linux-kernel - Re: [REPORT] cfs-v4 vs sd-0.44

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Message-ID: <20070421164241.GF2986@holomorphy.com>
Date:	Sat, 21 Apr 2007 09:42:41 -0700
From:	William Lee Irwin III <wli@...omorphy.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Willy Tarreau <w@....eu>, Ingo Molnar <mingo@...e.hu>,
	Con Kolivas <kernel@...ivas.org>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Gene Heskett <gene.heskett@...il.com>
Subject: Re: [REPORT] cfs-v4 vs sd-0.44

On Sat, 21 Apr 2007, Willy Tarreau wrote:
>> If you remember, with 50/50, I noticed some difficulties to fork many
>> processes. I think that during a fork(), the parent has a higher probability
>> of forking other processes than the child. So at least, we should use
>> something like 67/33 or 75/25 for parent/child.

On Sat, Apr 21, 2007 at 09:34:07AM -0700, Linus Torvalds wrote:
> It would be even better to simply have the rule:
>  - child gets almost no points at startup
>  - but when a parent does a "waitpid()" call and blocks, it will spread 
>    out its points to the childred (the "vfork()" blocking is another case 
>    that is really the same).
> This is a very special kind of "priority inversion" logic: you give higher 
> priority to the things you wait for. Not because of holding any locks, but 
> simply because a blockign waitpid really is a damn big hint that "ok, the 
> child now works for the parent".

An in-kernel scheduler API might help. void yield_to(struct task_struct *)?

A userspace API might be nice, too. e.g. int sched_yield_to(pid_t).


-- wli
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