[ltt-dev] liburcu cache line size

[alexandre.montplaisir@polymtl.ca (Alexandre Montplaisir)]

On 10-08-17 03:54 PM, Mathieu Desnoyers wrote:
> * Mathieu Desnoyers (compudj at krystal.dyndns.org) wrote:
>    
>> * David Goulet (david.goulet at polymtl.ca) wrote:
>>      
>>>
>>> On 10-08-17 02:51 PM, Mathieu Desnoyers wrote:
>>>        
>>>> * 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.
>>>>
>>>>          
>>> Yep I saw that and this is why I'm asking because only NUMA, P4 and vSMP
>>> machines are bigger then 64 bytes. The rest is 64 bytes (X86 generic,
>>> Core 2(Nehalem), Atom).
>>>
>>> So you are saying that you prefer use 128 bytes knowing that most of X86
>>> is lower or equal to 64 bytes?
>>>        
>> Yes. The performance degradation caused by cache-line bouncing is _way_
>> worse than extra cache pressure.
>>      
> Oh, and by the way, given that these are arrays made of one variable per
> cpu, the extra space allocated will not consume extra cache lines in any
> of the CPU. We're just wasting a bit a memory here, not adding to cache
> pressure.
>
> Mathieu
>
>    


Sorry to chime in, but wouldn't padding to 128 bytes on architectures 
with 64-byte cache lines "waste" an extra line every time, thus 
indirectly adding to cache pressure?

(relatively newbie here, please be gentle :) )

Alexandre


>> Mathieu
>>
>>      
>>>        
>>>> 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.
>>>>>
>>>>> PGP/GPG : 1024D/16BD8563
>>>>> BE3C 672B 9331 9796 291A  14C6 4AF7 C14B 16BD 8563
>>>>>
>>>>> _______________________________________________
>>>>> ltt-dev mailing list
>>>>> ltt-dev at lists.casi.polymtl.ca
>>>>> http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev
>>>>>
>>>>>            
>>>>          
>>> -- 
>>> David Goulet
>>> LTTng project, DORSAL Lab.
>>>
>>> PGP/GPG : 1024D/16BD8563
>>> BE3C 672B 9331 9796 291A  14C6 4AF7 C14B 16BD 8563
>>>
>>>        
>> -- 
>> Mathieu Desnoyers
>> Operating System Efficiency R&D Consultant
>> EfficiOS Inc.
>> http://www.efficios.com
>>
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>>      
>