Date: Fri, 27 May 2005 14:48:37 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Andi Kleen <ak@muc.de>
Cc: Sven-Thorsten Dietrich <sdietrich@mvista.com>, dwalker@mvista.com,
       bhuey@lnxw.com, nickpiggin@yahoo.com.au, hch@infradead.org,
       akpm@osdl.org, linux-kernel@vger.kernel.org
Subject: Re: RT patch acceptance
Message-ID: <20050527124837.GA7253@elte.hu>
References: <1116982977.19926.63.camel@dhcp153.mvista.com> <20050524184351.47d1a147.akpm@osdl.org> <4293DCB1.8030904@mvista.com> <20050524192029.2ef75b89.akpm@osdl.org> <20050525063306.GC5164@elte.hu> <m1br6zxm1b.fsf@muc.de> <1117044019.5840.32.camel@sdietrich-xp.vilm.net> <20050526193230.GY86087@muc.de> <20050526200424.GA27162@elte.hu> <20050527123529.GD86087@muc.de>
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* Andi Kleen <ak@muc.de> wrote:

> [...] Even normal kernels must have reasonably good latency, as long 
> as it doesnt cost unnecessary performance.

they do get reasonably good latency (within the hard constraints of the 
possibilities of a given preemption model), due to the cross-effects 
between the various preemption models, that i explained in detail in 
earlier mails. Something that directly improves latencies on 
CONFIG_PREEMPT improves the 'subsystem-use latencies' on PREEMPT_RT.  
Also there's the positive interaction between scalability and latencies 
as well.

but it's certainly not for free. Just like there's no zero-cost
virtualization, or there's no zero-cost nanokernel approach either,
there's no zero-cost single-kernel-image deterministic system either.

and the argument about binary kernels - that's a choice up to vendors
and users. Right now PREEMPT_NONE is dominant, so do you argue that
CONFIG_PREEMPT should be removed? It's certainly not zero-cost even on
the source code, witness all the preempt_disable()/preempt_enable() or
get_cpu()/put_cpu() uses.

	Ingo
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