linux-kernel - Re: Update cacheline size on X86

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Date:	Fri, 10 Oct 2008 14:28:23 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Dave Jones <davej@...hat.com>
Cc:	x86@...nel.org, Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: Update cacheline size on X86_GENERIC

On Friday 10 October 2008 04:14, Dave Jones wrote:
> I just noticed that configuring a kernel to use CONFIG_X86_GENERIC
> (as is typical for a distro kernel) configures it to use a 128 byte
> cacheline size. This made sense when that was commonplace (P4 era) but
> current
> Intel, AMD and VIA cpus use 64 byte cachelines.

I think P4 technically did have 64 byte cachelines, but had some adjacent
line prefetching. And AFAIK core2 CPUs can do similar prefetching (but
maybe it's smarter and doesn't cause so much bouncing?).

Anyway, GENERIC kernel should run well on all architectures, and while
going too big causes slightly increased structures sometimes, going too
small could result in horrible bouncing.

Lastly, I think x86 will go to 128 byte lines in the next year or two, so
maybe at this point we can just keep 128 byte alignment?

/random thoughts

>
> Signed-off-by: Dave Jones <davej@...hat.com>
>
> --- linux-2.6.26.noarch/arch/x86/Kconfig.cpu~	2008-10-09 12:59:56.000000000
> -0400 +++ linux-2.6.26.noarch/arch/x86/Kconfig.cpu	2008-10-09
> 13:11:32.000000000 -0400 @@ -301,8 +301,8 @@ config X86_CPU
>  # Define implied options from the CPU selection here
>  config X86_L1_CACHE_BYTES
>  	int
> -	default "128" if GENERIC_CPU || MPSC
> -	default "64" if MK8 || MCORE2
> +	default "128" if MPENTIUM4 || MPSC
> +	default "64" if MK8 || MCORE2 || GENERIC_CPU
>  	depends on X86_64
>
>  config X86_INTERNODE_CACHE_BYTES
> @@ -316,10 +316,10 @@ config X86_CMPXCHG
>
>  config X86_L1_CACHE_SHIFT
>  	int
> -	default "7" if MPENTIUM4 || X86_GENERIC || GENERIC_CPU || MPSC
> +	default "7" if MPENTIUM4 || MPSC
>  	default "4" if X86_ELAN || M486 || M386 || MGEODEGX1
>  	default "5" if MWINCHIP3D || MWINCHIP2 || MWINCHIPC6 || MCRUSOE ||
> MEFFICEON || MCYRIXIII || MK6 || MPENTIUMIII || MPENTIUMII || M686 ||
> M586MMX || M586TSC || M586 || MVIAC3_2 || MGEODE_LX -	default "6" if MK7 ||
> MK8 || MPENTIUMM || MCORE2 || MVIAC7
> +	default "6" if MK7 || MK8 || MPENTIUMM || MCORE2 || MVIAC7 || X86_GENERIC
> || GENERIC_CPU
>
>  config X86_XADD
>  	def_bool y
--
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