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I am trying to find out specifics about caches (in particular which caches are shared between cores and which are not) and have stumpled onto a inconsistency.

sudo lshw says

     description: L1 cache
     physical id: a
     slot: Internal Cache
     size: 64KiB
     capacity: 64KiB
     capabilities: synchronous internal write-back
     description: L2 cache
     physical id: b
     slot: External Cache
     size: 8MiB
     capabilities: synchronous internal write-back

but lscpu claims

L1d cache:   32K
L1i cache:   32K
L2 cache:   256K
L3 cache:  8192K

I do not worry too much about instruction and data cache being added together, but where did L2 go?

Observed on a machine running Ubuntu 10.10, or to let uname -a speak:

Linux name 2.6.35-32-generic #66-Ubuntu SMP Mon Feb 13 21:04:32 UTC 2012 x86_64 GNU/Linux

This is a general question, but note that neither the most precise manufacturer spec I could find nor Wikipedia do have the necessary detail.

Unrelated bonus question: does External Cache mean the cache is shared between the (four) cores (and Internal Cache the opposite)?

share|improve this question
Two thoughts. First, what does the manufacturer say? My approach would probably be to look up the specs for the CPU and then try to reason forward from there. Second, if it's something you could do, I'd be curious to see what you got if you tried this after booting up a Live CD/USB for the most recent Ubuntu or Fedora. (Maybe it's a bug that is already fixed? I suppose there are other ways to check if a bug existed and was fixed, but ...) – irrational John May 14 '12 at 3:05
@irrationalJohn: Good point about official specs (though unrelated to the more general question). Sadly, specs don't even help (see edit). Also, I can't tamper with the machine. – Raphael May 14 '12 at 16:25
up vote 2 down vote accepted

Thank you for adding the extra information about the processor to your question. It helps to know that the examples you posted refer to an Intel Core i7-920 Processor.

The information provided by lscpu is more accurate because it includes all three levels of cache, L1, L2, and L3. It appears that lshw was only minimally modified to reflect Intel's addition of an L3 cache to their CPUs. Instead of displaying information about all three caches levels, the information about the size of the L3 cache is apparently reported as L2 cache.

I assume that the specs you looked at did not include L1 and L2 cache because within a given microarchitecture they are all the same. For example, for Nehalem this is "64 KB L1 cache/core (32 KB L1 Data + 32 KB L1 Instruction) and 256 KB L2 cache/core.".

I believe giving each core its own L1 and L2 with a single, much larger common L3 was first introduced as part of the Nehalem (microarchitecture) (in November 2008?).

I do not know why lshw uses the term External Cache to refer to the L3. But it strikes me as misleading since the L3 cache is on the CPU die and not what I would consider external. Again, this feels like trying to use old software to describe newer hardware while only making minimal changes to the software.

(Probably more could be learned by looking at the actual source code, but I did not have time to try to do that.)

Finally, yes the L3 cache is shared among the cores/threads. The following quote is from the Wikipedia article linked above, "Hyper-threading is reintroduced along with a reduction in L2, which has been incorporated as L3 Cache which is usable by all cores."

share|improve this answer
Thanks for the help regarding the specs. Still, having to hunt down specs sucks if we have tools that pretend to to what we want. – Raphael May 17 '12 at 0:25
@R I was not aware you were (still) looking for something. Frankly, the problem I usually have with Intel is that is too much information provided. What sort of info are you interested in? Looking to the right on the Intel i7-920 page you'll see Quick Links. The datasheet link takes you to a Technical documents page. There are two volumes on the i7-900 processors before they even list the mechanical & thermal specs. Software developer mans on another page. Too many to list. – irrational John May 17 '12 at 0:55
Well, I benchmarked parallel algorithms so cache architecture was important. It sucks that the GNU/Linux tools are apparently not (all) reliable. Anyway, the question was more about the tools than the specific machine I was working on. – Raphael May 17 '12 at 1:04
@Raphael I don't really know why lshw returns the cache results it does. But possibly they didn't want to change their output format out of concern they might break a script that parsed their output. You can paint yourself into a corner all too easily with software tools. – irrational John May 17 '12 at 1:07

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