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Subject: Re: The need to unmake move

Author: Robert Hyatt

Date: 14:17:39 08/27/03

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On August 26, 2003 at 18:38:34, Jeremiah Penery wrote:

>On August 26, 2003 at 11:42:03, Robert Hyatt wrote:
>
>>On August 26, 2003 at 03:00:52, Johan de Koning wrote:
>>
>>>On August 25, 2003 at 18:04:29, Robert Hyatt wrote:
>>>
>>>>A single cpu that will run crafty at 1M nps has a cache-cache and cache-memory
>>>>bandwidth of X bytes/second.  A single cpu that runs crafty at 2M nps has
>>>>exactly twice the cache-cache and cache-memory bandwidth and twice the clock
>>>>frequency.  A dual-cpu just needs two cpus, but the two cpus give twice the
>>>>cache-cache bandwidth, but _no_ improvement in cache-memory bandwidth.
>>>
>>>Nope, C2M bandwith is constant, regardless of n and f (hence constant :-).
>>
>>Actually it isn't.  Some duals use interleaving.  Some don't.  All quads I
>>have here use 4-way interleaving to ramp up the bandwidth significantly.
>>
>>All machines are _not_ created equal...
>
>A great many single-CPU motherboards you can buy offer dual-channel memory
>and/or 4-way memory interleaving.

1.  I don't know much about dual-channel, but it doesn't sound related to
interleaving.

2.  I have looked at most every single-cpu machine we have here and they
are _all_ plain memory boxes (non-interleaved).  The easiest way to catch
interleaving is to find a requirement that you add two (or four) DIMMS, one
per "bank".  If you have just one DIMM, or you have just one/two/three DIMM
slots, that MB isn't interleaving.



>  I don't know if many of the SMP (2-CPU,
>specifically) boards for Intel or Athlon have that stuff or not.  Opteron
>doesn't worry about it, as each CPU has its own dedicated (dual-channel) memory,
>and N CPUs have N times the aggregate bandwidth of a single-CPU machine.


Yes, but Opteron is NUMA, which has its _own_ problem issues to deal with.
Anytime you say "each processor has its own memory" you are talking NUMA.
I haven't seen any recent Intel MBs that didn't have interleaving.  If you
don't do that, you take a big performance hit as memory has a hard enough
time keeping up with one cpu, much less two, without some sort of bandwidth
increasing trickery.

All of my quads have 4-way interleaving without exception, going back to my
original quad pentium-pro 200 box I still have (ALR Revolution).



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