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Re: Moving Data in Memory

I believe a simpler translation of what Ian is saying is this: on the G5, code that wastes the cache (i.e. by abusing the cache associativity) pays a double penalty because the G5 will stall, because it will speculatively predict that the data in the L1 is the data it needs, and that prediction will always be wrong, because your data access pattern is a degenerate case. I'm not sure that was much simpler. Sorry, some things are pretty hard to explain :) :)

Shorter answer: Even if you're using mtrans, you might try padding out the width of your matrix to 1028 when it's first allocated, and see if mtrans is any faster. (You might need to make it 1028x1028; don't know how mtrans works.) It might surprise you.


On Oct 18, 2004, at 8:07 AM, Gohara, David wrote:

Thanks to all who replied.

  For the moment I went with the mtrans() function.  If I am following correctly, the section on the developer site regarding matrix transpose is what the mtrans function is doing (and is essentially what Holger had suggested as well). 

  Ian I'm not certain I am following the statement you made (pasted below).  Could you, or someone else, elaborate a bit?

  Thanks again for the help!

  Dave


The power of 2 issue is even more important on G5.  You can take
load/store dispatch group rejects (like what stalls float<->int
conversions) due to false aliasing from stores N*65536 bytes away from
loads in the same dispatch group.
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 >RE: Moving Data in Memory (From: "Gohara, David " <email@hidden>)



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