Messages in this thread | | | Date | Fri, 2 Mar 2012 09:12:09 +0100 | From | Ingo Molnar <> | Subject | Re: change last level cache alignment on x86? |
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* Alex Shi <alex.shi@intel.com> wrote:
> On Thu, 2012-03-01 at 16:33 +0800, Alex,Shi wrote: > > Currently last level defined in kernel is still 128 bytes, but actually > > I checked intel's core2, NHM, SNB, atom, serial platforms, all of them > > are using 64 bytes. > > I did not get detailed info on AMD platforms. Guess someone like to give > > the info here. So, Is if it possible to do the similar following changes > > to use 64 byte cache alignment in kernel? > > > > === > > diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu > > index 3c57033..f342a5a 100644 > > --- a/arch/x86/Kconfig.cpu > > +++ b/arch/x86/Kconfig.cpu > > @@ -303,7 +303,7 @@ config X86_GENERIC > > config X86_INTERNODE_CACHE_SHIFT > > int > > default "12" if X86_VSMP > > - default "7" if NUMA > > + default "7" if NUMA && (MPENTIUM4) > > default X86_L1_CACHE_SHIFT > > > > config X86_CMPXCHG > > In arch/x86/include/asm/cache.h, the INTERNODE_CACHE_SHIFT macro will > transfer to '__cacheline_aligned_in_smp' finally. > > #ifdef CONFIG_X86_VSMP > #ifdef CONFIG_SMP > #define __cacheline_aligned_in_smp \ > __attribute__((__aligned__(INTERNODE_CACHE_BYTES))) \ > __page_aligned_data > #endif > #endif
Note the #ifdef CONFIG_X86_VSMP - so the 128 bytes does not actually transform into __cacheline_aligned_in_smp.
> look at the following contents in Kconfig.cpu, I wondering if > it is possible to remove 'default "7" if NUMA' line. Then a > thin and fit cache alignment will be potential helpful on > performance. Anyone like to give some comments?
> config X86_INTERNODE_CACHE_SHIFT > int > default "12" if X86_VSMP > - default "7" if NUMA > default X86_L1_CACHE_SHIFT
Yes, removing that line would be fine I think - I think it was copied from the old L1 alignment of 128 bytes (which was a P4 artifact when that CPU was the dominant platform - that's not been the case for a long time already).
Could you please also do a before/after build of an x86 defconfig with NUMA enabled and see what the alignments in the before/after System.map are?
Thanks,
Ingo
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