lkml.org 
[lkml]   [2021]   [Oct]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v4 6/8] tracing/histogram: Optimize division by a power of 2
    On Mon, 25 Oct 2021 13:08:38 -0700
    Kalesh Singh <kaleshsingh@google.com> wrote:

    > == Results ==
    >
    > Divisor is a power of 2 (divisor == 32):
    >
    > test_hist_field_div_not_optimized | 8,717,091 cpu-cycles
    > test_hist_field_div_optimized | 1,643,137 cpu-cycles
    >
    > If the divisor is a power of 2, the optimized version is ~5.3x faster.
    >
    > Divisor is not a power of 2 (divisor == 33):
    >
    > test_hist_field_div_not_optimized | 4,444,324 cpu-cycles
    > test_hist_field_div_optimized | 5,497,958 cpu-cycles

    To optimize this even more, if the divisor is constant, we could make a
    separate function to not do the branch, and just shift or divide.

    And even if it is not a power of 2, for constants, we could implement a
    multiplication and shift, and guarantee an accuracy up to a defined max.


    If div is a constant, then we can calculate the mult and shift, and max
    dividend. Let's use 20 for shift.

    // This works best for small divisors
    if (div > max_div) {
    // only do a real division
    return;
    }
    shift = 20;
    mult = ((1 << shift) + div - 1) / div;
    delta = mult * div - (1 << shift);
    if (!delta) {
    /* div is a power of 2 */
    max = -1;
    return;
    }
    max = (1 << shift) / delta;

    We would of course need to use 64 bit operations (maybe only do this for 64
    bit machines). And perhaps even use bigger shift values to get a bigger max.

    Then we could do:

    if (val1 < max)
    return (val1 * mult) >> shift;

    -- Steve

    \
     
     \ /
      Last update: 2021-10-26 21:16    [W:4.176 / U:0.048 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site