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Subject: Re: [PATCH] Prevent users from disabling tickless
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Pavel Machek <pavel@ucw.cz>
Cc: Matthew Wilcox <matthew@wil.cx>, Thomas Gleixner <tglx@linutronix.de>,
        Arjan van de Ven <arjan@infradead.org>, linux-kernel@vger.kernel.org
In-Reply-To: <20110102085755.GA32469@atrey.karlin.mff.cuni.cz>
References: <20101221233942.GL1263@parisc-linux.org>
	 <20110102085755.GA32469@atrey.karlin.mff.cuni.cz>
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Date: Sun, 02 Jan 2011 10:15:42 +0100
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Le dimanche 02 janvier 2011 à 09:57 +0100, Pavel Machek a écrit :
> Hi!
> 
> > Disabling NO_HZ has a serious negative effect on performance -- an extra
> > 70us per I/O.  Prevent users from deselecting it.
> > 
> > Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
> 
> System with fixed HZ is really simpler etc... and 70us per i/o does
> not sound that bad.
> 
> Anyway, should not i/o overhead just be fixed?

Also, where are those 70us spent exactly ? Once per clock tick ?

For me, I/O can be an ethernet frame sent or received, and I never
noticed such a delay.

At least, commit message sounds very evasive too me.