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

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Date:	Tue, 02 Oct 2007 18:46:18 -0400
From:	Mark Lord <lkml@....ca>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Anders Bostr?m <anders@...trom.dyndns.org>,
	linux-kernel@...r.kernel.org, arjan@...ux.intel.com
Subject: Re: PROBLEM: high load average when idle

Linus Torvalds wrote:
> 
> On Tue, 2 Oct 2007, Andrew Morton wrote:
>> This is unexpected.  High load average is due to either a task chewing a
>> lot of CPU time or a task stuck in uninterruptible sleep.
> 
> Not necessarily.
> 
> We saw high loadaverages with the timer bogosity with "gettimeofday()" and 
> "select()" not agreeing, so they would do things like
> 
> 	date = time(..)
> 	select(.. , timeout = <time + 1> )
> 
> and when "date" wasn't taking the jiffies offset into account, and thus 
> mixing these kinds of different time sources, the select ended up 
> returning immediately because they effectively used different clocks, and 
> suddenly we had some applications chewing up 30% CPU time, because they 
> were in a loop that *tried* to sleep.
> 
> And I wonder if the same kind thing is effectively happening here: the 
> code is written so that it *tries* to sleep, but the rounding of the clock 
> basically means that it's trying to sleep using a different clock than the 
> one we're using to wake things up with, so some percentage of the time it 
> doesn't sleep at all!
> 
> I wonder if the whole "round_jiffies()" thing should be written so that it 
> never rounds down, or at least never rounds down to before the current 
> second!
...

On a related note, {set/get}itimer() currently are buggy (since 2.6.11 or so),
also due to this round_jiffies() thing I believe.

If one sets ITIMER_PROF to, say, 5.000000 seconds, and then reads it back
very shortly thereafter, it will give 5.200000 seconds as the value (HZ==1000).

AFAIK, this should *never* be possible --> any read of get_itimer should never
return a value higher than the starting value.  This makes ITIMER_PROF not very
useful for measuring one's own CPU usage, for example.

Cheers
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