linux-kernel - Re: [PATCH] x86: eliminate redundant/contradicting cache line size config options

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Date:	Mon, 16 Nov 2009 08:08:07 +0000
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Nick Piggin" <npiggin@...e.de>
Cc:	<mingo@...e.hu>, <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [PATCH] x86: eliminate redundant/contradicting cache line
	 size config options

>>> Nick Piggin <npiggin@...e.de> 16.11.09 05:14 >>>
>On Fri, Nov 13, 2009 at 11:54:40AM +0000, Jan Beulich wrote:
>> Rather than having X86_L1_CACHE_BYTES and X86_L1_CACHE_SHIFT (with
>> inconsistent defaults), just having the latter suffices as the former
>> can be easily calculated from it.
>> 
>> To be consistent, also change X86_INTERNODE_CACHE_BYTES to
>> X86_INTERNODE_CACHE_SHIFT, and set it to 7 (128 bytes) for NUMA to
>> account for last level cache line size (which here matters more than
>> L1 cache line size).
>
>I think if we're going to set it to 7 (128B, for Pentium 4), then
>we should set the L1 cache shift as well? Most alignments to
>prevent cacheline pingpong use L1 cache shift for this anyway?

But for P4 L1_CACHE_SHIFT already is 7.

>The internode thing is really just a not quite well defined thing
>because internode cachelines are really expensive and really big
>on vsmp so they warrant trading off extra space on some critical
>structures to reduce pingpong (but this is not to say that other
>structures that are *not* internode annotated do *not* need to
>worry about pingpong).

The internode one, as said in the patch description, should consider
the last level cache line size rather than L1, which 128 seems to be
a much better fit (without in introducing model dependencies like
for L1) than just using the L1 value directly.

Jan

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