Re: [LUG] L3 Cache disabled

[Gordon Henderson <gordon+dcglug@xxxxxxxxxx>]

On Mon, 15 Feb 2010, Simon Waters wrote:

> Noticed on one of our servers (SC1425) that the L3 cache is disabled 
> according to lshw (lshw -C memory).
>
> So far the DELL Poweredge Linux folk haven't come up with an answer.
>
> So anyone know under what conditions L3 cache is enabled/disabled in Linux?
>
> I assume this is deliberate, and due either to hardware choice, or SMP, or 
> some other feature. But I can't find any discussion/explanation of L3 cache 
> above and beyond what it is, and some Linux changelogs saying it needed 
> disabling on some AMD chips.
>
> Machine has 2 x Intel(R) Xeon(TM) CPU 2.80GHz but I'd really like an 
> explanation I can understand. Haven't tried reading the Linux source code, 
> but the documentation didn't help me (although I might have missed some).

There are versions of the Xeon with no L3 cache present - I suspect you've 
got one - like me:

extract from /proc/cpuinfo on a Xeon box: (dual HT core)

model name      : Intel(R) Xeon(TM) CPU 3.00GHz
stepping        : 3
cpu MHz         : 2992.647
cache size      : 2048 KB

        *-cache:0
             description: L1 cache
             physical id: 700
             size: 16KB
             capacity: 16KB
             capabilities: internal write-through data
        *-cache:1
             description: L2 cache
             physical id: 701
             size: 2MB
             capacity: 2MB
             capabilities: internal write-back unified
        *-cache:2 DISABLED
             description: L3 cache
             physical id: 702
             capabilities: internal write-back unified

extract from /var/log/dmesg:

CPU: Trace cache: 12K uops, L1 D cache: 16K
CPU: L2 cache: 2048K

This is an older Dell 2850..

I wonder if it's disabled because it's simply not present?

Not sure how to positively identify the "core" name though - not sure if 
you can unless you take the lid & heatsink off...


Gordon

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