linux-kernel - Re: [PATCH] Prevent users from disabling tickless

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Message-ID: <20110102085755.GA32469@atrey.karlin.mff.cuni.cz>
Date:	Sun, 2 Jan 2011 09:57:55 +0100
From:	Pavel Machek <pavel@....cz>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Prevent users from disabling tickless

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@...ux.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?
								Pavel

> diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
> index f06a8a3..55b9a04 100644
> --- a/kernel/time/Kconfig
> +++ b/kernel/time/Kconfig
> @@ -5,13 +5,9 @@ config TICK_ONESHOT
>  	bool
>  
>  config NO_HZ
> -	bool "Tickless System (Dynamic Ticks)"
> +	def_bool y
>  	depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
>  	select TICK_ONESHOT
> -	help
> -	  This option enables a tickless system: timer interrupts will
> -	  only trigger on an as-needed basis both when the system is
> -	  busy and when the system is idle.
>  
>  config HIGH_RES_TIMERS
>  	bool "High Resolution Timer Support"
> 

-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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