linux-kernel - Re: [PATCH (update 3)] timer: Run calc_load halfway through each round

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Date:	Fri, 2 Mar 2007 17:35:55 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Simon Arlott <simon@...e.lp0.eu>
Cc:	akpm@...ux-foundation.org, Bill Irwin <bill.irwin@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	arjan@...ux.intel.com
Subject: Re: [PATCH (update 3)] timer: Run calc_load halfway through each round_jiffies second

On Friday 02 March 2007 16:15, Simon Arlott wrote:
> Whenever jiffies is started at a multiple of 5*HZ or wraps, calc_load is
> run exactly on the second which is when tasks using round_jiffies will
> be scheduled to run. This has a bad effect on the load average, making
> it tend towards 1.00 if a task happens to run every time the load is
> being calculated.
>
> This changes calc_load so that it updates load half a second after any
> tasks scheduled using round_jiffies.
>

Simon

I believe this patch is too complex/hazardous and may break exp decay 
computation.
(Even if nobody care about avenrun[] those days :), do you ? )

You could just change LOAD_FREQ from (5*HZ) to (5*HZ+1)


#define LOAD_FREQ   (5*HZ+1)


Mathematical proof (well... sort of)

$ cat prog.c
#define FSHIFT          11              /* nr of bits of precision */
#define FIXED_1         ((double)(1<<FSHIFT))   /* 1.0 as fixed-point */

#include <math.h>
#include <stdio.h>
int main()
{
printf("Old values :\n");
printf("#define EXP_1  %g\n", FIXED_1/exp(5.0/60.0));
printf("#define EXP_5  %g\n", FIXED_1/exp(5.0/(5*60.0)));
printf("#define EXP_15 %g\n", FIXED_1/exp(5.0/(15*60.0)));
printf("New values :\n");
printf("%g\n", FIXED_1/exp(5.01/60.0));
printf("%g\n", FIXED_1/exp(5.01/(5*60.0)));
printf("%g\n", FIXED_1/exp(5.01/(15*60.0)));
return 0;
}

# gcc -o prog prog.c -lm
# ./prog
Old values :
#define EXP_1  1884.25
#define EXP_5  2014.15
#define EXP_15 2036.65
New values :
1883.94
2014.08
2036.63

You can see that 5.01 instead of 5.00 second gives the same EXP_xx values.

So (5*HZ + 1) is safe. (because HZ >= 100)

Eric
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