Cache sharing

Article: New Itanium Microarchitecture at ISSCC 2011
By: Paul A. Clayton (paaronclayton.delete@this.gmail.com), November 22, 2010 1:42 pm
Room: Moderated Discussions
IntelUser2000 (Intel_user2000@yahoo.ca) on 11/22/10 wrote:
---------------------------
[snip]
>The only problem with that estimate is that would mean the L2 cache would go from
>independent I and D on Montecito/Montvale/Tukwila back to shared ID with 256KB
>capacity. That's a significant reduction from 1MB+256KB on Montecito/Montvale and 512KB+256KB on Tukwila.

Sharing L2I between two cores could easily make sense (ISTM),
especially if the replacement policy has some (or can be
configured to use some) reuse frequency component.

Unless the L1D cache becomes significantly larger, sharing
the L2D cache might be less attractive; access patterns are
probably not as predictable as for instructions, so extra
latency from larger capacity and more banking might be more
of a problem. (Optimizing snoop for one nearest neighbor
and perhaps allowing capacity stealing might make more
sense.) (Also for HPC, some data blocking optimizations
may break with a shared cache of smaller total capacity.
Also the static scheduling of at least FP loads from L2
discourages changing the latency of that cache--unless a
latency-hiding mechanism is included.)

I do wonder why previous implementations did not share L3
cache. (Obvious possible reasons: core-internal bandwidth
is more important than modest capacity and inter-core
bandwidth benefits, core-internal latency . . ., lockstep
operation is simpler with separate cache hierarchies)



Paul A. Clayton
just a technophile
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