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

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Message-ID: <20070201180247.GA561@elte.hu>
Date:	Thu, 1 Feb 2007 19:02:47 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mark Lord <lkml@....ca>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Zach Brown <zach.brown@...cle.com>,
	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

* Mark Lord <lkml@....ca> wrote:

> >also, we context-switch kernel threads in 350 nsecs on current 
> >hardware and the -rt kernel is certainly happy with that and runs all 
> >hardirqs
> 
> Ingo, how relevant is that "350 nsecs on current hardware" claim?
> 
> I don't mean that in a bad way, but my own experience suggests that 
> most people doing real hard RT (or tight soft RT) are not doing it on 
> x86 architectures.  But rather on lowly 1GHz (or less) ARM based 
> processors and the like.

it's not relevant to those embedded boards, but it's relevant to the AIO 
discussion, which centers around performance.

> For RT issues, those are the platforms I care more about, as those are 
> the ones that get embedded into real-time devices.

yeah. Nevertheless if you want to use -rt on your desktop (under Fedora 
4/5/6) you can track an rpmized+distroized full kernel package quite 
easily, via 3 easy commands:

   cd /etc/yum.repos.d
   wget http://people.redhat.com/~mingo/realtime-preempt/rt.repo

   yum install kernel-rt.x86_64   # on x86_64
   yum install kernel-rt          # on i686

which is closely tracking latest upstream -git. (for example, the 
current kernel-rt-2.6.20-rc7.1.rt3.0109.i686.rpm is based on 
2.6.20-rc7-git1, so if you want to run a kernel rpm that has all of 
Linus' latest commits from yesterday, this might be for you.)

it's rumored to be a quite smooth kernel ;-) So in this sense, because 
this also runs on all my testboxes by default, it matters on modern 
hardware too, at least to me. Today's commodity hardware is tomorrow's 
embedded hardware. If a kernel is good on today's colorful desktop 
hardware then it will be perfect for tomorrow's embedded hardware.

	Ingo
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