[ltt-dev] liburcu cache line size

[compudj@krystal.dyndns.org (Mathieu Desnoyers)]

* David Goulet (david.goulet at polymtl.ca) wrote:
> Hi,
>
> I have some doubt about the value of #define CACHE_LINE_SIZE  
> (urcu/arch_x86.h) that is set to 128.
>
> After some research and looking on my computer, the x86 architecture  
> seems to have most of the time 64 bytes size. On my i7 920, here's what  
> I have :
>
> # getconf LEVEL1_DCACHE_LINESIZE
> 64
>
> # cat /sys/devices/system/cpu/cpu0/cache/index0/coherency_line_size
> 64
>
> Since the Intel NetBurst microarch., the Intel manual says 64 bytes also  
> and it has not changed apparently for Nehalem.
>
> So, Mathieu, why 128 bytes? UST is using that, if it's the wrong value  
> here for x86, it could have an effect on cache pressure since 2 lines  
> are required for structure less then 64 bytes.

See Linux kernel source:

arch/x86/Kconfig.cpu

#
# Define implied options from the CPU selection here
config X86_INTERNODE_CACHE_SHIFT
        int
        default "12" if X86_VSMP
        default "7" if NUMA
        default X86_L1_CACHE_SHIFT

and

config X86_L1_CACHE_SHIFT
        int
        default "7" if MPENTIUM4 || MPSC
        default "6" if MK7 || MK8 || MPENTIUMM || MCORE2 || MATOM ||
MVIAC7 || X86_GENERIC || GENERIC_CPU
        default "4" if X86_ELAN || M486 || M386 || MGEODEGX1
        default "5" if MWINCHIP3D || MWINCHIPC6 || MCRUSOE || MEFFICEON
|| MCYRIXIII || MK6 || MPENTIUMIII || MPENTIUMII || M686 || M586MMX ||
M586TSC || M586 || MVIAC3_2 || MGEODE_LX

So Pentium 4 seems to have 128 bytes cache lines.

Hopefully the ScaleMP vSMP machine are rare enough (they would require a
4k alignment).

NUMA is not that rare, and requires 128 bytes cache lines too.

Can you send a patch for userspace RCU that documents this briefly in
urcu/arch_x86.h ? (just a summary of the info I pasted here would be
fine)

Thanks,

Mathieu





>
> Thanks!
> -- 
> David Goulet
> LTTng project, DORSAL Lab.
>
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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com