linux-kernel - Re: [PATCH 2 of 4] Introduce i386 fibril scheduling

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Message-ID: <20070201083611.GC18233@elte.hu>
Date:	Thu, 1 Feb 2007 09:36:11 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Zach Brown <zach.brown@...cle.com>
Cc:	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 2 of 4] Introduce i386 fibril scheduling

* Zach Brown <zach.brown@...cle.com> wrote:

> This patch introduces the notion of a 'fibril'.  It's meant to be a 
> lighter kernel thread. [...]

as per my other email, i dont really like this concept. This is the 
killer:

> [...]  There can be multiple of them in the process of executing for a 
> given task_struct, but only one can every be actively running at a 
> time. [...]

there's almost no scheduling cost from being able to arbitrarily 
schedule a kernel thread - but there are /huge/ benefits in it.

would it be hard to redo your AIO patches based on a pool of plain 
simple kernel threads?

We could even extend the scheduling properties of kernel threads so that 
they could also be 'companion threads' of any given user-space task. 
(i.e. they'd always schedule on the same CPu as that user-space task)

I bet most of the real benefit would come from co-scheduling them on the 
same CPU. But this should be a performance property, not a basic design 
property. (And i also think that having a limited per-CPU pool of AIO 
threads works better than having a per-user-thread pool - but again this 
is a detail that can be easily changed, not a fundamental design 
property.)

	Ingo
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