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Subject: Re: The need to unmake move

Author: Johan de Koning

Date: 00:13:00 08/26/03

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On August 25, 2003 at 18:28:24, Robert Hyatt wrote:

>On August 25, 2003 at 18:14:15, Sune Fischer wrote:
>
>>
>>>(status quo).
>>
>>Right, saw that one too late :)
>>
>>>That is exactly what I said.  The issue is how many copy/makes have to be
>>>done per unit of time.  Or how many units of time it takes to do a single
>>>copy/make.  Depends on your time scale (1s or 1ns).
>>>
>>>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.
>>>
>>>This was all about memory bandwidth with respect to copy/make.
>>
>>Aaah, and here lies our source of disagreement.
>>
>>I was always talking about cache to cache, or as Johan wrote "C2C" bandwidth.
>>
>>I think the assumptions for make/uncopy to match make/umake is that the stack
>>fits in cache, or else it will be heavily outgunned, even on a single.
>>
>>I doubt anyone disagrees with you there.
>>
>>>>
>>>>You made it sound like 2.4 was much worse than 1, which is just a silly
>>>>comparison at best.
>>>
>>>Clearly 2.4 _is_ much worse than 1.  It requires 2.4X the bandwidth.
>>
>>Yes _if_ you need to go over the bus.
>>
>>-S.
>
>
>My stuff will _never_ fit in 512KB of L2, much less the far smaller L1/trace
>caches (on the PIV).
>
>I am _always_ going to be burning the bus.

Exactly!
The heavily used stuff will *always* be in the cache.
For the other stuff it doesn't matter much if the remaining cache space is
either 31 kB or 28 kB, now does it?

... Johan



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