linux-kernel - Re: PROBLEM: high load average when idle

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Date:	Wed, 03 Oct 2007 22:15:16 +0200 (CEST)
From:	Anders Boström <anders@...trom.dyndns.org>
To:	torvalds@...ux-foundation.org
Cc:	cebbert@...hat.com, arjan@...ux.intel.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: high load average when idle

>>>>> "LT" == Linus Torvalds <torvalds@...ux-foundation.org> writes:

 LT> On Wed, 3 Oct 2007, Chuck Ebbert wrote:
 >> 
 >> But we reduce the number of samples because some ticks just never
 >> happen when the timers get rounded:
 >> 
 >> No rounding:
 >> 
 >> tick ............... tick
 >> 1 running            1 running
 >> 
 >> Rounded:
 >> 
 >> tick
 >> 2 running
 >> 
 >> In the first case the average is 1, but it's 2 in the second.

 LT> In fact, I think this is it!

 LT> The load average is not calculated every tick, because that's not just 
 LT> expensive, but we also want to have some time-based decay. So it's 
 LT> calculated every LOAD_FREQ ticks.

 LT> And guess what: LOAD_FREQ is defined to be exactly five seconds.

 LT> So imagine if the timer gets to be in sync with another event that happens 
 LT> every five seconds - let's pick at random a 5-second JBD transaction 
 LT> thing?

 LT> Anders - does this idiotic patch make a difference for you?

Yes, it does, it fixes the load average!!! I guess we have something
here!

Why does this problem only show up on my computer? Any idea?

/ Anders

 LT> Without this, I can easily imagine that the rounding code tends to try to 
 LT> round to an even second, and the load-average code generally also runs at 
 LT> even seconds!

 LT> 		Linus

 LT> ---
 LT>  include/linux/sched.h |    2 +-
 LT>  1 files changed, 1 insertions(+), 1 deletions(-)

 LT> diff --git a/include/linux/sched.h b/include/linux/sched.h
 LT> index a01ac6d..643de0f 100644
 LT> --- a/include/linux/sched.h
 LT> +++ b/include/linux/sched.h
 LT> @@ -113,7 +113,7 @@ extern unsigned long avenrun[];		/* Load averages */

 LT>  #define FSHIFT		11		/* nr of bits of precision */
 LT>  #define FIXED_1		(1<<FSHIFT)	/* 1.0 as fixed-point */
 LT> -#define LOAD_FREQ	(5*HZ)		/* 5 sec intervals */
 LT> +#define LOAD_FREQ	(5*HZ+1)	/* ~5 sec intervals */
 LT>  #define EXP_1		1884		/* 1/exp(5sec/1min) as fixed-point */
 LT>  #define EXP_5		2014		/* 1/exp(5sec/5min) */
 LT>  #define EXP_15		2037		/* 1/exp(5sec/15min) */
-
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