Differences between L2 & L3 Cache

Discussion in 'Mac Pro' started by turtlebud, Mar 3, 2009.

  1. turtlebud macrumors 6502a

    Jul 17, 2002
    I noticed in the specs for the new Mac Pros that there is 8MB of fully shared L3 cache per processor, but no mention of L2 cache. Previous gen Mac Pros, had 12MB of L2 cache per processor, but no mention of L3 cache. I did some searching to see if I could understand the difference, but maybe someone here has a better explanation.

    (from wikipedia)
    Multi-level caches generally operate by checking the smallest Level 1 (L1) cache first; if it hits, the processor proceeds at high speed. If the smaller cache misses, the next larger cache (L2) is checked, and so on, before external memory is checked.
    As the latency difference between main memory and the fastest cache has become larger, some processors have begun to utilize as many as three levels of on-chip cache. For example, the Alpha 21164 (1995) had a 96 KB on-die L3 cache, the IBM POWER4 (2001) had a 256 MB L3 cache off-chip, shared among several processors, the Itanium 2 (2003) had a 6 MB unified level 3 (L3) cache on-die, Intel's Xeon MP product code-named "Tulsa" (2006) features 16 MiB of on-die L3 cache shared between two processor cores, the AMD Phenom (2007) has a 2 MB on-die L3 cache and the Intel Core i7 (2008) has an 8 MB on-die unified L3 cache that is inclusive, shared by all cores. The benefits of an L3 cache depend on the application's access patterns.
  2. Salavat23 macrumors 6502

    Feb 7, 2008
    Not really much to get here.

    L2=faster, less of it, each core has their own

    L3=slower, much more of it, all the cores share it
  3. turtlebud thread starter macrumors 6502a

    Jul 17, 2002
    okay, so if that's the case, then do the new Mac Pros have L2 cache? If not, that's a pretty big omissions - esp if they ONLY have L3 cache.
  4. Salavat23 macrumors 6502

    Feb 7, 2008
    Of course they do. They have 256kb of very fast L2 cache.
  5. turtlebud thread starter macrumors 6502a

    Jul 17, 2002
    Sorry if I'm being dense or missing something, but how come the previous gen mac pros have 12MB of shared L2 cache per processor and the new mac pros only have 256K of L2 cache?

    I guess my question is whether the 12MB of L2 cache in the prev mac pros are pretty much the same as the 8MB of L3 cache in the new mac pros or if the 12MB of L2 cache is the same as the 256K of L2 cache that you are talking about.
  6. Dr.Pants macrumors 65816


    Jan 8, 2009
    I think that to have an L3 cache, an L2 has to exist. Correct me if I am wrong.

    Also, considering the large L3 cache size and the low L2 cache size, methinks it has something to do with how the new chipset operates:confused: I'm stuck on this one, too. However, it makes sense to have a large L3 in a dual-processor-die machine, since the work of the processors is far more serial in nature.
  7. Eidorian macrumors Penryn


    Mar 23, 2005
    The reduction in L2 cache size also has more to do with the new integrated memory control more than anything else. Check out AMD's processors.

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