lkml.org 
[lkml]   [2016]   [Apr]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 1/4] sched/fair: Optimize sum computation with a lookup table
From
Date
A few comments:

On Mon, 2016-04-11 at 06:36 +0800, Yuyang Du wrote:
> __compute_runnable_contrib() uses a loop to compute sum, whereas a
> table loopup can do it faster in a constant time.

lookup typo

> The following python script can be used to generate the constants:

Thanks for including the script.
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
[]
> @@ -2603,6 +2603,15 @@ static const u32 runnable_avg_yN_sum[] = {
>  };
>  
>  /*
> + * Precomputed \Sum y^k { 1<=k<=n, where n%32=0). Values are rolled down to
> + * lower integers.
> + */
> +static const u32 __accumulated_sum_N32[] = {
> +     0, 23371, 35056, 40899, 43820, 45281,
> + 46011, 46376, 46559, 46650, 46696, 46719,
> +};
> +
> +/*
>   * Approximate:
>   *   val * y^n,    where y^32 ~= 0.5 (~1 scheduling period)
>   */
> @@ -2650,14 +2659,9 @@ static u32 __compute_runnable_contrib(u64 n)
>   else if (unlikely(n >= LOAD_AVG_MAX_N))
>   return LOAD_AVG_MAX;
>  
> - /* Compute \Sum k^n combining precomputed values for k^i, \Sum k^j */
> - do {
> - contrib /= 2; /* y^LOAD_AVG_PERIOD = 1/2 */
> - contrib += runnable_avg_yN_sum[LOAD_AVG_PERIOD];
> -
> - n -= LOAD_AVG_PERIOD;
> - } while (n > LOAD_AVG_PERIOD);
> -
> + /* Since n < LOAD_AVG_MAX_N, n/LOAD_AVG_PERIOD < 11 */
> + contrib = __accumulated_sum_N32[n>>5]; /* =n/LOAD_AVG_PERIOD */

Perhaps the variable name could be improved and
the comment block around the runnable_avg_yN
declarations should include this new declaration.

\
 
 \ /
  Last update: 2016-04-12 01:21    [W:0.219 / U:1.084 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site