > 16 51 34 17(33%) > 17 51 35 16(31%) > 18 52 37 15(28%) > 19 51 38 13(25%) > 20 52 39 13(25%) > 21 52 40 12(23%) > 22 51 42 9(17%) > 23 51 46 5(9%) > 24 52 35 17(32%) > 25 52 37 15(28%) > 26 52 38 14(26%) > 27 52 39 13(25%) > 28 52 40 12(23%) > 29 53 42 11(20%) > 30 52 43 9(17%) > 31 52 44 8(15%) > 32 51 36 15(29%) > 33 51 38 13(25%) > 34 51 39 12(23%) > 35 51 41 10(19%) > 36 52 41 11(21%) > 37 52 43 9(17%) > 38 51 44 7(13%) > 39 52 46 6(11%) > 40 51 37 14(27%) > 41 50 38 12(24%) > 42 50 39 11(22%) > 43 50 40 10(20%) > 44 50 42 8(16%) > 45 50 43 7(14%) > 46 50 43 7(14%) > 47 50 45 5(10%) > 48 50 37 13(26%) > 49 49 38 11(22%) > 50 50 40 10(20%) > 51 50 42 8(16%) > 52 50 42 8(16%) > 53 49 46 3(6%) > 54 50 46 4(8%) > 55 49 48 1(2%) > 56 50 39 11(22%) > 57 50 40 10(20%) > 58 49 42 7(14%) > 59 50 42 8(16%) > 60 50 46 4(8%) > 61 50 47 3(6%) > 62 50 48 2(4%) > 63 50 48 2(4%) > 64 51 38 13(25%) > > Above 64 bytes the gain fades away. > > Very similar values are collectd for __copy_to_user(). > UDP receive performances under flood with small packets using recvfrom() > increase by ~5%.
What CPU model(s) were used for the performance testing and was it performance tested on several different types of CPUs? Please add a comment here: + if (len <= 64) + return copy_user_generic_unrolled(to, from, len); + ... because it's not obvious at all that this is a performance optimization, not a correctness issue. Also explain that '64' is a number that we got from performance measurements. But in general I like the change - as long as it was measured on reasonably modern x86 CPUs. I.e. it should not regress on modern Intel or AMD CPUs. Thanks, Ingo