linux-kernel - Re: x86 status was Re: -mm merge plans for 2.6.23

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Message-ID: <Pine.LNX.4.64.0707131728070.1817@scrub.home>
Date:	Fri, 13 Jul 2007 19:23:41 +0200 (CEST)
From:	Roman Zippel <zippel@...ux-m68k.org>
To:	Mike Galbraith <efault@....de>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrea Arcangeli <andrea@...e.de>,
	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: x86 status was Re: -mm merge plans for 2.6.23

Hi,

On Fri, 13 Jul 2007, Mike Galbraith wrote:

> > The new scheduler does _a_lot_ of heavy 64 bit calculations without any 
> > attempt to scale that down a little...
> 
> See prio_to_weight[], prio_to_wmult[] and sysctl_sched_stat_granularity.
> Perhaps more can be done, but "without any attempt..." isn't accurate.

Calculating these values at runtime would have been completely insane, the 
alternative would be a crummy approximation, so using a lookup table is 
actually a good thing. That's not the problem.
BTW could someone please verify the prio_to_wmult table, especially [16] 
and [21] look a little off, like a digit was cut off.

While I'm at this, the 10% scaling there looks a little much (unless there 
are other changes I haven't looked at yet), the old code used more like 
5%. This would mean a prio -20 task would get 98.86% cpu time compared to 
a prio 0 task, that was previously about the difference between -20 and 
19 (and it would have previously gotten only 88.89%), now a prio -20 task 
would get 99.98% cpu time compared to a prio 19 task.
The individual levels are unfortunately not that easily comparable, but at 
the overall scale the change looks IMHO a little drastic.

bye, Roman
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