linux-kernel - Re: [PATCH 7/8] sched: non-zero lag renice

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Date:	Fri, 24 Oct 2008 22:28:03 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Chris Friesen <cfriesen@...tel.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, efault@....de,
	vatsa@...ibm.com
Subject: Re: [PATCH 7/8] sched: non-zero lag renice

On Fri, 2008-10-24 at 11:47 -0600, Chris Friesen wrote:
> Peter Zijlstra wrote:
> 
>  > Then renicing, esp when lowering nice value (getting heavier), its possible
>  > to get into a starvation scenario. If you got too much runtime as a very
>  > light task, you get shot way far too the right, which means you'll have to
>  > wait for a long time in order to run again.
>  >
>  > If during that wait you get reniced down, fairness would suggest you get run
>  > earlier, because you deserve more time.
>  >
>  > This can be solved by scaling the vruntime so that we keep the real-time
>  > lag invariant.
> 
> If we've already been shot way out to the right, presumably that would give us 
> a large real-time lag.  If we renice to a lower nice level, wouldn't we want 
> to reduce the real-time lag rather than make it constant?

Ah, see but a 1ms real-time lag might be gigantic on weight=15, but
nearly nothing on weight=88761.

1e6 * 1024/15 is massively larger than 1e6 * 1024/88761.

1000000*1024/15 = 68266666
1000000*1024/88761 = 11536




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