[CentOS] OT: general question re processor, l2 and l3 cache etc

Sat Apr 7 18:38:31 UTC 2007
John R Pierce <pierce at hogranch.com>

R Lists06 wrote:
> Greetings
> Please forgive the OT question yet I highly value the experience and wisdom
> on this list
> I am wondering if anyone here can address the performance difference between
> having a processor board with say 256KB L2 *and* 2048KB L3 cache *VERSUS*
> just having the same processor board with just the L2 cache in a centos
> server environment...
> Please figure that all other necessary and related componentry is well above
> needed amounts and high performance specs for a professional server
> environment.

Thats absolutely unanswerable.       many servers are far more network 
and/or disk IO bound than they are CPU bound, so that extra cache would 
do nothing.

even on a pure compute workload, many inner loops fit almost entirely 
into the L2 cache and the L3 cache wouldn't be doing much at all.

even if you are getting lots of cache hits from this L3 cache, the 
actual benefit would depend on how slow the external memory latency  is 
vs. this L3 cache.

today's modern CPUs often have 1-4MB of L2 cache, so L3 is even more 
pushed out...  larger caches are a classic case of diminishing returns

The only way you'll get meaningful data on this would be to take these 
two systems, configured identically, and run the same application 
workloads on them, and measure the performance.